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POPULAR  LECTURES 


SCIENCE    AKD    AET 


VOLUME    11. 


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POPULAR  LECTURES 


SCIE:^CE    aid    ART; 


DELIVERED   IN    THE    PRINCIPAL 


CITIES  AND  TOWNS  OF  THE  UNITED  STATES. 


DIONYSIUS    LAilDNER, 

DOCTOR   OF   CIVIL   LAW,   FELLOW   OF   THE    ROYAL   SOCIETIES    OF    LONDON   AND    EDINBURGH- 
OF   THE    ROYAL    IRISH    ACADEMY,    MEiMBER   OF    THE    PRINCIPAL    EUROPEAN    SOCIETIES 
FOR  THE  ADVANCEMENT  OF  SCIENCE,  AND    FORMERLY   PROFESSOR  OF  ASTRON- 
OMY  AND    NATURAL    PHILOSOPHY   IN   THE    UNIVERSITY   OF   LONDON. 


"  The  most  obvious  means  of  elevating  the  people,  is  to  provide  for  them  works  on  popular  and  prac- 
tical science,  freed  from  mathematical  symbols  and  technical  terms,  written  in  simple  and  perspicuous 
language,  and  illustrated  by  facts  and  experiments  which  are  level  to  the  capacity  of  ordinary  minds." 

London  Quaeterly  Review. 


IN    TWO    VOLUMES. 

VOL.  11. 

BOSTON  COLLEGE 

PHYSICS  DEPT, 

NEW    YORK: 

GREELEY  &  McELRATH,  TRIBUNE  BUILDINGS. 


1846. 


BOSTOK  COLtEGE  LIBRAr. 
,  Cja^STHUT  MILL,  MAS^ 


Entered,  according  to  Act  of  Congress,  in  the  year  1846, 

By    GREELEY    &    McELRATH, 

in  the  Clerk's  Office  of  the  District  Court  of  the  United  States,  in  and  for  the  Southern 

District  of  New  York. 


STEREOTVPED    BY    REDFIELD    &   SAVAGE 
13  Chambers  Street,  N.  Y. 


CONTENTS    OF   VOLUME   I. 


[Note. — For  Analytical  Index,  see  first  Volume.] 


THE  PLURALITY  OF  WORLDS  ., page    49 

Contemplation  of  the  Firmament.- — Reflections  thereby  suggested. — Limited  Powers  of  the  Tele- 
scope.— What  it  can  do  for  us. — Its  Effect  on  the  Appearances  of  the  Planets. — Are  the  Planets 
inhabited  ? — Circumstantial  Evidence. — Analogies  of  the  Planets  to  the  Earth. — Plan  of  the  Solar 
System. — Uniform  Supply  of  Light  and  Warmth. — Expedient  for  securing  it. — Different  Dis- 
tances of  the  Planets  do  not  necessarily  infer  different  Temperatures,  nor  diflPerent  Degrees  of 
Light. — Admirable  Adaptatioaof  the  Rotation  of  the  Earth  to  the  Organization  of  its  Inhabitants. 
— The  same  Provision  exists  on  the  Planets. — Minor  and  Major  Planets. — Short  Days  on  the  lat- 
ter.— The  Seasons. — Similar  Arrangement  on  the  Planets. — The  Atmosphere. — Similar  Append- 
age to  the  Planets. — Many  Uses  of  the  Atmosphere. — Clouds. — Rain,  Hail,  and  Snow. — Mountains 
on  the  Planets.— Land  and  Water. — Weights  of  Bodies  on  the  Planets  analogous  to  Weight  on 
the  Earth. — Appearances  of  the  San. — Conclusion. 

THE  SUN 65 

The  most  interesting  Object  in  the  Firmament. — Its  Distance. — How  measured. — Its  Magnitude. — 
How  ascertained. — Its  Bulk  and  Weight. — Its  Density. — Form. — Time  of  Rotation. — Spots. — 
Its  Physical  Constitution. — Nature  of  the  Spots. — Luminous  Coating. — Its  Thickness. — Probable 
Temperature  of  the  Surface  of  the  Sun. — Nature  of  its  Luminous  Matter. 

ECLIPSES 77 

Lunar  and  Solar  Eclipses. — Their  Causes.— Shadow  of  the  Earth. — Shadow  of  the  Moon. — Mag- 
nitude of  Eclipses. — When  they  can  happen. — Central  Solar  Eclipse. — Great  Solar  Eclipse 
described  by  Halley. — Ecliptic  Limits. 

THE  AURORA  BOREALIS 87 

Origin  of  the  Name. — Produced  by  Electricity. — General  Phenomena  of  Auroras. — Various  Exam- 
ples of  this  Meteor. — Blot's  Excursion  to  the  Shetland  Isles  to  observe  the  Aurora. — Lottin's 
Observations  in  1838-39. — Various  Auroras  seen  by  him. — Theory  of  Biot  to  explain  these 
Meteors. — Objections  to  it. — Hypothesis  of  Faraday. — Auroras  seen  on  the  Polar  Voyage  of 
Captain  Franklin. 

ELECTRICITY 101 

Electric  Phenomena  observed  by  the  Ancients.— Thales. — Gilbert  de  Magnete. — Otto  Guericke's 
Electric  Machine. — Hawkesbee's  Experiments.— Stephen  Grey's  Discoveries  on  Electrics  and 
Non-Electrics. — Wheeler  and  Grey's  Experiments. — Dufaye  discovers  the  Resinous  and  Vitreous 
Electricities. — Invention  of  the  Leyden  Phial. — Singular  Effects  of  the  first  Electric  Shocks. — 
Experiments  of  Watson  and  Bevis. — Experiments  on  Conductors. — Franklin's  Experiments  and 
Letters. — His  celebrated  Theoi-y  of  Positive  and  Negative  Electricity. — His  Experiments  on  the 
Leyden  Phial. — His  Discovery  of  the  Identity  of  Lightning  and  Electricity. — Reception  of  his 
Suggestions  by  the  Royal  Society.— His  Kite  Experiment— His  Right  to  this  Discovery  denied 
by  Arago.— His  Claim  vindicated.— Invention  of  Conductors.— Death  of  Richmann.— Beccaria's 
Obsei-vations.- Canton's  Experiments.— Discovery  of  Induction.- Invention  of  the  Condenser.— 
Works  of  ^pinus. — Theory  of  Symmer. — Experiments  of  Coulomb. — Balance  of  Torsion. — 
Electricity  of  the  Atmosphere. — Effects  of  Flame. — Experiments  of  Volta. — Lavoisier  and  La- 
place.:— Analytical  Woi'k  of  Poissou. 


THE  MINOR  PLANETS page  141 

Classification  of  the  Planets. — Mercary. — Transit  over  the  Sun. — Relative  Position  with  regard  to 
the  Sun. — Difficulty  of  observing  it. — Venus. — Diurnal  Motion  of  Venus  and  Mercury  indicated 
by  the  Shadows  of  Mountains. — Direction  of  the  Axis  of  Rotation. — Seasons,  Climates,  and 
2;ones — Orbits  and  Transits  of  Mercury  and  Venus. — Mountains  on  Mercury  and  Venus. — Influ- 
ence of  the  Sun  at  Mercury  and  Venus. — Twilight  on  Mercury  and  Venus. — Mars. — Atmosphere 
of  Mars. — Physical  Constitution  of  Mars.— Has  Mars  a  Satellite? — Appearance  of  the  Sun  at 
Mars. — Its  close  Analogy  to  the  Earth. 

WEATHER  ALMANACS 157 

Merits  of  Weather  Almanacs. — Excitability  of  the  London  Public. — Fright  produced  by  Biela's 
Comet. — London  Water  Panic. — London  Air  Panic. — London  Bread  Panic — Rage  for  Weather 
Almanacs. — Patrick  Murphy's  Pretensions — Examination  of  the  Predictions  of  the  Weather 
Almanac. — Their  Absurdity. — Comparison  of  the  Predictions  with  the  Event. — Morrison's 
^^''eather  Almanac. — Charlatanism  of  these  Publications. — Great  Frost  of  1838  in  London. — 
Other  Visitations  of  Cold. 

HALLEY'S  COMET 169 

Predictions  of  Science. — Structure  of  the  Solar  System. — Motion  of  Comets. — How  to  identify 
them. — Intervals  of  their  Appearance. — Halley's  Comet. — Its  History. — Newton's  Conjectures. — 
Sagacity  of  Voltaire.— Halley's  Researches — Foretells  the  Reappearance  of  the  Comet  in  1759. — 
Principle  of  Gravitation  applied  to  its  Motion  by  Clairaut. — Researches  of  that  Mathematician. — 
Anecdotes  of  Lalande  and  Madame  Lepaute.— Minute  and  circumstantial  Prediction  of  the  Re- 
appearance of  Halley's  Comet. — Di.scovery  of  the  Planet  Herschel  anticipated  by  Clairaut. — 
Reappearance  of  the  Comet  at  the  predicted  Time. — Second  Prediction  of  its  Return  in  1835. — 
Prediction  fulfilled. — Observations  on  its  Appearance  in  1835. 

THE  ATMOSPHERE 191 

Atmospheric  Air  is  material.- — Its  Color. — Cause  of  the  blue  Sky. — Cause  of  the  green  Sea. — Air 
has  Weight. — Experimental  Proofs. — Air  has  Inertia. — Examples  of  its  Resistance.— It  acquires 
moving  Force. — Examples  of  its  Impact. — Air  is  impenetrable. — Experimental  Proofs. — Elastic 
and  compressing  Forces  equal. — Limited  Height  of  the  Atmosphere. — Elasticity  proportioned  to 
the  Density. — Experimental  Proofs. — Internal  and  external  Pressure  on  close  Vessels  contain- 
ing Air. 

THE  NEW  PLANETS 203 

Indications  of  a  Gap  in  the  Solar  System. — Bode's  Analogy. — Prediction  founded  upon  it. — Piazzi 
discovers  Ceres. — Dr.  Olbers  discovers  Pallas.— Harding  discovers  Juno. — Dr.  Olbers  discovers 
Vesta. — Indications  afforded  by  these  Bodies  of  the  Truth  of  Bode's  Predictions. — Fragments 
of  a  broken  Planet. — Others  probably  still  undiscovered. — Their  ultra-zodiacal  Motions.- — Their 
Eccentricities. — They  are  probably  not  globular. — Other  Singularities  of  their  Appearance. 

THE  TIDES , o ....  209 

Correspondence  between  the  Tides  and  Phases  of  the  Moon  shown  by  Kepler. — Erroneous  popular 
Notion  of  the  Moon's  Influence. — Actual  Manner  in  which  the  Moon  operates. — Influence  of  the 
Sun. — Combined  Action  of  the  Sun  and  Moon. — Spring  Tides. — Counter-action  of  the  Sun  and 
Moon. — Neap  Tides. — Priming  and  Lagging  of  the  Tides — Discussions  at  the  British  Association. 
— Whewell's  Researches. — EiFect  of  Continents  and  Islands  on  the  Tides. — General  Progress 
of  the  Great  Tidal  Wave.— Velocity  of  the  Tidal  Wave.— Range  of  the  Tide. 

LIGHT 221 

Structure  of  the  Eye. — Manner  in  which  distant  Objects  become  visible. — Corpuscular  Theory. — 
Undulatory  Theory. — Its  general  Reception. — Velocity  of  Light. — Account  of  its  Discovery  by 
Roemer. — Measurement  of  the  Waves  of  Light  by  Newton. — Color  produced  by  Waves  of 
different  Magnitudes. — Magnitudes  of  Waves  of  dift'erent  Color. — Summary  View  of  the  Corpus- 
cular Theory. — Summary  View  of  the  undulatory  Theory. — These  Theories  compared. — Discov- 
eries of  Dr.  Young. — Discoveries  of  Malus,  Arago,  Poisson,  Herschel,  and  Airy. — Relations  of 
Light  and  Heat. 

THE  MAJOR  PLANETS 235 

Space  between  Mars  and  Jupiter. — Jupiter's  Distance  and  Period — His  Magnitude  and  Weight. 
— His  Velocity. — Appearance  of  his  Disk. — Day  and  Night  on  Jupiter. — Position  of  his  Axis. — 
Absence  of  Seasons. — His  telescopic  Appearance. — His  Belts.-^Causes  of  his  Belts. — Currents 
in  his  Atmosphere. — Madler's  telescopic  Views  of  Jupiter. — Appearance  of  the  Sun  as  seen  from 
Jupiter. — His  Satellites. — The  Variety  of  his  Months. — Magnificent  Appearance  of  the  Moons  as 
Been  from  Jupiter. — Their  Eclipses. — Saturn. — His  diurnal  Rotation. — Appearance  of  the  Sun 
as  seen  from  him. — His  Atmosphere.— His  Rings. — Their  Dimensions. — B lot's  Explanation  of 
their  Stability. — Herscbel's  Theory  of  the  same. — Appearances  and  Disappearances  of  the 
Rings. — Various  Phases  of  the  Rings. — Saturn's  Satellites. — Herschel  or  Uranus. — His  Dis- 
tance and  Magnitude. — His  Moons. — Reasons  why  there  is  no  Planet  beyond  his  Orbit. 

REFLECTION  OF  LIGHT 257 

Ray  of  Light. — Pencil  of  Light. — Reflection. — Irregular  Reflection. — Regular  Reflection. — Different 
Powers  of  Reflection  in  different  Bodies. — B.eflection  at  plane  Surfaces. — Its  Laws. — Image  of 
an  Object  in  a  plane  Reflector. — Reflection  of  curved  Surfaces. — Concave  Reflectors. — Convex 
Reflectors. — Images  in  sphericai  Reflectors. — Illusion  of  the  Air-drawn  Dagger.— Effects  of  com- 


CONTENTS   OF   VOLUME   I. 


mon  Looking-Glasses  analyzed. — A  flattering  Glass  explained. — Metallic  Specula. — Reflection  in 
Liquids. — Image  of  the  Banks  of  a  Lake  or  River. 

PROSPECTS  OF  STEAM-NAVIGATION page     267 

Retrospect  of  Atlantic  Steamers. — Origin  of  the  Great  Western. — Cunard  Steamers. — Can  Steam 
Packet-Ships  be  successful? — Difficulties  attending  their  Operation. — In  a  commercial  Sense. — 
In  a  mechanical  and  nautical  Sense. — Great  Expedition  must  be  given  up. — Defects  of  common 
Paddle-Wheels. — Defects  of  the  present  Steam- Vessels  as  applicable  to  War. — Difficulty  of  long 
Ocean-Voyages. — Ericsson's  Propeller. — Loper's  Propeller. — Advantages  of  Submerged  Propel- 
ler.— Method  of  raising  the  Propeller  out  of  the  Water. — Fuel. — Form  and  Arrangement  of  the 
proposed  Steam  Packet-Ships. — War-Steamers.— The  Princeton. — Effects  to  ensue  from  the  new 
Steamships. — Conclusion. 

THE  BAROMETER 283 

Maxim  of  the  Ancients. — Abhorrence  of  a  Vacuum. — Suction. — Galileo's  Investigations. — Torricelli 
discovers  the  Atmospheric  Pressure. — The  Barometer. — Pascal's  Experiment. — Requisites  for  a 
good  Barometer. — Means  of  securing  them. — Diagonal  Barometer. — Wheel  Barometer. — Ver- 
nier.— Uses  of  the  Barometer. — Variation  of  Atmospheric  Pressure. — Weather- Glasses. — Rules  in 
common  Use  absurd. — Correct  Rules. — Measurement  of  Heights. — Pressure  on  Bodies. — Why 
not  apparent. — Effect  of  a  Leather  Sucker. — How  Flies  adhere  to  Ceilings  and  Fishes  to  Rocks. — 
Breathing. — Common  Bellows. — Forge  Bellows. — Vent  Peg. — Teapot.— Kettle. — Ink-Bottles. — 
Pneumatic  Trough. — Gurgling  Noise  in  decanting  Wine. 

THE  MOON 305 

P:)pular  Interest  attached  to  the  Moon. — Its  Distance. — Its  Rotation. — Same  Face  always  toward 
';he  Earth. — Its  Phases. — Its  changes  of  Position  with  regard  to  the  Sun. — Has  it  an  Atmosphere  ? 
— Optical  Test  to  determine  it. — Physical  dualities  of  Moonlight. — Is  Moonlight  w^arm  or 
cold? — Does  Water  exist  on  the  Moon? — Does  the  Moon  influence  the  Weather? — Mode  of 
determining  this. — Physical  Condition  of  the  lunar  Surface. — Absence  of  Air  and  Gases. — Ab- 
sence of  Liquids. — Appearance  of  the  Earth  as  seen  from  the  Moon. — Prevalence  of  Mountains 
ujon  it. — Their  general  volcanic  Character. — Appearance  of  the  Mountain  Tycho. — Heights  of 
lunar  Mountains  and  Depths  of  Ravines. — Telescopic  Views  of  the  Moon  by  Beer  and  Madler. — 
Detached  Views  of  the  lunar  Surface. — Condition  of  a  lunar  Crater  deduced  from  Analogy. 

HEAT 323 

Heat  as  a  Branch  of  elementary  Physics  neglected. — Has  as  strong  Claims  as  Light,  Electricity,  or 
Majnetism. — Is  a  universal  Agent  in  Nature. — In  Art. — In  Science. — Astronomy. — Chemistry. — 
In  every  Situation  of  Life. — Applications  of  it  in  Clothing  and  artificial  Warming  and  Cooling. — 
Ligiiting. — Admits  of  easy  explanation. — Dilatation. — Examples. — Thermometer. — Melting  and 
boiling  Points. — Evaporation. — Specific  Heat. — Heat  produced  by  Compression. — Radiation. — 
Conduction. — Incandescence. 

THE  ATLANTIC  STEAM  QUESTION.. 335 

The  Project  proposed  in  1835. — Previous  Condition  of  Steam  Navigation  in  Europe. — Practicability 
of  the  Atlantic  Voyage  not  denied  or  doubted. — Report  of  the  Meeting  of  the  British  Association 
at  Bristol. — Extract  from  the  London  Times. — Ocean  Voyages  for  Steamers  and  sailing  Vessels 
compared. — Effect  of  the  westerly  Winds  in  the  Atlantic. — The  British  Postoffice  Contract 
necessary  for  the  commercial  Success  of  the  Project. — Review  of  the  Pi-oceedings  since  1837. — 
Cunard  Line  of  Steamers. — The  Support  received  by  them  from  the  British  Postoffice. — Total 
Failure  of  the  Project  to  establish  New  York  and  Liverpool  Steam-Liners. — Essay  on  the  Clues- 
tion,  "  Has  Atlantic  Steam  Navigation  been  successful?"  published  in  the  London  Civil  Engineer 
and  Architect's  Journal. 

GALVANISM 359 

Origin  of  the  Discovery. — Galvani  Professor  at  Bologna. — Accidental  Effect  on  Frogs. — Ignorance 
of  Galvani. — His  Experiments  on  the  Frog. — Accidental  Discovery  of  the  Effect  of  metallic 
Contact. — Animal  Electricity. — Galvani  opposed  by  Volta. — Volta's  Theoi-y  of  Contact  prevails. — 
Fabroni's  Experiments — Invention  of  the  Voltaic  Pile. — La  Couronne  des  Tasses. — Napoleon's 
Invitation  to  Volta. — Physiological  Effects  of  the  Pile. — Anecdote  of  Napoleon. — Decomposition 
of  Water. — Cruickshank's  Experiments. — Davy  commences  his  Researches. — Effect  of  Chemical 
Action  discovered. — Ritter's  Secondary  Pile. — Calorific  Effects  of  the  Pile. — Hypothesis  of 
Grotthus. — Davy's  celebrated  Bakerian  Lecture. — Prize  awarded  him  by  the  French  Academy. — 
His  Discovery  of  the  Transferring  Power  of  the  Pile  in  chemical  Action. — His  Electro-Chemical 
Theory. — Decomposition  of  Potash  and  Soda. — New  Metals,  Potassium  and  Sodium. — Discovery 
of  Barium. — Strontium,  Calcium,  and  Magnesium. — Rapid  Discovery  of  the  other  new  Metals. — 
Dry  Piles. 

THE  MOON  AND  THE  WEATHER 403 

Ancient  Prognostics  of  Aristotle,  Theophrastus,  Aratus,  Tbeon,  Pliny,  Virgil. — Recent  Predictions. — 
Theory  of  Lunar  Attraction  not  in  accordance  with  popular  Opinion. — Changes  of  Weather  com- 
pared with  Changes  of  the  Moon. — Prevalence  of  Rain  compared  with  lunar  Phases. — Direction 
of  the  Wind. — Height  of  Barometer  compared  with  lunar  Phases. — Erroneous  Notions  of  Cycles 
of  nineteen  and  nine  Years.. — Cycle  of  four  and  eight  Years  mentioned  by  Pliny. 

PERIODIC  COMETS 421 

Encke's  Comet.— Its  Period  and  Orbit. — How  its  Motion  shows  the  Existence  of  a  resisting 
Medium. — This  Result  corroborated  by  the  Theory  of  Light. — Newton's  Conjectures  respecting 


CONTENTS  OF  VOLUME  I. 


Comets. — Biela's  Comet. — Its  Period  and  Orbit — Lexell's  Comet. — Causes  of  its  Appearance  and 
Disappearance. — Whiston's  Comet. — His  Theory. — Did  this  Comet  produce  the  Deluge? — Orbit 
of  this  Comet. 

RADIATION  OF  HEAT page    435 

Radiation  a  Property  of  Heat. — Prismatic  Spectrum. — Invisible  Rays. — Two  Hypotheses. — Invis- 
ible Rays  alike  in  their  Properties  to  luminous  Rays. — Discoveries  of  Leslie. — Differential  Ther- 
mometer.— Radiation,  Reflection,  and  Absorption. — Effect  of  Screens. — Supposed  Rays  of  Cold.— 
Common  Phenomenon  explained. — Theory  of  Dew. 

METEORIC  STONES  AND  SHOOTING  STARS 457 

Inductive  Method. — Appearances  accompanying  Meteorites. — Theories  to  explain  them. — Exam- 
ination of  these  Theories. — Shooting  Stars. — November  and  August  Meteors. — Orbits  and  Dis- 
tances.— Heights. — Chladni's  Hypothesis. 

THE  EARTH 475 

A  difficult  Subject  of  Investigation. — Form  of  the  Earth. — How  proved  globular. — Its  Magnitude. — 
Its  annual  Motion. — Elliptic  Form  of  its  Orbit. — Proofs  of  its  annual  Motion  from  the  Theory  of 
Gravitation. — From  the  Motion  of  Light. — The  Earth's  diarnal  Motion. — Inequalities  of  Day  and 
Night. — Weight  of  the  Earth. — Maskelyne's  Experiment. — Cavendish's  Experiment. — Their 
Accordance. — Density  of  the  Earth. — The  Seasons. — Calorific  Effect  of  the  Sun's  Rays. — Whj 
the  longest  is  not  the  hottest  Day. — Why  the  shortest  Day  is  not  the  colde,st. — The  hottest  Seasoa 
takes  place  when  the  Sun  is  farthest  from  the  Earth. — Proofs  of  the  diurnal  Rotation. — Spheroidal 
Form  of  the  Earth  proved  by  Theory  and  by  Observation. 

LUNAR  INFLUENCES 499 

The  red  Moon. — Supposed  Effect  of  the  Moon  on  the  Movement  of  Sap  in  Plants. — Prejudice 
respecting  the  time  for  felling  Timber. — Extent  of  this  Prejudice.— Its  Prevalence  among  "Triins- 
atlantic  People. — Prejudices  respecting  Effects  on  Grain. — On  Wine. — On  the  Complexion.— On 
Putrefaction. — On  Wounds. — On  the  size  of  Oysters  and  Shellfish. — On  the  Marrow  of  Animafs. — 
On  the  \Veight  of  the  human  Body. — On  the  Time  of  Births. — On  the  Hatching  of  Eggg.— On 
Human  Maladies. — On  Insanity. — On  Fevers. — On  Epidemics. — Case  of  Vallisnieri. — Case  of 
Bacon. — On  Cutaneous  Diseases,  Convulsions,  Paralysis,  Epilepsy,  &c. — Observations  of  Dr. 
Olbers. 

PHYSICAL  CONSTITUTION  OF  COMETS 511 

Orbitual  Motion  of  Comets. — Their  Number. — Their  Light. — Explanation  of  this. — Theory  of  Her- 
gcbel. — Constitution  of  Comets. — Nebulosity. — Nucleus. — Tail. — Comets  of  1811 — 1680 — 1769 — 
1744—1843-1844. 

THUNDER-STORMS 529 

The  Deficiency  of  our  present  Knowledge. — Of  common  Thunder- Clouds. — Character  and  electric 
Charge  of  Clouds. — Discharge  between  vicinal  Clouds. — Conditions  for  such  Discharge. — Dis- 
charge between  the  Clouds  and  the  Earth. — Mutual  Attraction  or  Repulsion  of  electrized  Clouds. 
— Characters  of  the  upper  and  of  the  lower  Surface  of  Clouds. — Negative  Testimony  respecting  i 
Thunder  from  an  isolated  Cloud. — Cases  of  Lightning  from  an  isolated  Cloud. — A  fresh  Case  ( 
related  by  M.  Duperrey. — Obvious  Inferences  from  the  above  Cases. — Of  volcanic  Thunder-  y 
Clouds. — Lightning  from  the  Ashes,  Smoke,  and  Vapor  of  Volcanoes. — Theoretical  Ideas  of  its  ( 
Origin — Of  the  Height  of  stormy  Clouds. — Mode  of  Observation, — Ascending  Flashes  of  Light-  S 
ning. — Minor  Limits  of  the  Height  of  Storm-Clouds. — Inefficiency  of  many  recorded  Observations.  S 
— Table  of  Observations  as  collected  by  Arago — Flash  of  Lightning  from  a  Cloud  upward. — Of 
Lightning. — Varieties  of  Lightning. — Zigzag" Lightning.— Forked  Lightning. — Deficiency  in  our 
Vocabulary  of  Terms. — Sheet  Lightning. — Table  of  Instances  of  Ball-Lightning. — Mr.  Harris's 
■  Explanation  of  Ball-Lightning. — On  the  Speed  of  Lightning. — Theory  of  Vision  illustrated  bj 
a  rotating  Disk. — Wheatstone's  Experiments. — Observations  of  the  Velocity  of  Lightning.— 
Silent  Lightning. — Heat  Lightning. — Thunder-Bursts.— O/"  luminous  Clouds.-— Clouds  them- 
selves faintly  luminous. — Possession  of  the  duality  in  various  Degrees. — Clouds  visibly  luminouf. 
— Various  Observations  of  luminous  Clouds. — Sabine's  Observations. — Of  Thunder. — RoUiife 
of  Thunder. — Duration  and  Intensity  of  rolling  Thunder. — Violent  Thunder  from  Ball-Lightning.-t- 
Interval  between  Lightning  and  Thunder. — A  Case  in  which  they  were  almost  simultaneous.-i- 
Thunder  without  Lightning. — Noise  attendant  on  Earthquakes. — Of  the  Attempts  to  explain  the 
Phenomena  of  Thunder  and  Lightning. — Identity  of  Lightning  and  Electricity. — Whether  pon- 
derable Matter,  or  a  Propagation  of  Undulations. — Difficulties  of  the  undulatory  Hypothesis. — 
Ball-Lightning  and  the  Inferences  to  which  it  leads. — Bituminous  Matter  accompanying  a  Case 
of  Lightning  Discharge. — Explanations  of  silent  Lightnings. — Observations  of  silent  Lightnings.— 
Difficulties  in  the  Explanation  of  silent  Lightnings. — Arago's  Suggestion  for  Observations. — 
Lightning  hidden  by  dense  Clouds. — Place  of  the  Sound  of  Thunder. — Greatest  Distance  at 
which  Thunder  is  heard. — Case  of  Distance  beyond  which  it  was  inaudible. — Distance  at  which 
other  Sounds  have  been  heard.— Effects  of  Heat,  Cold,  Wind,  &c. — On  the  Transmission  of 
Sound. — Thunder  heard  when  no  Cloud  was  visible. — Hypothesis  of  the  Cause  of  Thunder  from 
the  Creation  of  a  Vacuum. — Contractions  and  Dilatations  of  the  Air  assigned  as  the  Cause. — 
Pouillet's  Theory  of  Decompositions  and  Recompositions. — Influence  of  Echo  in  causing  the 
ExiU. — Duration  of  an  Echo. — Duration  of  the  Roll  of  Thunder  at  Sea. — Dr.  Robison's  Explana- 
tion of  the  Roll. — Application  of  the  Theory  to  Zigzag  Lightning. — Inefficiency  of  the  Theory. — 
Means  of  obtaining  a  minor  Limit  of  the  Length  of  a  Flash. 


CONTENTS  OF  VOLUME  I 


THE  LATITUDES  AND  LONGITUDES page     559 

Definition  of  the  Equator  and  Poles. — Northern  and  SQuthem  Hemispheres. — Latitude  of  a  Place  — 
Parallel  of  Latitude. — Meridian  of  a  Place. — Longitude  of  a  Place. — Standard  Meridian. — Meth- 
ods of  determining  Latitude  and  Longitude  various. — To  find  the  Latitude. — Methods  applicable 
in  Observatories. — At  Sea. — Hadley's  Sextant. — To  determine  the  Longitude. — How  to  find  the 
Time  of  Day  at  Land. — At  Sea. — Use  of  Chronometers. — Lnnar  Method  of  finding  the  Longi- 
tude.— Apparatus  provided  at  Greenwich  for  giving  the  exact  Time  to  Ships  leaving  the  Port  of 
London. — Method  of  determining  Longitude  by  Moon-culminating  Stars. 

THEORY  OF  COLORS . .   573 

Refraction  of  a  Ray  of  Light. — At  plane  Surfaces. — By  a  Prism. — The  Prismatic  Spectrum. — The 
Decomposition  of  Light. — Newton's  Discoveries. — Colors  of  the  Spectrum. — Brewster's  Discoveiy 
of  three  Colors. — How  three  Colors  can  produce  the  Spectrum. — Colors  of  natural  Bodies. — How 
they  are  produced. 

THE  VISIBLE  STARS 583 

What  occupies  the  Space  beyond  the  Limits  of  the  Solar  System. — Wide  Vacuity  between  this 
System  and  the  Stars. — Indications  of  this  observable  in  the  Motions  of  the  Planets. — Indications 
in  the  Motions  of  the  Comets. — The  immence  Distance  of  the  Stars  proved  by  the  Earth's  annual 
Motion. — Observations  made  at  Greenwich. — Bessel's  Discovery  of  the  Parallax. — The  conse- 
quent Distance  of  the  Stars — Illustrations  of  the  Magnitude  of  this  Distance. — The  different 
Orders  and  Magnitudes  of  the  Stars. — How  accounted  for. — Why  those  of  the  lowest  Magnitude 
are  most  numerous. — The  real  Magnitude  of  the  Stars. — The  Telescope  unable  to  magnify 
them. — Dr.  Wollaston's  Investigations  of  the  comparative  Brightness  and  Magnitude. of  the  Stars 
in  relation  to  the  Sun. — Their  stupendous  Magnitude. — Application  of  this  to  the  Dog-Star. 

WATERSPOUTS  AND  WHIRLWINDS 597 

Character  and  Effects  of  Water-Spouts. — Ditference  between  Water  and  Land-Spouts. — Land- 
Spout  at  Montpellier. — Land-Spout  at  Esclades. — Columns  of  Sand  on  the  Steppes  of  South 
America. — Meteor  at  Carcassonne. — Meteor  at  Dreux  and  Mantes. — Land-Spout  at  Ossonval. — 
Meteor  witnessed  and  described  by  M.  Peltier. — Conversion  of  a  Storm  into  a  Land-Spout. — 
M.  Peltier's  Tables  of  Water-Spouts  and  Land-Spouts. — Analysis  of  the  above  Tables. — Water- 
Spouts  seen  by  Captain  Beechy. — Experimental  Illustration  of  the  Phenomena. — Illustration 
of  the  gyratory  Motion  of  Water-Spouts. — M.  Peltier's  Deductions  concerning  Water  Spouts. — 
Action  of  charged  Clouds  on  light  Bodies. — Noise  attending  Water  and  Land  Spouts. — Trans- 
ition from  direct  to  gyratory  Motion. — Effect  of  Induction  on  watery  Surfaces — Disappearance 
of  Pools,  &c. 


CONTENTS    OF   VOLUME   II. 


[Note. — For  Analytical  Index,  see  first  Volume.] 


MATTER  AND  ITS  PHYSICAL  PROPERTIES page     17 

Divisibility. — Unlimited  Divisibility. — Wollaston's  Micrometric  Wire. — Method  of  Making  it. — 
Thickness  of  a  Soap-Babble. — Wings  of  Insects. — Gilding  of  Embroidery. — Globules  of  the 
Blood. — Animalcules. — Their  minute  Organization. — Ultimate  Atoms. — Crystals. — Porosity. — Vol- 
ume.— Den.sity. — duicksilver  passing  through  .Pores  of  V/ood. — Filtration. — Porosity  of  Hydro- 
phane. — Compressibility. — Elasticity.— Dilatability. — Heat. — Contraction  of  Metal  used  to  restore 
the  Perpendicular  to  Walls  of  a  Building. — Impenetrability  of  Air. — Compressibility  of  it. — Elas- 
ticity of  it. — Liquidsnot  absolutely  Incompressible — Experiments. — Elasticity  of  Fluids. — Aeriform 
Fluids. — Domestic  Fire-Box. — Evolution  of  Heat  by  compressed  Air. — Inertia. — Matter  incapable 
of  spontaneous  Change. — Impediments  to  Motion. — Motion  of  the  Solar  Sy.stem. — Law  of  Nature. 
— Spontaneous  Motion. — Immateriality  of  the  thinking  and  willing  Principles. — Language  used  to 
express  Inertia  sometimes  faulty. — Familiar  Examples  of  Inertia. 

ELASTICITY  OF  AIR 39 

Exhausting  Syringe. — Rate  of  Exhaustion. — Impo.ssible  to  produce  a  perfect  Vacuum. — Mechanical 
Defects. — The  Air-Pump. — Barometer-Gauge. — Siphon-Gauge. — Various  Forms  of  Air-Pump. — 
Pump  without  Suction- Valve. — Experiments  with  Air-Pump. — Bladder  burst  by  atmospheric 
Pressure. — Bladder  burst  by  Elasticity  of  Air. — Dried  Fruit  inflated  by  fixed  Air. — Flaccid  Blad- 
der swells  by  Expansion. — Water  raised  by  Elastic  Force. — A  Pump  cannot  act  in  the  Absence 
of  atmospheric  Pressure. — Suction  ceases  when  this  Pressure  is  removed. — The  Magdeburg  Hem- 
isphere.— Guinea  and  Feather  Experiment. — Cupping.— Effervescing  Liquors. — Sparkling  of 
Champagne,  &c. — Presence  of  Air  necessary  for  the  Transmission  of  Sound. — The  condensing 
Syringe. — The  Condenser. 

EFFECTS  OF  LIGHTNING 61 

Classification  of  the  Effects  of  Lightning. —  The  sulphureous  Odor  developed  by  Lightning. — Cases 
collected  by  M.  Arago.— Nature  of  the  Odor. — Chemical  Changes  operated  by  Lightning. — Nitric 
Acid  formed  by  the  Electric  Spark;  also  Ammonia  and  Nitric  Acid  produced  during  ^Thunder- 
Storms. — Fusion  and  Contraction  of  i/e^(7&.— Observations  of  the  Ancients. — Franklin's  cold 
Fusion. — Evidence  against  cold  Fusion. — Masses  of  Metal  melted  by  Lightning. —  Viirefactions 
and  Fulgurites. — Heights  at  which  Vitrefactions  have  been  found. — Facts  collected  by  M.  Arago. 
— Fulminary  Tubes,  or  Fulgurites. — Characters  of  Fulgurites. — Variations  dependent  on  the  Na- 
ture of  the  Soil  where  they  are  found. — Four  Hypotheses  to  explain  their  Oi-igin. — Their  Forma- 
tions in  some  Cases  are  recent. — Sand  fused  by  artificial  Heat  into  the  State  of  the  Fulgurites. — 
Artificial  Fulgurites  formed  by  the  Electrical  Battery.— The  further  Condition  essential  to  explain 
the  Origin  of  Fulgurites. — Recent  Formation  of  Fulgurites  ohserYeA.— Mechanical  Effects.— In- 
etances  of  the  Mechanical  Action  of  Lightning. — The  Action  is  exerted  in  all  Directions. — Induc- 
tive Action  of  Lightning.— M.  Arago's  Explanation  of  the  Effect  as  due  to  Vaporization.— Objec- 
tions to  the  Explanation. — Decompositions  of  the  natural  Electricities  of  Bodies. — Induction 
between  the  Clouds  and  the  Earth. — Upwari  Flashes  and  Mechanical  Effects. — Arago's  Explana- 
tion.— Magnetic  Effects. — To  be  explained  in  Electro-Mag.n-etisji.- iJ/erfs  of  conducting  Bodies 
on  Lightning.-'  Conducting  Properties  of  Metallic  Bodies. — Lightning  pas.sing  along  Conductors 
in  Preference  to  NonConductors. — Protection  afforded  by  conducting  Bodies. — Lightning  selects 
conducting  Bodies  from  among  others.— Lightning  Conductors  should  descend  to  a  humid  Soil.— 


Necessity  of  Continuity  in  a  Conductor. — Effects  proceeding  from  the  Surface  of  tlie  Earth. — As- 
cent or  Ebullition  of  Water. — Inundations  from  subterranean  Sources. — Mosaic  Account  of  the 
Deluge;  Analogous  natural  Phenomena. — Electrical  State  of  the  Atmosphere  fovorable  to  the 
Process  of  barking  Trees  — Effect  of  Thunder  on  fermented  Liquors,  &c. — Return  Stroke  reported 
by  Brydone. — Theory  of  such  EflPects. — Flame  appearing  on  the  Ground. — Not  extinguishable  by 
Water. — Superposed  Clouds  not  necessary  to  its  Appearance. — Stationary  luminous  Appearance. 
• — Lightning  rising  from  the  Earth  like  a  Rocket. — Flames  observed  ou  exposed  Points. — Lumin- 
ous Ilain. — Cases  collected  by  M.  Arago. — Luminous  Dust. 

POPULAR  FALLACIES page     83 

Fallacious  Indications  of  Senses. — Errors  of  the  Sense  of  Feeling. — Erroneous  Impressions  of  Heat 
and  Cold. — Explanation  of  the.se  by  the  Principle  of  Conduction. — Why  a  Fan  is  cooling, — Feats 
of  the  Fire-King  explained. — Horizontal  Appearance  of  the  Sun  and  Moon. — Deceptive  oval  Disk 
in  the  Horizon. — Deceptions  of  Vision — of  Taste — of  Smelling. 

PROTECTION  FROM  LIGHTNING 97 

Danger  Proportionate  to  the  Magnitude,  not  to  the  Frequency  of  the  Evil. — Ancient  Methods  of 
averting  Lightning. — Persons  in  Bed  not  secure,  as  some  think. — Augustus's  sealskin  Cloak  as  a 
Lightning  Protector. — Influence  of  Color  on  the  Electric  Fluid. — Tiberius's  Crown  of  Laurel  as  a 
Lightning  Protector. — The  Danger  of  taking  Shelter  beneath  Trees. — Futility  of  taking  Shelter 
in  Glass  Cages. — Metal  about  the  Person  destroyed  by  Lightning. — Metal  Appendages  to  be  laid 
aside. — Lightning  Explosions  occur  at  the  Points  where  it  leaves  or  enters  a  Metal. — Part  of  a 
Room  which  is  most  Safe. — Lightning  more  likely  to  discharge  among  a  Crowd  than  on  a  single 
Individual. — Influence  of  the  Vapor  of  Transpiration,  &c. — Certain  Individuals  are  comparative 
Non-Conductors. — Thunder-Clouds  have  been  traversed  with  Impunity. — Thunder-Storms  below 
the  Place  of  Observation. — Para.tonneiires,  or  Lightning  Conductors. — Lightning  Conductors 
protective  even  when  no  Flash  strikes  them  —Sparks  at  the  Interval  where  a  Conductor  is  dis- 
jointed.— Lightning  Conductors  drain  ofi'  the  Electricity  of  Clouds. — Sparks  or  luminous  Aigrettes 
on  the  Points  of  Conductors. — More  frequent  Occurrence  at  Sea. — Influence  of  Elevation  of  a 
Paratonnerre. — Experimental  Illustration. — Electric  Kites. — Captive  Balloons  as  Paragreles  and 
for  Meteorological  Research. — Pointed  and  blunt  Conductors. — GLuantity  of  Lightning  drawn  down 
by  a  Conductor.— Mr.  Harris's  Conductors  for  Ships. — Assumed  Extent  of  the  protecting  Power 
of  a  Paratonnerre. — Not  based  on  experimental  Grounds. — Cases  against  its  general  Application. 
— Lightning  does  not  alway  strike  the  highest  Points. — Lightning  Conductors  with  many  Points.- 
A  Lightning  Conductor  must  have  sufficient  Capacity. — A  Lightning  Conductor  must  be  in  good 
Connexion  with  the  moist  Sub-Soil. — Charcoal  Beds  to  receive  the  Base  of  the  Conductor. — Vici 
nal  metallic  Conductors. — Conductors  of  metallic  Wire-Rope  ;  Insulation  not  needed. — Conductors 
for  Powder  Magazines. — Efficacy  of  Lightning  Conductors. — Lateral  or  divided  Discharge  de- 
fined; its  Cause. — More  readily  obtained  from  Conductors  than  from  Leyden  Discharges. — Line 
or  Lines  of  least  Resistance. — Absolute  Necessity  of  connecting  the  Conductor  with  vicinal  Bodies. 
— Artificial  Means  of  producing  the  Electrical  Odor. — Chemical  Changes. — Fusion. — Fulgurites. — 
Mechanical  Effects. — Effects  of  conducting  Bodies. 

MAGNETISM 109 

Magnetic  Attraction  and  Polarity. — Magnetic  Meridian,  Variation. — Dip  of  the  Magnetic  Needle. — 
Magnetic  Attraction  known  to  the  Ancients. — Invention  of  the  Mariner's  Compass  of  uncertain 
Date. — Discovery  of  the  Variation. — Tables  of  Variation  constructed. — Robert  Norman  discovers 
the  Dip — Invention  of  the  Dipping  Needle. — The  Variation  of  the  Variation  discovered.- — Influ- 
ence of  Magnets  on  soft  Iron  observed. — Polarity  of  Magnets  observed.  Construction  of  artificial 
Magnets. — Magnetism  imparted  to  Iron  by  the  Earth. — Laws  of  Magnetic  Attraction  discovered 
by  Coulomb. — Methods  of  making  artificial  Magnets — consequent  Points. — Knight's  improved 
Method. — Duhamel's  Improvement. — Coulomb's  Researches  on  artificial  Magnets. — Influence  of 
Heat  on  Magnetism. — Local  and  periodical  Changes  of  the  Variation. — Diurnal  Variation. — Cas- 
sini's  Observations  at  Paris. — Advancement  of  Magnetic  Geography. — Magnetic  Equator. — Mag- 
netic Poles. 

ELECTRO-MAGNETISM 117 

Electro-Magnetism  very  recently  discovered. — Oersted's  Experiments  at  Copenhagen. — The  Law 
according  to  which  the  Needle  is  deflected. — The  Law  of  Attraction  and  Repulsion  of  Electric 
Currents. — Supposes  Electric  Currents  circulating  round  the  Globe. — Arago  shows  that  the  con- 
ducting Wire  has  Magnetic  Properties. — Needles  magnetized  by  the  Electric  Current. — The 
Variation  of  the  Attraction  of  the  Current  at  different  Distances  determined. — Laplace  reduces 
this  result  to  an  analytical  Formula. — The  whole  Body  of  Electro-Magnetic  Phenomena  reduced 
to  analytical  Calculation. — Faraday's  Researches. — Rotation  imparted  to  Mercury  by  means  of  the 
Magnet  and  Electric  Current. — The  Multiplier  or  Galvanometer. — Its  Construction  and  Applica- 
tion.— The  Earth  affects  Electric  Currents  in  the  same  Manner  as  it  affects  Magnets. — Ampere's 
Theory  of  Terrestrial  Magnetism. — Researches  of  M.  de  la  Rive. — Magnetizing  Power  of  the 
Cun'ent  at  different  Distances,  and  the  Law  of  its  Variation. — The  Effect  produced  by  transmit- 
ting it  through  Metals. — The  undulatory  Theory  of  Electricity  similar  to  that  of  Light. —  Thermo- 
Electridty. — Thermo-Electric  Effects  observed  by  Professor  Seebeck. — His  Experiment  with 
Antimony  and  Copper. — Researches  of  Yelin,  Marsh,  and  Gumming. — Oersted  and  Fourier  con- 
struct a  ThermoEiectric  Pile.— Becquerel  decomposes  Water  with  such  an  Instrument. — Thermo- 
Electric  Scale  of  Metals. 

THE  THERMOMETER 129 

Advantages  of  a  mercurial  Thermometer. — Method  of  constructing  one. — To  purify  the  Mercury. — 
Formation  of  the  Tube. — To  fill  the  Tube. — Determination  of  the  freezing  and  boiling  Points. — 


Modes  of  Graduation. — Alcohol  Thermometers. — Difficulty  of  fixing  the  boiling  Point. — Useful- 
ness of  the  Thermometer. — History  of  its  Invention. — Methods  of  comparing  Scales  of  different 
Thermometers. 

ATMOSPHERIC  ELECTRICITY page  147 

On  the  Electric-ily  of  the  Atmosphere  in  clear  Weather. — Connexion  between  Electricity  and  Me- 
teorology.— Apparatus  for  ohservins(the  Electricity  of  the  Atmosphere. — Insulated  elevated  Rod. 
— Portable  Apparatus  made  of  a  fishing  Rod. — Saussure's  Electroscope  and  his  Mode  of  estimating 
tlio  Value  of  the  Divergences. — Occasional  Use  of  the  Galvanometer. —  The  ordinary  State  of  the 
Atmosphere — Volta's  Theory  of  the  Origin  of  Atmospheric  Electricity. — Inadequacy  ot  the  Theo- 
ry of  Chemical  Origin. — The  Author's  Suggestion  of  the  probable  Influence  of  Friction. — Diurnal 
Variation  of  the  Electricity. — Periodical  hourly  Variation. — Representation  of  the  Rate  of  Varia- 
tion.— Maxima  and  Minima  at  a  given  Parallel. — Schiibler's  Observations. — Anmial  Variation  of 
the  Electricity. — Variation  of  the  daily  Maxima  and  Minima. — Arago's  Repetition  of  Schiibler's 
Observations. — Local  Variations  oftJie  Electricity. — Influence  of  particular  Localities,  Buildings, 
&c. — No  satisfactory  Explanation  yet  given  of  the  Variations. — Correspondence  between  Electric 
and  Magnetic  Variations. — Becquerel's  Explanation  of  the  Phenomena  of  Variation. — Distribu- 
tion of  Electricity  of  the  Air. — Negative  State  of  the  Earth. — Character  of  the  lower  Stratum  of 
Air. — Increase  of  Electric  Charge  in  the  higher  Strata  of  Air. — Decrease  in  the  lower  Strata. — 
Comparative  Electric  Character  of  difierent  Strata. — Pomiute  for  the  comparative  Electricity  of 
two  Strata. — Electricity  of  the  Air  in  clouded  Weather. — Preliminary. — Schiibler's  Observations. 
— Table  of  Observations  explained. 

EVAPORATION 161 

Erroneously  ascribed  to  Chemical  Combination. — Takes  place  from  the  Surface. — Law  discovered 
by  Dalton  extended  to  aU  Liquids. — Limit  of  Evaporation  conjectured  by  Faraday. — Hygrome- 
ters.— Various  Phenomena  explained  by  Evaporation. — Leslie's  Method  of  freezing. — Examples 
in  the  useful  Arts. — Methods  of  Cooling  by  Evaporation. — Dangerous  Effects  of  Dampness. — 
Wollaston's  Cryophorus. — Pneumatic  Ink-Bottle. — Clouds. — Dew. 

CONDUCTION  OF  HEAT 177 

Conducting  Powers  of  Bodies. — Liquids  Non-Conductors. — Effect  of  Feathers  and  Wool  on  Ani- 
mals.— Clothing. — Familiar  Examples. 

RELATION  OF  HEAT  AND  LIGHT 185 

Probable  Identity  of  Heat  and  Light. — Incandescence. — Probable  Temperature  of — Gases  cannot 
be  made  Incandescent. — The  Absorption  and  Reflection  of  Heat  depend  on  Color. — Burning 
Glass — Heat  of  Sun's  Rays. — Heat  of  artificial  Light. — Moonlight. — Phosphorescence. 

ACTION  AND  REACTION 195 

Inertia  in  a  single  Body. — Consequences  of  Inertia  in  two  or  more  Bodies. — Examples. — Effects  of 
Impact. — Motion  not  estimated  by  Speed  or  Velocity  alone. — Examples. — Rule  for  estimating  the 
Quantity  of  Motion. — Action  and  Reaction. — Examples  of. — Velocity  of  two  Bodies  after  Impact. 
— Magnet  and  Iron. — Feather  and  Cannon-Bail  impinging. — Newton's  Laws  of  Motion. — Inu- 
tility of 

COMPOSITION  AND  RESOLUTION  OF  FORCE 205 

Motion  and  Pressure. — Force — Attraction. — Parallelogram  of  Forces. — Resultant. — Components  — 
Composition  of  Force. — Resolution  of  Force. — Illustrative  Experiments. — Composition  of  Pres- 
sures.— Theorems  regulating  Pressures  also  regulate  Motion. — Examples. — Resolution  of  Motion. 
— Forces  in  Equilibrium. — Composition  of  Motion  and  Pressure — Illustrations. — Boat  in  a  Cur- 
rent.— Motions  of  Fishes. — Flight  of  Birds. — Sails  of  a  Vessel. — Tacking. — Equestrian  Feats. — 
Absolute  and  relative  Motion. 

CENTRE  OF  GRAVITY 219 

Terrestrial  Attraction  the  combined  Action  of  parallel  Forces. — Single  equivalent  Force. — Exam- 
ples.— Method  of  finding  the  Centre  of  Gravity. — Line  of  Direction. — Globe. — Oblate  Spheroid. — 
Prolate  Spheroid. — Cube. — Straight  Wand. — Flat  Plate. — Triangular  Plate. — Centre  of  Gravity 
not  always  within  the  Body. — A  Ring. — Experiments. — Stable,  instable,  and  neutral  Equilibrium. 
— Motion  and  Position  of  the  Arms  and  Feet. — Effect  of  the  Knee- Joint. — Positions  of  a  Dancer. — 
Porter  under  a  Load. — Motion  of  a  Quadruped. — Rope  Dancing. — Centre  of  Gravity  of  two  Bod- 
ies separated  from  each  other. — Mathematical  and  experimental  Examples. — The  Conservation 
of  the  Motion  of  the  Centre  of  Gravity. — Solar  System. — Centre  of  Gravity  sometimes  called  Cen- 
tre of  Inertia. 


THE  LEVER  AND  WHEEL-WORK 241 

Simple  Machine. — Statics. — Dynamics. — Force. — Power. — Weight. — The  Lever. — Cord. — Inclined 
Plane. — Arms. — Fulcrum. — Three  kinds  of  Lever. — Crow-Bar. — Handspike. — Oar. — Nut-Crack- 
ers.— Turning  Lathe. — Steelyard. — Rectangular  Lever. — Hammer. — Load  between  two  Bearers. 
— Combination  of  Levers. — Equivalent  Lever. — Wheel  and  Axle. — Thicknessof  the  Rope. — "Ways 
of  applying  the  Power. — Projecting  Pins. — Windlass. — Winch. — Axle. — Horizontal  Wheel. — 
Tread-Mill.— Cranes.— Water-Wheels. — Paddle-Wheel.—Rachet-Wheel.— Rack— Spring  of  a 
Watch — Fusee. — Straps  or  Cords. — Examples  of — Turning  Lathe. — Revolving  Shafts. — Spinning 
Machinery. — SawMill. — Pinion.  —  Leaves. —  Crane. — Spur- Wheels. — Crown-Wheels. — Bevelled 
Wheels. — Hunting-Cog. — Chronometers. — Hair-Spring.— Balance- Wheel. 


THE  PULLEY page  269 

Cord. — Sheave. — Fixed  Pulley —Fire-Escapes. — Single  moveable  Pulley. — Systems  of  Pulleys. — 
Smeaton's  Tackle. — White's  Pulley. — Advantage  of. — Runner. — Spanish  Bartons. 

THE  INCLINED  PLANE,  WEDGE,  AND  SCREW 281 

Inclined  Plane. — Effect  of  a  Weight  on. — Power  of. — Roads. — Power  oblique  to  the  Plane. — Plane 
sometimes  moves  under  the  Weight. — Wedge. — Sometimes  formed  of  two  inclined  Planes. — 
More  powerful  as  its  Angle  is  acute. — Where  used. — Limits  to  the  Angle. — Screw. — Hunter's 
Screw. — Examples. — Micrometer  Screw. 

EBULLITION 295 

Process  of  boiling. — Vaporization  and  Condensation. — Latent  Heat  of  Steam. — Experiments  of 
Black. — Effect  of  atmospheric  Pressure  on  the  boiling  Point. — Ebullition  under  increased  Pressure 
— under  diminished  Pressure, — Relation  between  the  Barometer  and  boiling  Point. — Effect  of  the 
Altitude  of  the  Station  of  the  boiling  Point. — Elasticity  of  Steam.- — Its  Lightness. — Sum  of  the  latent 
and  sensible  Heat  always  the  same. — Effect  of  the  Compression  of  Steam  without  Loss  of  Heat. — 
Steam  cannot  be  liquefied  by  mere  Pressure. — Boiling  Points  and  latent  Heat  of  other  Liquids. — 
Condensation  of  Vapor. — Principle  of  the  Steam-Engine. — Nature  of  permanent  Gases. — Examples 
of  the  Application  of  the  Properties  of  Steam. 

COMBUSTION 319 

Flame  produced  by  chemical  Combination. — Supporters  of  Combustion  and  Combustibles. — Oxygen 
chief  Supporter. — Heat  of  Combustion. — Flame. — Its  illuminating  Powers. — Combustion  without 
Flame. — Property  of  spongy  Platinum. — Table  of  Heat  evolved  in  Combustion. — Theory  of  La- 
voisier.— Of  Hook  and  others. — Electric  Theory. 

HOW  TO  OBSERVE  THE  HEAVENS 329 

Interesting  Nature  of  the  Subject. — Diurnal  Rotation. — Circumpolar  Stars. — Ursa  Major. — Forms 
of  the  Constellations. — The  Pointers. — The  Pole-Star. — Cassiopeia. — Capella. — The  Swan. — 
Equatorial  Constellations — Orion. — Simis,  or  the  Dogstar. — Aldebaran. — Procyon. — Auriga. — 
Coluviba. — Herschel's  Observations  on  Sirius. — Dr.  W"ollaston's  Observations. — Aspect  of  the 
Heavens  at  different  Seasons  of  the  Year. — Uses  of  the  Celestial  Globe. — To  Ascertain  the 
Aspect  of  the  Heavens  on  any  Night — at  any  Hour. — Effect  of  the  Telescope  on  Fixed  Stars. 
— Relative  Brightness  of  the  Stars. — Theory  of  refracting  and  reflecting  Telescopes,  as  applied 
to  the  Stars. — Manner  in  which  Sir  W.  Herschel  applied  it. — Method  of  estimating  the  Bright- 
ness of  small  Stars. — Method  of  observing  variable  Stars. — Double  Stars. — Description  of  the  Mi- 
crometer. 

THE  STELLAR  UNIVERSE— first  lecture 355 

Range  of  Vision. — Augmented  by  the  Telescope. — Periodic  Stars. — Examples  of  this  Class. — Vari- 
ous Hypotheses  to  explain  these  Appearances. — Their  InsuiEciency. —  Temporary  Stars. — Re- 
markable Examples  of  this  Cla.ss. — These  may  possibly  be  periodic  Stars. — Double  Stars. — Their 
vast  Number. — 'They  are  physically  connected. — Tele,scopic  Views  of  them. — How  they  may  in- 
dicate the  annual  Parallax. — Researches  of  Sir  W.  Herschel. — Discovery  of  the  orbitual  Motions 
of  double  Stars. — Binary  Stars. — Extension  of  Gravitation  to  the  Stars. — Their  elliptic  Orbits  dis- 
covered.— Effects  of  double  and  colored  Suns — Proper  Motions  of  tlte  Stars. — Probable  Motion 
of  the  Solar  System. — Analysis  of  its  Effects. — Suggestion  of  Mr.  Pond. — Independent  Motions  of 
the  Stars. — Proper  Motions  of  double  Stars. — Probable  Amount  of  the  real  Motions  of  the  Stars. 

THE  STELLAR  UNIVERSE— second  lecture 375 

Form  and  Arrangement  of  the  Mass  of  visible  Stars. — Sir  W.  Herschel's  Analysis  of  the  Heavens. 
— The  Milky  Way. — The  vast  Numbers  of  Stars  in  it. — Form  and  Dimensions  of  this  Mass  of 
Stars. — NebulfE  and  Clusters. — Various  Forms  and  Appearance  of  Nebulas. — Great  Nebula  in 
Orion. — Megallanic  Clouds. — Planetary  Nebulas. — Vast  Number  of  Nebula. — Herschel's  Cata- 
logue.— Structure  of  the  Universe. — Laplace's  nebular  Hypothesis. — Examination  of  its  moral 
Tendency. 

THE  STEAM-ENGINE— FIRST  lecture 397 

The  SteamEngine  a  Subject  of  popular  Interest. — Effects  of  Steam. — Great  Power  of  Steam. — 
Mechanical  Properties  of  Fluids. — Elastic  and  inelastic  Fluids. — Elasticity  of  Gases. — Effects  of 
Heat. — Savory's  Engine. — Boilers  and  their  Appendages. — Working  Apparatus. — Mode  of  Op- 
eration.— Defects  of  Savory's  Engine. — Newcomen  and  Cawley's  Patent. — Accidental  Discovery 
of  Condensation  by  Injection. — Potter's  Invention  of  the  Method  of  working  the  Valves. — His  Con- 
trivance improved  by  the  Substitution  of  the  Plug-Frame. 

THE  STEAM-ENGINE— second  lecture 417 

Mechanical  Force  of  Steam. — Facts  to  be  remembered. — Watt  finds  Condensation  in  the  Cylinder 
incompatible  with  a  due  Economy  of  Fuel. — Conceives  the  Notion  of  Condensing  out  of  the  Cyl- 
inder.— Discovers  separate  Condensation. — Invents  the  Air-Pump. — Substitutes  Steam  Pressure 
for  Atmospheric  Pressure. — Invents  the  Steam  Case  or  Jacket. — His  first  Experiments  to  realize 
these  Inventions. — His  Experimental  Apparatus. — His  Models  at  Delft  Hou.se. — Difficulties  of 
bringing  the  improved  Engines  into  Use. — Watt  employed  by  Roebuck. — His  Partnership — His 
first  Patent. — His  Single- Acting  Engine. — Discovery  of  the  Expansive  Action  of  Steam — Its  Me- 
chanical Effects. — Its  Variable  Action. — Methods  of  Equalizing  it. — Its  extensive  Application  in 
the  Cornish  Engines — Extension  of  the  SteamEngine  to  Manufactures. — Attempts  of  Papin, 
Saveiy,  Hull,  Champion,  Stewart,  and  'V\-''ashborough. — Watt's  second  Patent. — Sun-and  Planet 
Wheels. — Valves  of  Double-Actiug  Engine. 


CONTENTS  OF  VOLUME  II. 


15 


THE  STEAM-ENGINE— THIRD  lecture 451 

Methods  of  Connecting  the  Piston-Rod  and  Beam  in  the  Double- Acting  Engine. — Rack  and  Sector. 
— Parallel  Motion. — Connexion  of  Piston-Rod  and  Beam. — Connecting  Rod  and  Crank. — Flj^- 
"Wheel. — Throttle-Valve. — Governor. — Construction  and  Operation  of  the  Double- Acting  Engine. 
— Eccentric. — Cocks  and  Valves. — Single-Clack  Valves. — Double-Clack  Valves. — Conical  Valves. 
— Slide  Valves. — Murray's  Slides. — The  D  Valves. — Seaward's  Slides. — Single  Cock. — Tvs'o-way 
Cock. — Four-v7ay  Cock. — Pistons. — Common  hemp-packed  Piston. — Woolf's  Piston. — Metallic 
Pistons. — Cartwright's  Engine. — Cartwxight's  Piston. — Barton's  Piston. 

THE  STEAM-ENGINE— FOTTRTH  lecture 491 

Analysis  of  Coal. — Process  of  Combustion. — Heat  evolved  in  it. — Form  and  Structure  of  Boiler. — 
Wagon-Boiler. — Furnace. — Method  of  Feeding  it. — Combustion  of  Gas  in  Flues. — Williams's 
Patent  for  Method  of  Consuming  unburned  Gases. — Construction  of  Grate  and  Ash-Pit. — Magni- 
tude of  Heating  Surface  of  Boiler. — Steam-Space  and  Water-Space  in  Boiler. — Position  of  Flues. 
— Method  of  Feeding  Boiler. — Method  of  Indicating  the  Level  of  Water  in  the  Boiler. — Lever 
Gauges. — Self  Regulating  Feeders. — Steam-Gauge. — Barometer- Gauge. — Watt's  Invention  of  the 
Indicator. — Counter. — Safety-Valve. — Fusible  Plugs. — Self-Regulating  Damper. — Brunton's  Self- 
Regulating  Furnace. — Gross  and  Useful  Effect  of  an  Engine. — Power  and  Duty  of  Engines. — 
Horse-Power  of  Steam-Engines. — Table  exhibiting  the  Mechanical  Power  of  Water  converted 
into  Steam  at  various  Pressures. — Evaporation  proportional  to  Horse-Power. — Sources  of  Loss  of 
Power. — Absence  of  good  Practical  Rules  for  Power. — Common  Rules  followed  by  Engine-Makers. 
— Duty  distinguished  from  Power. — Duty  of  Boilers. — Proportion  of  Stroke  to  Diameter  of  Cylin- 
der.— Duty  of  Engines. — Cornish  System  of  Inspection. — Table  showing  the  Improvement  of  Cor- 
nish Engines. — Beneficial  Effects  of  Cornish  Inspection. — Successive  Improvements  on  which  the 
increased  Duty  of  Engines  depends,  traced  by  John  Taylor  in  his  "  Records  of  Mining." 

THE  STEAM-ENGINE— FIFTH  lecture ' 525 

Railways. — Effects  of  Railway  Transport. — History  of  the  Locomotive  Engine. — Construction  of 
Locomotive  Engine  by  Blinkinsop. — Messrs.  Chapman's  Contrivance. — Walking  Engine. — Mr. 
Stephenson's  Engines  at  Killingworth. — Liverpool  and  Manchester  Railway. — Experimental  Trial 
of  the  "Rocket,"  "  Sanspareil,"  and  "Novelty." — Method  of  Subdividing  the  Flue  into  Tubes. — 
Progressive  Improvement  of  Locomotive  Engines. — Adoption  of  Brass  'Tubes. — Detailed  Descrip- 
tion of  the  most  Improved  Locomotive  Engines. — Power  of  Locomotive  Engines. — Position  of  the 
Eccentrics. — Pressure  of  Steam  in  the  Boiler. — Dr.  Lardner's  Experiments  in  1838. — Resistance 
to  Railway  Trains. — Dr.  Lardner's  Experiments  on  the  Great-Western  Railway. — Experiments 
on  Resistance. — Restrictions  on  Gradients. — Compensating  Effect  of  Gradients. — Experiment 
with  the  '•  Hecla." — Disposition  of  Gradients  should  be  Uniform. — Methods  of  surmounting  Steep 
Inclinations. 


MATTER  &  ITS  PHYSICAL  PROPERTIES. 


Divisibility, — Unlimited  Divisibility. — Wollaston's  Micrometric  Wire. — Method  of  making  it. — 
Thickness  of  a  Soap-Babble. — Wings  of  Insects. — Gilding  of  Embroidery. — Globules  of  the 
Blood. — Animalcules. — Their  minute  Organization. — Ultimate  Atoms. — Crystals. — Porosity. — Vol- 
ume.— Density. — duicksilver  passing  through  Pores  of  Wood. — Filtration. — Porosity  of  Hydro-  ( 
phane. — Compressibility. — Elasticity.— Dilatability. — Heat. — Contraction  of  Metal  used  to  restore 
the  Perpendicular  to  Walls  of  a  Building. — Impenetrabihty  of  Air. — Compressibility  of  it. — Ela.s- 
ticity  of  it. — Liquids  not  absolutely  Incompressible. — Experiments. — Elasticity  of  Fluids. — Aeri- 
form Fluids. — Domestic  Fire  Box. — Evolution  of  Heat  by  compressed  Air.^Inertia. — Matter  in- 
capable of  spontaneous  Change. — Impediments  to  Motion. — Motion  of  the  Solar  System. — Law 
of  Nature — Spontaneous  Motion. — Immateriality  of  the  thinking  and  willing  Principles. — Lan- 
guage used  to  express  Inertia  sometimes  faulty. — Familar  Examples  of  Inertia. 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


19 


MATTER  &  ITS  PHYSICAL  PROPERTIES. 


Placed  in  the  material  world,  man  is  continually  exposed  to  the  action  of 
an  infinite  variety  of  objects  by  which  he  is  surrounded.  The  body,  to  which 
the  thinking  and  living  principles  have  been  united,  is  an  apparatus  exquisitely 
contrived  to  receive  and  to  transmit  these  impressions.  Its  various  parts  are 
organized  with  obvious  reference  to  the  several  external  agents  by  which  it  is 
to  be  affected.  Each  organ  is  designed  to  convey  to  the  mind  immediate  notice 
of  some  peculiar  action,  and  is  accordingly  endued  with  a  corresponding  sus- 
ceptibility. This  adaptation  of  the  organs  of  sense  to  the  particular  influences 
of  material  agents,  is  rendered  still  more  conspicuous  when  we  consider  that, 
however  delicate  its  structure,  each  organ  is  wholly  insensible  to  every  influ- 
ence except  that  to  which  it  appears  to  be  specially  appropriated.  The  eye, 
so  intensely  susceptible  of  impressions  from  light,  is  not  at  all  affected  by  those 
of  sound  ;  while  the  fine  mechanism  of  the  ear,  so  sensitively  alive  to  every 
effect  of  the  latter  class,  is  altogether  insensible  to  the  former.  The  splendor 
of  excessive  light  may  occasion  blindness,  and  deafness  may  result  from  the 
roar  of  a  cannonade  ;  but  neither  the  sight  nor  the  hearing  can  be  injured  by 
the  most  extreme  action  of  that  principle  which  is  designed  to  affect  the  other. 

Thus  the  organs  of  sense  are  instruments  by  which  the  mind  is  enabled  to 
determine  the  existence  and  the  qualities  of  external  things.  The  eflfects 
which  these  objects  produce  upon  the  mind  through  the  organs,  are  called 
sensations,  and  these  sensations  are  the  immediate  elements  of  all  human 
knowledge.  Matter  is  the  general  name  that  has  been  given  to  that  sub- 
stance which,  under  forms  infinitely  various,  aff'ects  the  senses.  Metaphysi- 
cians have  differed  in  defining  this  principle.  Some  have  even  doubted  of  its 
existence.  But  these  discussions  are  beyond  the  sphere  of  mechanical  phi- 
losophy, the  conclusions  of  which  are  in  no  wise  affected  by  them.  Our  in- 
vestigations here  relate,  not  to  matter  as  an  abstract  existence,  but  to  those 
qualities  which  we  discover  in  it  by  the  senses,  and  of  the  existence  of  which 
we  are  sure,  however  the  question  as  to  matter  itself  may  be  decided.     When 


20 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


we  speak  of  "  bodies,"  we  mean  those  things,  whatever  they  be,  which  excite 
in  our  minds  certain  sensations ;  and  the  powers  to  excite  those  sensations  are 
called  "  properties,"  or  "  qualities." 

To  ascertain,  by  observation,  the  properties  of  bodies,  is  the  first  step  toward 
obtaining  a  knowledge  of  nature.  Hence  man  becomes  a  natural  philosopher 
the  moment  he  begins  to  feel  and  to  perceive.  The  first  stage  of  life  is  a  state 
of  constant  and  curious  excitement.  Observation  and  attention,  ever  awake, 
are  engaged  upon  a  succession  of  objects  new  and  wonderful.  The  large  re- 
pository of  the  memory  is  opened,  and  every  hour  pours  into  it  unbounded 
stores  of  natural  facts  and  appearances,  the  rich  materials  of  future  knowledge. 
The  keen  appetite  for  discovery,  implanted  in  the  mind  for  the  highest  ends, 
continually  stimulated  by  the  presence  of  what  is  novel,  renders  torpid  every 
other  faculty,  and  the  powers  of  reflection  and  comparison  are  lost  in  the  in- 
cessant activity  and  unexhausted  vigor  of  observation.  After  a  season,  how- 
ever, the  more  ordinary  classes  of  phenomena  cease  to  excite  by  their  novelty. 
Attention  is  drawn  from  the  discovery  of  what  is  new,  to  the  examination  of 
what  is  familiar.  From  the  external  world  the  mind  turns  in  upon  itself,  and 
the  feverish  astonishment  of  childhood  gives  place  to  the  more  calm  contem- 
plation of  incipient  maturity.  The  vast  and  heterogeneous  mass  of  phenomena 
collected  by  past  experience  is  brought  under  review.  The  great  work  of  com- 
parison begins.  Memory  produces  her  stores,  and  reason  arranges  them. 
Then  succeed  those  first  attempts  at  generalization  which  mark  the  dawn  of 
science  in  the  mind. 

To  compare,  to  classify,  to  generalize,  seem  to  be  instinctive  propensities 
peculiar  to  man.  They  separate  him  from  inferior  animals  by  a  wide  chasm. 
It  is  to  these  powers  that  all  the  higher  mental  attributes  may  be  traced,  and 
it  is  from  their  right  application  that  all  progress  in  science  must  arise.  With- 
out these  powers,  the  phenomena  of  nature  would  continue  a  confused  heap  of 
crude  facts,  with  which  the  memory  might  be  loaded,  but  from  which  the  in- 
tellect would  derive  no  advantage.  Comparison  and  generalization  are  the 
great  digestive  organs  of  the  mind,  by  which  only  nutrition  can  be  extracted 
from  this  mass  of  intellectual  food,  and  without  which,  observation  the  most  ex- 
tensive, and  attention  the  most  unremitting,  can  be  productive  of  no  real  or 
useful  advancement  in  knowledge. 

Upon  reviewing  those  properties  of  bodies  which  the  senses  most  frequently 
present  to  us,  we  observe  that  very  few  of  them  are  essential  to,  and  insepa- 
rable from,  matter.  The  greater  number  may  be  called  particular  or  peculiar 
qualities,  being  found  in  some  bodies,  but  not  in  others.  Thus  the  property  of 
attracting  iron  is  peculiar  to  the  loadstone,  and  not  observable  in  other  sub- 
stances. One  body  excites  the  sensation  of  green,  another  of  red,  and  a  third 
is  deprived  of  all  color.  A  few  characteristic  and  essential  qualities  are,  how- 
ever, inseparable  from  matter  in  whatever  state  or  under  whatever  form  it 
exist.  Such  properties  alone  can  be  considered  as  tests  of  materiality.  Where 
their  presence  is  neither  manifest  to  sense,  nor  demonstrable  by  reason,  there 
matter  is  not.  The  principal  of  these  qualities  are  magnitude  and  impenetra- 
hility. 

MAGNITUDE, 


Every  body  occupies  space  :  that  is,  it  has  magnitude.  This  is  a  property 
observable  by  the  senses  in  all  bodies  which  are  not  so  minute  as  to  elude 
them,  and  which  the  understanding  can  trace  to  the  smallest  particle  of  matter. 
It  is  impossible,  by  any  stretch  of  imagination,  even  to  conceive  a  portion  of 
matter  so  minute  as  to  have  no  magnitude. 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


21 


The  quantity  of  space  which  a  body  occupies  is  sometimes  called  its  magni- 
tude. In  colloquial  phraseology,  the  word  size  is  used  to  express  this  notion  ; 
but  the  most  correct  term,  and  that  which  we  shall  generally  adopt,  is  volume. 
Thus  we  say,  the  volume  of  the  earth  is  so  many  cubic  miles,  the  volume  of 
this  room  is  so  many  cubic  feet. 

The  external  limits  of  the  magnitude  of  a  body  are  lines  and  surfaces,  lines 
being  the  limits  which  separate  the  several  surfaces  of  the  same  body.  The 
linear  limits  of  a  body  are  also  called  edges.  Thus  the  line  which  separates 
the  top  of  a  chest  from  one  of  its  sides  is  called  an  edge. 

The  quantity  of  a  surface  is  called  its  area,  and  the  quantity  of  a  line  is 
called  its  length.  Thus  we  say,  the  area  of  a  field  is  so  many  acres,  the  length 
of  a  rope  is  so  many  yards.  The  word  "magnitude"  is,  however,  often  used 
indifferently  for  volume,  area,  and  length.  If  the  objects  of  investigation  were 
of  a  more  complex  and  subtle  character,  as  in  metaphysics,  this  unsteady  ap- 
plication of  terms  might  be  productive  of  confusion,  and  even  of  error  ;  but  in 
this  science,  the  meaning  of  the  term  is  evident,  from  the  way  in  which  it  is 
applied,  and  no  inconvenience  is  found  to  arise. 

IMPENETRABILITY. 

This  property  will  be  most  clearlyexplained  by  defining  the  positive  quality 
from  which  it  takes  its  name,  and  of  which  it  merely  signifies  the  absence.  A 
substance  would  be  penetrable  if  it  were  such  as  to  allow  another  to  pass 
through  the  space  which  it  occupies,  without  disturbing  its  component  parts. 
Thus,  if  a  comet,  striking  the  earth,  could  enter  it  at  one  side,  and,  passing 
through  it,  emerge  from  the  other  without  separating  or  deranging  any  bodies 
on  or  within  the  earth,  then  the  earth  would  be  penetrable  by  the  comet. 
When  bodies  are  said  to  be  impenetrable,  it  is  therefore  meant  that  one  cannot 
pass  through  another  without  displacing  some  or  all  of  the  component  parts  of 
that  other.  There  are  many  instances  of  apparent  penetration  ;  but  in  all  these 
the  parts  of  the  body  which  seem  to  be  penetrated  are  displaced.  Thus,  if  the 
point  of  a  needle  be  plunged  in  a  vessel  of  water,  all  the  water  which  previ- 
ously filled  the  space  into  which  the  needle  enters  will  be  displaced,  and  the 
level  of  the  water  will  rise  in  the  vessel  to  the  same  height  as  it  would  by  pour- 
ing in  so  much  more  water  as  would  fill  the  space  occupied  by  the  needle. 


FIGURE. 

If  the  hand  be  placed  upon  a  solid  body,  we  become  sensible  of  its  impene- 
trability, by  the  obstruction  which  it  opposes  to  the  entrance  of  the  hand  within 
its  dimensions.  We  are  also  sensible  that  this  obstruction  commences  at  cer- 
tain places  ;  that  it  has  certain  determinate  limits  ;  that  these  limitations  are 
placed  in  certain  directions  relatively  to  each  other.  The  mutual  relation  which 
is  found  to  subsist  between  these  boundaries  of  a  body,  gives  us  the  notion  of 
its  figure.  The  figure  and  volume  of  a  body  should  be  carefully  distinguished. 
Each  is  entirely  independent  of  the  other.  Bodies  having  very  different  vol- 
umes may  have  the  same  figure  ;  and  in  like  manner  bodies  differing  in  figure 
may  have  the  same  volume.  The  figure  of  a  body  is  what  in  popular  language 
is  called  its  shape  or  form.  The  volume  of  a  body  is  that  which  is  commonly 
called  its  size.  It  will  hence  be  easily  understood  that  one  body  (a  globe,  for 
example)  may  have  ten  times  the  volume  of  another  (globe),  and  yet  have  the 
same  figure  ;  and  that  two  bodies  (as  a  die  and  a  globe)  may  have  figures  alto- 
gether different,  and  yet  have  equal  volumes.  What  we  have  here  observed 
of  volumes  will  also  be  applicable  to  lengths  and  areas.     The  arc  of  a  circle 


22 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


and  a  straight  line  may  have  the  same  length,  although  they  have  different 
figures  ;  and,  on  the  other  hand,  two  arcs  of  different  circles  may  have  the  same 
figure,  but  very  unequal  lengths.  The  surface  of  a  ball  is  curved,  that  of  the 
table  plane  ;  and  yet  the  area  of  the  surface  of  the  ball  may  be  equal  to  that  of 
the  table. 

ATOMS MOLECULES, 

Impenetrability  must  not  be  confounded  w^ith  inseparability.  Every  body 
which  has  been  brought  under  human  observation  is  separable  into  parts  ; 
and  these  parts,  however  small,  are  separable  into  others  still  more  minute. 
To  this  process  of  division  no  practical  limit  has  ever  been  found.  Neverthe- 
less, many  of  the  phenomena  which  the  researches  of  those  who  have  success- 
fully examined  the  laws  of  nature  have  developed,  render  it  highly  probable 
that  all  bodies  are  composed  of  elementary  parts  which  are  indivisible  and  un- 
alterable. The  component  parts,  which  may  be  called  atoms,  are  so  minute  as 
altogether  to  elude  the  senses,  even  when  improved  by  the  most  powerful  aids 
of  art.  The  word  molecule  is  often  used  to  signify  component  parts  of  a  body 
so  small  as  to  escape  sensible  observation,  but  not  ultimate  atoms,  each  mole- 
cule being  supposed  to  be  formed  of  several  atoms,  arranged  according  to  some 
determinate  figure.  Particle  is  used  also  to  express  small  component  parts, 
but  more  generally  is  applied  to  those  which  are  not  too  minute  to  be  discover- 
able by  observation. 


FORCE. 

If  the  particles  of  matter  were  endued  with  no  property  in  relation  to  one 
another,  except  their  mutual  impenetrability,  the  universe  would  be  like  a  mass 
of  sand,  without  variety  of  state  or  form.  Atoms,  when  placed  in  juxtaposition, 
would  neither  cohere,  as  in  solid  bodies,  nor  repel  each  other,  as  in  aeriform 
substances.  We  find,  on  the  other  hand,  that,  in  some  cases,  the  atoms  which 
compose  bodies  are  not  simply  placed  together,  but  a  certain  effect  is  mani- 
fested in  their  strong  coherence.  If  they  were  merely  placed  in  juxtaposition, 
their  separation  would  be  effected  as  easily  as  any  component  particle  could 
be  removed  from  one  place  to  another.  Take  a  piece  of  iron,  and  attempt  to 
separate  its  parts  :  the  effort  will  be  strongly  resisted,  and  it  will  be  a  matter 
of  much  greater  facility  to  remove  the  whole  mass.  It  appears,  therefore,  that 
in  such  cases  the  parts  which  are  in  juxtaposition  cohere,  and  resist  their  mutual 
separation.  This  effect  is  denominated  force  ;  and  the  constituent  atoms  are 
said  to  cohere  with  a  greater  or  less  degree  of  force,  according  as  they  oppose 
a  greater  or  less  resistance  to  their  mutual  separation. 

The  coherence  of  particles  in  juxtaposition  is  an  effect  of  the  same  class  as 
the  mutual  approach  of  particles  placed  at  a  distance  from  each  other.  It  is 
not  difficult  to  perceive  that  the  same  influence  which  causes  the  bodies  A  and 
B  to  approach  each  other,  when  placed  at  some  distance  asunder,  will,  when 
they  unite,  retain  ihem  together,  and  oppose  a  resistance  to  their  separation. 
Hence  this  effect  of  the  mutual  approximation  of  bodies  toward  each. other  is 
also  called  force. 

Force  is  generally  defined  to  be  "  whatever  produces  or  opposes  the  produc- 
tion of  motion  in  matter."  In  this  sense,  it  is  a  name  for  the  unknown  cause  of 
a  known  effect.  It  would,  however,  be  more  philosophical  to  give  the  name, 
not  to  the  cause,  of  which  we  are  ignorant,  but  to  the  effect,  of  which  we  have 
sensible  evidence.  To  observe  and  to  classify  is  the  whole  business  of  the 
natural  philosopher.     When  causes  are  referred  to,  it  is  implied  that  effects  of 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


23 


the  same  class  arise  from  the  agency  of  the  same  cause.  However  probable 
this  assumption  may  be,  it  is  altogether  unnecessary.  All  the  objects  of  sci- 
ence, the  enlargement  of  mind,  the  extension  and  improvement  of  knowledge, 
the  facility  of  its  acquisition,  are  obtained  by  generalization  alone,  and  no  good 
can  arise  from  tainting  our  conclusions  with  the  possible  errors  of  hypotheses. 

It  may  be  here,  once  for  all,  observed,  that  the  phraseology  of  causation  and 
hypotheses  has  become  so  interwoven  with  the  language  of  science,  that  it  is 
impossible  to  avoid  the  frequent  use  of  it.  Thus  we  say,  "  The  magnet  attracts 
iron  :"  the  expression  attract  intimating  the  cause  of  the  observed  effect.  In 
such  cases,  however,  we  must  be  understood  to  mean  the  effect  itself,  finding 
it  less  inconvenient  to  continue  the  use  of  the  received  phrases,  rhodifying 
their  signification,  than  to  introduce  new  ones. 

Force,  when  manifested  by  the  mutual  approach  or  cohesion  of  bodies,  is 
also  called  attraction,  and  it  is  variously  denominated,  according  to  the  circum- 
stances under  which  it  is  observed  to  act.  Thus  the  force  which  holds  to- 
gether the  atoms  of  solid  bodies  is  called  cohesive  attraction.  The  force  which 
draws  bodies  to  the  surface  of  the  earth,  when  placed  above  it,  is  called  the 
attraction  of  gravitation.  The  force  which  is  exhibited  by  the  mutual  approach 
or  adhesion  of  the  loadstone  and  iron,  is  called  magnetic  attraction,  and  so  on. 

When  force  is  manifested  by  the  remotion  of  bodies  from  each  other,  it  is 
called  repulsion.  Thus,  if  a  piece  of  glass,  having  been  briskly  rubbed  with 
a  silk  handkerchief,  touch,  successively,  two  feathers,  these  feathers,  if  brought 
near  each  other,  will  move  asunder.  This  effect  is  called  repulsion,  and  the 
feathers  are  said  to  repel  each  other. 

The  influence  which  forces  have  upon  the  form,  state,  arrangement,  and  mo- 
tions, of  material  substances,  is  the  principal  object  of  physical  science.  In  its 
strict  sense,  mechanics  is  a  term  of  very  extensive  signification.  According 
to  the  more  popular  usage,  however,  it  has  been  generally  applied  to  that  part 
of  physical  science  which  includes  the  investigation  of  the  phenomena  of  motion 
and  rest,  pressure,  and  other  effects  developed  by  the  mutual  action  of  solid 
masses.  The  consideration  of  similar  phenomena,  exhibited  in  bodies  of  the 
liquid  form,  is  consigned  to  hydrostatics,  and  that  of  aeriform  fluids  to  pneu- 
matics. 

divisibility. 


Observation  and  experience  prove  that  all  bodies  of  sensible  magnitude,  even 
the  most  solid,  consist  of  parts  which  are  separable.  To  the  practical  sub- 
)  division  of  matter  there  seems  to  be  no  assignable  limit.  Numerous  examples 
of  the  division  of  matter,  to  a  degree  almost  exceeding  belief,  may  be  found  in 
experimental  inquiries  instituted  in  physical  science  ;  the  useful  arts  furnish 
many  instances  not  less  striking  ;  but  perhaps  the  most  conspicuous  proofs 
which  can  be  produced,  of  the  extreme  minuteness  of  which  the  parts  of  mat- 
ter are  susceptible,  arise  from  the  consideration  of  certain  parts  of  the  organ- 
ized world. 

The  relative  places  of  stars  in  the  heavens,  as  seen  in  the  field  of  view  of  a 
telescope,  are  marked  by  fine  lines  of  wire  placed  before  the  eyeglass,  and 
which  cross  each  other  at  right  angles.  The  stars  appearing  in  the  telescope 
as  mere  lucid  points  without  sensible  magnitude,  it  is  necessary  that  the  wires 
which  mark  their  places  should  have  a  corresponding  tenuity.  But  these 
wires,  being  magnified  by  the  eyeglass,  would  have  an  apparent  thickness, 
which  would  render  them  inapplicable  to  this  purpose,  unless  their  real  dimen- 
sions were  of  a  most  uncommon  degree  of  minuteness.  To  obtain  wire  for 
this  purpose,  Dr.  WoUaston  invented  the  following  process  ;    A  piece  of  fine 


platinum  wire  is  extended  along  the  axis  of  a  cylindrical  mould.  Into  this 
mould,  molten  silver  is  poured.  Since  the  heat  necessary  for  the  fusion  of  pla- 
tinum is  much  greater  than  that  which  retains  silver  in  the  liquid  form,  the 
platinum  wire  remains  solid,  while  the  mould  is  filled  with  the  silver.  When 
the  metal  has  become  solid  by  being  cooled,  and  has  been  removed  from  the 
mould,  a  cylindrical  bar  of  silver  is  obtained,  having  a  platinum  wire  in  its 
axis.  This  bar  is  then  wiredrawn,  by  forcing  it  successively  through  holes 
diminishing  in  magnitude,  the  first  hole  being  a  little  less  than  the  wire  at  the 
beginning  of  the  process.  By  these  means,  the  platinum  is  wiredrawn  at  the 
same  time  and  in  the  same  proportion  with  the  silver ;  so  that  whatever  be  the 
original  proportion  of  the  thickness  of  the  platinum  wire  to  that  of  the  mould, 
the  same  will  be  the  proportion  of  the  platinum  wire  to  all  the  successive  thick- 
nesses to  which  it  is  reduced.  If  we  suppose  the  mould  to  be  ten  times  the 
thickness  of  the  platinum  wire,  then  the  silver  wire  throughout  the  whole  pro- 
cess will  be  ten  times  the  thickness  of  the  platinum  wire  which  it  includes 
within  it.  The  silver  wire  may  be  drawn  to  a  thickness  not  exceeding  the 
three  hundredth  of  an  inch.  The  platinum  will  thus  not  exceed  the  three 
thousandth  of  an  inch. 

It  now  remains  to  disengage  this  fine  filament  of  platinum  from  the  surround- 
ing silver.  For  this  purpose,  the  wire  is  bent  into  the  form  of  a  loop,  as  rep- 
resented in  the  figure,  with  hooks  at  A  B  for  suspending  it.     The  part  C  D  E 


E- 


-C 


D 


is  now  immersed  in  nitric  acic,  by  which  the  silver  is  dissolved,  and  the  pla- 
tinum remains  suspended  in  a  thread  so  fine  as  to  be  invisible  without  the  aid 
of  the  microscope. 

By  this  method,  Dr.  WoUaston  succeeded  in  obtaining  wire  the  diameter  of 
which  did  not  exceed  the  eighteen  thousandth  of  an  inch.  A  quantity  of  this 
wire,  equal  in  bulk  to  a  common  die  used  in  games  of  chance,  would  extend 
from  New  York  to  New  Orleans. 

Newton  succeeded  in  determining  the  thickness  of  very  thin  laminae  of 
transparent  substances  by  observing  the  colors  which  they  reflect.  A  soap- 
bubble  is  a  thin  shell  of  water,  and  is  observed  to  reflect  different  colors  from 
different  parts  of  its  surface.  Immediately  before  the  bubble  bursts,  a  black 
spot  may  be  observed  near  the  top.  At  this  part  the  thickness  has  been  proved 
not  to  exceed  the  two  million  five  hundred  thousandth  of  an  inch. 

The  transparent  wings  of  certain  insects  are  so  attenuated  in  their  structure, 
that  fifty  thousand  of  them  placed  over  each  other  would  not  form  a  pile  a  quar- 
ter of  an  inch  in  height. 

In  the  manufacture  of  embroidery  it  is  necessary  to  obtain  very  fine  gilt  sil- 
ver threads.  To  accomplish  this,  a  cylindrical  bar  of  silver,  weighing  three 
hundred  and  sixty  ounces,  is  covered  with  about  two  ounces  of  gold.    This  gilt 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


25 


bar  is  then  wiredrawn,  as  in  the  first  example,  until  it  is  reduced  to  a  thread 
so  fine  that  thirty-four  hundred  feet  of  it  weigh  less  than  an  ounce.  The  wire 
is  then  flattened,  by  passing  it  between  rollers  under  a  severe  pressure,  a  pro- 
cess which  increases  its  length,  so  that  about  four  thousand  feet  shall  weigh 
one  ounce.  Hence  one  foot  will  weigh  the  four  thousandth  part  of  an  ounce. 
The  proportion  of  the  gold  to  the  silver  in  the  original  bar  was  that  of  two  to 
three  hundred  and  sixty,  or  one  to  one  hundred  and  eighty.  Since  the  same 
proportion  is  preserved  after  the  bar  has  been  wiredrawn,  it  follows  that  the 
quantity  of  gold  which  covers  one  foot  of  the  fine  wire  is  the  one  hundred  and 
eightieth  part  of  the  four  thousandth  of  an  ounce  :  that  is  the  seven  hundred 
and  twenty  thousandth  part  of  an  ounce. 

The  quantity  of  gold  which  covers  one  inch  of  the  wire  will  be  twelve  times 
less  than  that  which  covers  one  foot.  Hence  this  quantity  will  be  the  eight 
million  six  hundred  and  forty  thousandth  part  of  an  inch.  If  this  inch  be  again 
divided  into  one  hundred  equal  parts,  every  part  will  be  distinctly  visible  with- 
out the  aid  of  microscopes.  The  gold  which  covers  this  small  but  visible  por- 
tion is  the  eight  hundred  and  sixty-four  millionth  part  of  an  ounce. 

But  we  may  proceed  even  further.  This  portion  of  the  wire  may  be  viewed 
by  a  microscope  which  magnifies  five  hundred  times,  so  that  the  five  hundredth 
part  of  it  will  thus  become  visible.  In  this  manner,  therefore,  an  ounce  of 
gold  may  be  divided  into  four  hundred  and  thirty-two  thousand  million  parts. 
Each  of  these  parts  will  possess  all  the  characters  and  qualities  which  are 
found  in  the  largest  masses  of  the  metal.  It  retains  its  solidity,  texture,  and 
color  ;  it  resists  the  same  agents,  and  enters  into  combination  with  the  same 
substances.  If  the  gilt  wire  be  dipped  in  nitric  acid,  the  silver  within  the 
coating  will  be  dissolved,  but  the  hollow  tube  of  gold  which  surrounded  it  will 
still  cohere  and  remain  suspended. 

The  organized  world  offers  still  more  remarkable  examples  of  the  inconceiv- 
able subtilty  of  matter. 

The  blood  which  flows  in  the  veins  of  animals  is  not,  as  it  seems  to  be,  a  uni- 
formly red  liquid.  It  consists  of  small  red  globules,  floating  in  a  transparent 
fluid  called  serum.  In  different  species  these  globules  differ  both  in  figure  and 
in  magnitude.  In  man,  and  all  animals  which  suckle  their  young,  they  are 
perfectly  round  or  spherical ;  in  birds  and  fishes,  they  are  of  an  oblong  sphe- 
roidal form.  In  the  human  species,  the  diameter  of  the  globules  is  about  the 
four  thousandth  part  of  an  inch.  "Hence  it  follows  that  in  a  drop  of  blood  which 
would  remain  suspended  from  the  point  of  a  fine  needle,  there  must  be  about  a 
million  of  globules. 

Small  as  these  globules  are,  the  animal  kingdom  presents  beings  whose  whole 
bodies  are  still  more  minute.  Animalcules  have  been  discovered,  whose  mag- 
nitude is  such,  that  a  million  of  them  do  not  exceed  the  bulk  of  a  grain  of  sand, 
and  yet  each  of  these  creatures  is  composed  of  members  as  curiously  organized 
as  those  of  the  largest  species  ;  they  have  life  and  spontaneous  motion,  and  are 
endued  with  sense  and  instinct.  In  the  liquids  in  which  they  live,  they  are 
observed  to  move  with  astonishing  speed  and  activity  ;  nor  are  their  motions 
blind  or  fortuitous,  but  evidently  governed  by  choice  and  direction  to  an  end. 
They  use  food  and  drink,  from  which  they  derive  nutrition,  and  are  therefore 
furnished  with  a  digestive  apparatus.  They  have  great  muscular  power,  and 
are  furnished  with  limbs  and  members  of  strength  and  flexibility.  They  are 
susceptible  of  the  same  appetites,  and  obnoxious  to  the  same  passions,  the  grati- 
fication of  which  is  attended  with  the  same  results  as  in  our  own  species. 
Spallanzani  observes  that  certain  animalcules  devour  others  so  voraciously  that 
they  fatten,  and  become  indolent  and  sluggish,  by  over-feeding. 

After  a  meal  of  this  kind,  if  they  be  confined  in  distilled  water,  so  as  to  be 


26 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


deprived  of  all  food,  their  condition  becomes  reduced  ;  they  regain  their  spirit  ) 

and  activity,  and  amuse  themselves  in  the  pursuit  of  the  more  minute  animals  < 

which  are  supplied  to  them  ;  they  swallow  these   without  depriving   them  of  ; 

life,  for,  by  the  aid  of  the  microscope,  the  one  has  been  observed  moving  within  f 

the  body  of  the  other.     These  singular  appearances  are  not  matters  of  idle  and  ) 

curious  observation.  They  lead  us  to  inquire  what  parts  are  necessary  to  pro-  ' 
duce  such  results.  Must  we  not  conclude  that  these  creatures  have  heart, 
arteries,  veins,  muscles,  sinews,  tendons,  nerves,  circulating  fluids,  and  all  the 
concomitant  apparatus  of  a  living  organized  body  ?  And  if  so,  how  inconceiv- 
ably minute  must  not  those  parts  be  !  If  a  globule  of  their  blood  bears  the  same 
proportion  to  their  whole  bulk  as  a  globule  of  our  blood  bears  to  our  magnitude, 
what  powers  of  calculation  can  give  an  adequate  notion  of  its  minuteness  ? 

These  and  many  other  phenomena  observed  in  the  immediate  productions 
of  nature,  or  developed  by  mechanical  and  chemical  processes,  prove  that  the 
materials  of  which  bodies  are  formed  are  susceptible  of  minuteness  which  infi- 
nitely exceeds  the  powers  of  sensible  observation,  even  when  those  powers 
have  been  extended  by  all  the  aids  of  science.  Shall  we,  then,  conclude  that 
matter  is  infinitely  divisible,  and  that  there  are  no  original  constituent  atoms  of 
determinate  magnitude  and  figure  at  which  all  subdivision  must  cease  ?  Such 
an  inference  would  be  unwarranted,  even  had  we  no  other  means  of  judging 
the  question  except  those  of  direct  observation  ;  for  it  would  be  imposing 
that  limit  on  the  works  of  nature  which  she  has  placed  upon  our  powers  of 
observing  them.  Aided  by  reason,  however,  and  a  due  consideration  of  certain 
phenomena  which  come  within  our  immediate  powers  of  observation,  we  are 

frequently  able  to  determine  other  phenomena  which  are  beyond  those  powers.  ^ 

The  diurnal   motion  of  the  earth  is  not  perceived   by  us,  because  all  things  \ 

around  us  participate  in  it,  preserve  their  relative  position,  and  appear  to  be  at  / 

rest.     But  reason  tells  us  that  such  a  motion  must  produce  the  alternations  of  S 

day  and  night,  and  the  rising  and  setting  of  all  the  heavenly  bodies — appear-  ) 

ances  which  are  plainly  observable,  and  which  betray  the  cause  from  which  S 

they  arise.     Again,  we  cannot  place  ourselves  at  a  distance  from  the  earth,  ) 

and  behold  the  axis  on  which  it  revolves,  and  observe  its  peculiar  obliquity  to  s 

the  orbit  in  which  the  earth  moves  ;  but  we  see  and  feel  the  vicissitudes  of  the  / 

seasons,  an  eftfect  which  is  the  immediate  consequence  of  that  inclination,  and  s 

by  which  we  are  able  to  detect  it.  ) 

So  it  is  in  the  present  case.     Although  we  are  unable  by  direct  observation  s 

to  prove  the  existence  of  constituent  material  atoms  of  determinate  figure,  yet  / 

there  are   many  observable   phenomena  which  render  their  existence  in  the  \ 

highest  degree  probable,  if  not  morally  certain.     The  most  remarkable  of  this  / 

class  of  effects  is  observed  in  the  crystallization  of  salts.     When  salt  is  dis-  \ 

solved  in  a  sufficient  quantity  of  pure  water,  it  mixes  with  the  water  in  such  a  ) 

manner  as  wholly  to  disappear  to  the  sight  and  touch,  the  mixture  being  one  S 

uniform  transparent  liquid  like  the  water  itself  before  its  union  with  the  salt.  / 

The  presence  of  the  salt  in  the  water  may,  however,  be  ascertained  by  weigh-  ) 

I  ing  the   mixture,  which  will  be   found   to  exceed  the  original  weight  of  the  ) 

1  water  by  the  exact  amount  of  the  weight  of  the  salt.     It  is  a  well-known  fact  s 

I  that  a  certain  degree  of  heat  will  convert  water  into  vapor,  and  that  the  same  ) 

I  degree  of  heat  does  not  effect  any  change  in  the  form  of  salt.     The  mixture  of  s 

'  salt  and  water  being  exposed  to  this   temperature,  the  water  will   gradually  ) 

I  evaporate,  disengaging  itself  from  the  salt  with  which  it  has  been  combined,  s 

*  When  so  much  of  the  water  has  evaporated  that  what  remains  is  insufficient  to  ) 

)  keep  in  solution  the  whole  of  the  salt,  a  part  of  it  thus  disengaged  from  the  S 

[  water  will  return  to  the  solid  state.     The  saline  particles  will  not  in  this  case  \ 

)  collect  in  irregular  solid  molecules,  but  will  exhibit  themselves  in  regular  fig-  S 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


27 


iires,  terminated  by  plane  surfaces,  the  figures  being  always  the  same  for  the 
same  species  of  salt,  but  different  for  different  species.  There  are  several 
circumstances  in  the  formation  of  these  crystals  which  merit  attention. 

If  one  of  the  crystals  be  detached  from  the  others,  and  the  process  of  its 
formation  observed,  it  will  be  found  gradually  to  increase,  always  preserving 
its  original  figure.  Since  its  increase  must  be  caused  by  the  continued  acces- 
sion of  saline  particles  disengaged  by  the  evaporation  of  the  water,  it  follows 
that  these  particles  must  be  so  formed,  that,  by  attaching  themselves  successively 
to  the  crystal,  they  maintain  the  regularity  of  its  bounding  planes,  and  preserve 
their  mutual  inclinations  unvaried. 

Suppose  a  crystal  to  be  taken  from  the  liquid  during  the  progress  of  crystal- 
lization, and  a  piece  broken  from  it  so  as  to  destroy  the  regularity  of  its  form ; 
if  the  crystal  thus  broken  be  restored  to  the  liquid,  it  will  be  observed  gradu- 
ally to  resume  its  regular  form,  the  atoms  of  salt  successively  dismissed  by  the 
vaporizing  water  filling  up  the  irregular  cavities  produced  by  the  fracture. 
Hence  it  follows  that  the  saline  particles  which  compose  the  surface  of  the 
crystal,  and  those  which  form  the  interior  of  its  mass,  are  similar,  and  exert 
similar  attractions  on  the  atoms  disengaged  by  the  water. 

All  these  details  of  the  process  of  crystallization  are  very  evident  indications 
of  a  determinate  figure  in  the  ultimate  atoms  of  the  substances  which  are  crys- 
tallized. But  besides  the  substances  which  are  thus  reduced  by  art  to  the  form 
of  crystals,  there  are  larger  classes  which  naturally  exist  in  that  state.  There 
are  certain  planes,  called  planes  of  cleavage,  in  the  directions  of  which  natural 
crystals  are  easily  divided.  These  planes,  in  substances  of  the  same  kind, 
always  have  the  same  relative  position,  but  differ  in  different  substances.  The 
surfaces  of  the  planes  of  cleavage  are  quite  invisible  before  the  crystal  is  di- 
vided ;  but  when  the  parts  are  separated,  these  surfaces  exhibit  a  most  intense 
polish,  which  no  effort  of  art  can  equal. 

We  may  conceive  crystallized  substances  to  be  regular  mechanical  struc- 
tures formed  of  atoms  of  a  certain  figure,  on  which  the  figure  of  the  whole  struc- 
ture must  depend.  The  planes  of  cleavage  are  parallel  to  the  sides  of  the  con- 
stituent atoms,  and  their  directions  therefore  form  so  many  conditions  for  the 
determination  of  its  figure.  The  shape  of  the  atoms  being  thus  determined,  it 
is  not  difficult  to  assign  all  the  various  ways  in  which  they  may  have  been 
arranged,  so  as  to  produce  figures  which  are  accordingly  found  to  correspond 
with  the  various  forms  of  crystals  of  the  same  substance. 

When  these  phenomena  are  duly  considered  and  compared,  little  doubt  can 
remain  that  all  substances  susceptible  of  crystallization  consist  of  atoms  of  de- 
terminate figure.  This  is  the  case  with  all  solid  bodies  whatever  which  have 
come  under  scientific  observation,  for  they  have  been  severally  found  in,  or  re- 
duced to,  a  crystallized  form.  Liquids  crystallize  in  freezing ;  and  if  aeriform 
fluids  could  by  any  means  be  reduced  to  the  solid  form,  they  would  probably 
also  manifest  the  same  effect.  Hence  it  appears  reasonable  to  presume  that 
all  bodies  are  composed  of  atoms  ;  that  the  different  qualities  with  which  we 
find  different  substances  endued,  depend  on  the  magnitude  and  figure  of  these 
atoms  ;  and  these  atoms  are  indestructible  and  immutable  by  any  natural  pro- 
cess, for  we  find  the  qualities  which  depend  on  them  unchangpably  the  same 
under  all  the  influences  to  which  they  have  been  submitted  since  their  crea- 
tion ;  that  these  atoms  are  so  minute  in  their  magnitude,  that  they  cannot  be 
observed  by  any  means  which  human  art  has  yet  contrived,  but  still  that  there 
are  limits  of  magnitude  which  they  do  not  exceed.  \ 

It  is  proper,  however,  to  observe  here,  that  the  various  theories  of  mechani-  ' 
cal  science  do  not  rest  upon  any  hypotheses  concerning  these  atoms  as  a  basis.  \ 
They  are  not  inferred  from  this  or  any  other  supposition,  and  therefore  their  ' 


28 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


truth  would  not  be  in  anywise  disturbed,  even  thougli  it  should  be  established 
that  matter  is  physically  divisible  ad  infinitum.  The  basis  of  mechanical  sci- 
ence is  observed  facts ;  and  since  the  reasoning  is  demonstrative,  the  con- 
clusions have  the  same  degree  of  certainty  as  the  facts  from  which  they  are 
deduced. 

POROSITY. 


The  volume  of  a  body  is  the  quantity  of  space  included  within  its  external 
surface,  The  mass  of  a  body  is  the  collection  of  atoms  or  material  particles 
of  which  it  consists.  Two  atoms  or  particles  are  said  to  be  in  contact,  when 
they  have  approached  each  other  until  arrested  by  their  mutual  impenetrability. 
If  the  component  particles  of  a  body  were  in  contact,  the  volume  would  be  com- 
pletely occupied  by  the  mass.  But  this  is  not  the  case.  We  shall  presently 
prove  that  the  component  particles  of  no  known  substance  are  in  absolute  con- 
tact. Hence  it  follows  that  the  volume  consists  partly  of  material  particles  and 
partly  of  interstitial  spaces,  which  spaces  are  either  absolutely  void  and  empty 
or  filled  by  some  substance  of  a  different  species  from  the  body  in  question. 
These  interstitial  spaces  are  called  pores. 

In  bodies  which  are  constituted  uniformly  throughout  their  entire  dimen- 
sions, the  component  particles  and  the  pores  are  uniformly  distributed  through 
the  volume  ;  that  is,  a  given  space  in  one  part  of  the  A^olume  will  contain  the 
same  quantity  of  matter  and  the  same  quantity  of  pores  as  an  equal  space  in 
another  part. 

The  proportion  of  the  quantity  of  matter  to  the  magnitude  is  called  the  den- 
sity. Thus,  if  of  two  substances,  one  contains  in  a  given  space  twice  as  much 
matter  as  the  other,  it  is  said  to  be  "  twice  as  dense."  The  density  of  bodies 
is  therefore  proportionate  to  the  closeness  or  proxirnity  of  their  particles,  and 
it  is  evident  that  the  greater  the  density,  the  less  will  be  the  porosity. 

The  pores  of  a  body  are  frequently  filled  with  another  body  of  a  more  subtile 
nature.  If  the  pores  of  a  body  on  the  surface  of  the  earth,  and  exposed  to  the 
atmosphere,  be  greater  than  the  particles  of  air,  then  the  air  will  pervade  the 
pores.  This  is  found  to  be  the  case  of  many  sorts  of  wood  which  have  open 
grains.  If  a  piece  of  such  wood,  or  of  chalk,  or  of  sugar,  be  pressed  to  the 
bottom  of  a  vessel  of  water,  the  air  which  fills  the  pores  will  be  observed  to 
escape  in  bubbles,  and  to  rise  to  the  surface,  the  water  pervading  the  pores  and 
taking  its  place. 

If  a  tall  vessel  or  lube,  having  a  wooden  bottom,  be  filled  with  quicksilver, 
the  liquid  metal  will  be  forced  by  its  own  weight  through  the  pores  of  the 
wood,  and  will  be  seen  escaping  in  a  silver  shower  from  the  bottom. 

The  process  of  filtration  in  the  arts  depends  on  the  presence  of  pores  of  such 
a  magnitude  as  to  allow  a  passage  to  the  liquid,  but  to  refuse  it  to  those  impu- 
rities from  which  it  is  to  be  disengaged.  Various  substances  are  used  as  filters  ; 
but  whatever  be  used,  this  circumstance  should  always  be  remembered,  that  no 
substance  can  be  separated  from  a  liquid  by  filtration  except  one  whose  parti- 
cles are  larger  than  those  of  the  liquid.  In  general,  filters  are  used  to  separate 
solid  impurities  from  a  liquid.  The  most  ordinary  filters  are  soft-stone  paper 
and  charcoal.     * 

All  organized  substances  in  the  animal  and  vegetable  kingdoms  are,  from 

their  very  nature,  porous  in  a  high  degree.     Minerals  are   porous  in  various 

degrees.     Among  the  silicious  stones  is  one  called  hydrophane,  which  mani- 

I  fests  its  porosity  in  a  very  remarkable  manner.     The  stone  in   its  ordinary 

\  state  is  semi-transparent.     If,  however,  it  be  plunged  in  water,  when  it  is  with- 

I  drawn  it  is  as  translucent  as  glass.     The  pores  in  this   case  previously  filled 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


29 


with  air,  are  pervaded  by  the  water,  between  which  and  the  stone  there  sub- 
sists a'  physical  relation  by  which  the  one  renders  the  other  perfectly  trans- 
parent. 

Larger  mineral  masses  exhibit  degrees  of  porosity  not  less  striking.  Water 
percolates  through  the  sides  and  roofs  of  caverns  and  grottoes  ;  and  being  im- 
pregnated with  calcareous  and  other  earths,  forms  stalactites  or  pendent  protu- 
berances, which  present  a  curious  appearance. 

COMPRESSIBILITY. 

That  quality  in  virtue  of  which  a  body  allows  its  volume  to  be  diminished 
without  diminishing  its  mass,  is  called  cornpressibiliiy.  This  effect  is  produced 
by  bringing  the  constituent  particles  more  closely  together,  and  thereby  in- 
creasing the  density  and  diminishing  the  pores.  This  effect  maybe  produced 
in  several  ways,  but  the  name  compressibility  is  applied  to  it  when  it  is  caused 
by  the  agency  of  mechanical  force,  as  by  pressure  or  percussion.  AH  known 
bodies,  whatever  be  their  nature,  are  capable  of  having  their  dimensions  re- 
duced without  diminishing  their  mass,  and  this  is  one  of  the  most  conclusive 
proofs  that  all  bodies  are  porous,  or  that  the  constituent  atoms  are  not  in  con- 
tact ;  for  the  space  by  which  the  volume  may  be  diminished,  must,  before  the 
diminution,  consist  of  pores.  Some  bodies,  when  compressed  by  the  agency 
of  mechanical  force,  will  resume  their  former  dimensions  with  a  certain  force 
when  relieved  from  the  operation  of  the  force  which  has  compressed  them. 
This  property  is  called  elasticity,  and  it  follows  from  this  definition  that  all  elas- 
tic bodies  must  be  compressible,  although  the  converse  is  not  true  compressi- 
bility— not  necessarily  implying  elasticity. 

DILATABILITY. 


This  quality  is  the  opposite  of  compressibility.  It  is  the  capability  observed 
in  bodies  to  have  their  volume  enlarged  without  increasing  their  mass.  This 
effect  may  be  produced  in  several  ways.  In  ordinary  circumstances,  a  body 
may  exist  under  the  constant  action  of  a  pressure  by  which  its  volume  and 
density  are  determined.  It  may  happen  that  on  the  occasional  removal  of  that 
pressure  the  body  will  dilate,  by  a  quality  inherent  in  its  constitution.  This  is 
the  case  with  common  air.  Dilatation  may  also  be  the  effect  of  heat,  as  will 
presently  appear.  The  several  qualities  of  bodies  which  we  have  noticed  in 
this  chapter,  when  viewed  in  relation  to  each  other,  present  many  circum- 
stances worthy  of  attention.  It  is  a  physical  law,  to  which  there  is  no  real 
exception,  that  an  increase  in  the  temperature  or  degree  of  heat  by  which  a 
body  is  affected,  is  accompanied  by  an  increase  of  volume,  and  that  a  diminu- 
tion of  temperature  is  accompanied  by  a  diminution  of  volume.  The  apparent 
exceptions  to  this  law  will  be  noticed  and  explained  in  our  discourses  on  heat. 
Hence  it  appears  that  the  reduction  of  temperature  is  an  effect  which,  consid- 
ered mechanically,  is  equivalent  to  compression  or  condensation,  since  it  di- 
minishes the  volume  without  altering  the  mass  ;  and  since  this  is  an  effect  of 
which  all  bodies  whatever  are  susceptible,  it  follows  that  all  bodies  whatever 
have  pores. 

The  fact  that  the  elevation  of  temperature  produces  an  increase  of  volume, 
is  manifested  by  numerous  experiments. 

If  a  flaccid  bladder  be  tied  at  the  mouth  so  as  to  stop  the  passage  of  air,  and 
be  then  held  before  a  fire,  it  will  gradually  swell  and  assume  the  appearance 
of  being  fully  inflated.     The  small  quantity  of  air  contained  in  the  bladder  is. 


30 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


in  this  case,  so  much  dilated  by  the  heat,  that  it  occupies  a  considerably  in- 
creased space,  and  fills  the  bladder,  of  which  it  before  only  occupied  a  small 
part.  When  the  bladder  is  removed  from  the  fire,  and  allowed  to  resume  its 
former  temperature,  the  air  returns  to  its  former  dimensions,  and  the  bladder 
becomes  again  flaccid. 

Let  a  glass  tube,  with  the  bulb  at  the  end,  have  the  bnlb  and  a  part  of  the 
tube  filled  with  any  liquid,  colored  so  as  to  be  visible.  If  the  bulb  be  exposed 
to  heat,  by  being  plunged  in  hot  water,  the  level  of  the  liquid  will  rapidly  rise. 
This  eff'ect  is  produced  by  the  dilatation  of  the  liquid  in  the  bulb,  which,  filling 
a  greater  space,  a  part  of  it  is  forced  into  the  tube.  This  experiment  may 
easily  be  made  with  a  common  glass  tube  and  a  little  port  wine. 

Thermometers  are  constructed  on  this  principle,  the  ascent  of  the  liquid  in  the 
tube  being  used  as  an  indication  of  the  degree  of  heat  which  causes  it.  A  par- 
ticular account  of  these  useful  instruments  will  be  found  in  our  discourse  on 
them. 

The  change  of  dimensions  of  solids  produced  by  changes  of  temperature 
being  much  less  than  that  of  bodies  in  the  liquid  or  aeriform  state,  is  not  so 
easily  observable.  A  remarkable  instance  occurs  in  the  process  of  shoeing 
the  wheels  of  carriages.  The  rim  of  iron  with  which  the  wheel  is  to  be  bound 
is  made  in  the  first  instance  of  a  diameter  somewhat  less  than  that  of  the 
wheel  ;  but  being  raised  by  the  application  of  fire  to  a  very  high  temperature, 
its  volume  receives  such  an  increase,  that  it  will  be  sufficient  to  embrace  and 
surround  the  wheel.  When  placed  upon  the  wheel,  it  is  cooled,  and  suddenly 
contracting  its  dimensions,  binds  the  parts  of  the  wheel  firmly  together,  and 
becomes  securely  seated  in  its  place  upon  the  face  of  the  felloes. 

It  frequently  happens  that  the  stopper  of  a  glass  bottle  or  decanter  becomes 
fixed  in  its  place  so  firmly,  that  the  exertion  of  force  sufficient  to  withdraw  it 
would  endanger  the  vessel.  In  this  case,  if  a  cloth  wetted  with  hot  water  be 
applied  to  the  neck  of  the  bottle,  the  glass  will  expand,  and  the  neck  will  be 
enlarged  so  as  to  allow  the  stopner  to  be  easily  withdrawn. 

The  contraction  of  metal  consequent  upon  change  of  temperature  has  been 
applied  some  time  ago  in  Paris  to  restore  the  walls  of  a  tottering  building  to 
their  proper  position.  In  the  Conservatuire  des  Arts  et  Metiers,  the  walls  of  a 
part  of  the  building  were  forced  out  of  the  perpendicular  by  the  weight  of  the 
roof,  so  that  each  wall  was  leaning  outward.  M.  Molard  conceived  the  notion 
of  applying  the  irresistible  force  with  which  metals  contract  in  cooling,  to  draw 
the  walls  together.  Bars  of  iron  were  placed  in  parallel  directions  across  the 
building,  and  at  right  angles  to  the  direction  of  the  walls.  Being  passed  through 
the  walls,  nuts  were  screwed  on  their  ends  outside  the  building.  Every  alter- 
nate bar  was  then  heated  by  lamps,  and  the  nuts  screwed  close  to  the  walls. 
The  bars  were  then  cooled,  and  the  lengths  being  diminished  by  contraction, 
the  nuts  on  their  extremities  were  drawn  together,  and  with  them  the  walls 
were  drawn  through  an  equal  space.  The  same  process  was  repeated  with 
the  intermediate  bars,  and  so  on  alternately,  until  the  walls  were  brought  into 
a  perpendicular  position. 

Since  there  is  a  continual  change  of  temperature  in  all  bodies  on  the  surface 
of  the  globe,  it  follows  that  there  is  also  a  continual  change  of  magnitude.  The 
substances  which  surround  us  are  constantly  swelling  and  contracting  under 
the  vicissitudes  of  heat  and  cold.  They  grow  smaller  in  winter,  and  dilate  in 
summer  ;  they  swell  their  bulk  in  a  warm  day,  and  contract  it  in  a  cold  one 
These  curious  phenomena  are  not  noticed,  only  because  our  ordinary  means  of 
observation  are  not  sufficiently  accurate  to  appreciate  them.  Nevertheless,  in 
some  instances,  the  efl^ect  is  very  obvious.  In  warm  weather,  the  flesh  swells, 
the  vessels  appear  filled,  the  hand  is  plump,  and  the  skin  distended.     In  cold 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


31 


weather,  when  the  body  has  been  exposed  to  the  open  air,  the  flesh  appears  to 
contract,  the  vessels  shrink,  and  the  skin  shrivels. 

The  phenomena  attending  change  of  temperature  are  conclusive  proofs  of 
the  universal  porosity  of  material  substances,  but  they  are  not  the  only  proofs. 
Many  substances  admit  of  compression  by  the  mere  agency  of  mechanical  force. 
Let  a  small  piece  of  cork  be  placed  floating  on  the  surface  of  water  in  a  basin  or 
other  vessel,  and  an  empty  glass  goblet  be  inverted  over  the  cork  so  that  its  edge 
just  meets  the  water.  A  portion  of  air  will  then  be  confined  in  the  goblet  and 
detached  from  the  remainder  of  the  atmosphere.  If  the  goblet  be  now  pressed 
downward  so  as  to  be  entirely  immersed,  it  will  be  observed  that  the  water 
will  not  fill  it,  being  excluded  by  the  impenetrability  of  the  air  enclosed  in  it. 
This  experiment,  therefore,  is  decisive  of  the  fact  that  air,  one  of  the  most 
subtile  and  attenuated  substances  we  know  of,  possesses  the  quality  of  impene- 
trability. It  absolutely  excludes  every  other  body  from  the  space  which  it 
occupies  at  any  given  moment. 

But  although  the  water  does  not  fill  the  goblet,  yet  if  the  position  of  the  cork 
which  floats  upon  its  surface  be  noticed,  it  will  be  found  that  the  level  of  the 
water  within  has  risen  above  its  edge  or  rim.  In  fact,  the  water  has  partially 
filled  the  goblet,  and  the  air  has  been  forced  to  contract  its  dimensions.  This 
ei!"ect  is  produced  by  the  pressure  of  the  incumbent  water  forcing  the  surface 
in  the  goblet  against  the  air,  which  yields  until  it  is  so  far  compressed  that  it 
acquires  a  force  able  to  withstand  this  pressure.  Thus  it  appears  that  air  is 
capable  of  being  reduced  in  its  dimensions  by  mechanical  pressure,  indepen- 
dently of  the  agency  of  heat.     It  is  compressible. 

That  this  efl^ect  is  the  consequence  of  the  pressure  of  the  liquid,  will  be 
easily  made  manifest  by  showing  that,  as  the  pressure  is  increased,  the  air  is 
proportionally  contracted  in  its  dimensions  ;  and  as  it  is  diminished,  the  dimen- 
sions are,  on  the  other  hand,  enlarged.  If  the  depth  of  the  goblet  in  the  water 
be  increased,  the  cork  will  be  seen  to  rise  in  it,  showing  that  the  increased 
pressure  at  the  greater  depth  causes  the  air  in  the  goblet  to  be  more  condensed. 
If,  on  the  other  hand,  the  goblet  be  raised  toward  the  surface,  the  cork  will 
be  observed  to  descend  toward  the  edge,  showing  that  as  it  is  relieved  from 
the  pressure  of  the  liquid,  the  air  gradually  approaches  to  its  primitive  dimen- 
sions. 

These  phenomena  also  prove  that  air  has  the  property  of  elasticity.  If  it 
were  simply  compressible,  and  not  elastic,  it  would  retain  the  dimensions  to 
which  it  was  reduced  by  the  pressure  of  the  liquid  ;  but  this  is  not  found  to  be 
the  result.  As  the  compressing  force  is  diminished,  so  in  the  same  proportion 
does  the  air,  by  its  elastic  virtue,  exert  a  force  by  which  it  resumes  its  former 
dimensions. 

That  it  is  the  air  alone  which  excludes  the  water  from  the  goblet  in  the  pre- 
ceding experiments,  can  easily  be  proved.  When  the  goblet  is  sunk  deep  in 
the  vessel  of  water,  let  it  be  inclined  a  little  to  one  side  until  its  mouth  is  pre- 
sented toward  the  side  of  the  vessel ;  let  this  inclination  be  so  regulated  that 
the  surface  of  the  water  in  the  goblet  shall  just  reach  its  edge.  Upon  a  slight 
increase  of  inclination,  air  will  be  observed  to  escape  from  the  goblet,  and  to  rise 
in  bubbles  to  the  surface  of  the  water.  If  the  goblet  be  then  restored  to  its 
position,  it  will  be  found  that  the  cork  will  rise  higher  in  it  than  before  the 
escape  of  the  air.  The  water  in  this  case  rises  and  fills  the  space  which  the 
air,  allowed  to  escape,  has  deserted.  The  same  process  may  be  repeated  until 
all  the  air  has  escaped,  and  then  the  goblet  will  be  completely  filled  by  the 
water. 

Liquids  are  compressible  by  mechanical  force  in  so  slight  a  degree,  that  they 
are  considered  in  all  hydrostatical  treatises  as  incompressible  fluids.     They  ; 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


are,  however,  not  absolutely  incompressible,  but  yield  slightly  to  very  intense 
pressure. 

The  question  of  the  compressibility  of  liquids  was  raised  at  a  remote  period 
in  the  history  of  science.  Nearly  two  centuries  ago  an  experiment  was  insti- 
tuted at  the  Academy  del  Cimento  in  Florence,  to  ascertain  whether  water  be 
compressible.  With  this  view,  a  hollow  ball  of  gold  was  filled  with  thd  liquid, 
and  the  aperture  exactly  and  firmly  closed.  The  globe  was  then  submitted  to 
a  very  severe  pressure,  by  which  its  figure  was  slightly  changed.  Now,  it  is 
proved  in  geometry  that  a  globe  has  this  peculiar  property,  that  any  change  what- 
ever in  its  figure  must  necessarily  diminish  its  volume  or  contents.  Hence  it  was 
inferred  that  if  the  water  did  not  issue  through  the  pores  of  the  gold,  or  burst  the 
globe,  its  compressibility  would  be  established.  The  result  of  the  experiment 
was  that  the  water  did  ooze  through  the  pores,  and  covered  the  surface  of  the 
globe,  presenting  the  appearance  of  dew  or  of  steam  cooled  by  the  metal.  But 
this  experiment  was  inconclusive.  It  is  quite  true  that  if  the  water  had  not 
escaped,  upon  the  change  of  figure  of  the  globe,  the  compressibility  of  the  liquid 
would  have  been  established.  This  escape  of  the  water  does  not,  however, 
prove  its  incompressibility .  To  accomplish  this,  it  would  be  necessary  first  to 
measure  accurately  the  volume  of  water  which  transuded  by  compression,  and 
next  to  measure  the  diminution  of  volume  which  the  vessel  suffered  by  its 
change  of  figure.  If  this  diminution  were  greater  than  the  volume  of  water 
which  escaped,  it  would  follow  that  the  water  remaining  in  the  globe  had  been 
compressed,  notwithstanding  the  escape  of  the  remainder.  But  this  could 
never  be  accomplished  with  the  delicacy  and  exactitude  necessary  in  such  an 
experiment,  and  consequently,  as  far  as  the  question  of  the  compressibility  of 
water  was  concerned,  nothing  was  proved.  It  forms,  however,  a  very  striking 
illustration  of  the  porosity  of  so  dense  a  substance  as  gold,  and  proves  that  its 
pores  are  larger  than  the  elementary  particles  of  water,  since  they  are  capable 
of  passing  through  them. 

It  has  since  been  proved  that  water  and  other  liquids  are  compressible.  In 
the  year  1761,  Canton  communicated  to  the  Royal  Society  the  results  of  some 
experiments  which  proved  this  fact.  He  provided  a  glass  tube  with  a  bulb, 
like  that  of  a  common  thermometer,  and  filled  the  bulb  and  a  part  of  the  tube 
with  the  liquid  well  purified  from  air.  He  then  placed  this  in  an  apparatus 
called  a  condenser,  by  which  he  was  enabled  to  submit  the  surface  of  the  liquid 
in  the  tube  to  a  very  intense  pressure  of  condensed  air.  He  found  that  the 
level  of  the  liquid  in  the  tube  fell  in  a  perceptible  degree  upon  the  appli- 
cation of  the  pressure.  The  same  experiment  established  the  fact  that  liquids 
are  clastic  ;  for,  upon  removing  the  pressure,  the  liquid  rose  to  its  original  level, 
and  therefore  resumed  its  former  dimensions. 

Elasticity  does  not  always  accompany  compressibility.  If  lead  or  iron  be 
submitted  to  the  hammer,  it  may  be  hardened  and  diminished  in  its  volume, 
but  it  will  not  resume  its  former  volume  after  each  stroke  of  the  hammer. 

There  are  some  bodies  which  maintain  the  state  of  density  in  which  they 
are  commonly  found  by  the  continual  agency  of  mechanical  pressure,  and  such 
bodies  are  endued  with  a  quality  in  virtue  of  which  they  would  enlarge  their 
dimensions  without  limit  if  the  pressure  which  confines  them  were  removed. 
Such  bodies  are  called  elastic  fluids  or  gases,  and  always  exist  in  the  form  of 
\  .common  air,  in  whose  mechanical  properties  they  participate.  They  are  hence 
I  called  aeriform  fluids- 

\  Those  who  are  provided  with  an  air-pump  can  easily  establish  this  property 
•  experimentally.  Take  a  flaccid  bladder  and  place  it  under  the  glass  receiver 
,  of  an  air-pump.  By  this  instrument  we  shall  be  able  to  remove  the  air  which 
I  surrounds  the  bladder  under  the  receiver  so  as  to  relieve  the  small  quantity  of 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


33 


air  which  is  enclosed  in  the  bladder  from  the  pressure  of  the  external  air.  When 
this  is  accomplished,  the  bladder  Avill  be  observed  to  swell  as  if  it  were  in- 
flated, and  will  become  perfectly  distended.  The  air  contained  in  it,  there- 
fore, has  a  tendency  to  dilate,  which  takes  effect  when  it  ceases  to  be  resisted 
by  the  pressure  of  surrounding  air. 

It  has  been  stated  that  the  increase  or  diminution  of  temperature  is  accom- 
panied by  an  increase  or  diminution  of  volume.  Related  to  this  there  is  anoth- 
er phenomenon,  too  remarkable  to  pass  unnoticed,  although  this  is  not  the  proper 
place  to  dwell  upon  it :  it  is  the  converse  of  the  former,  viz.,  that  all  increase 
or  diminution  of  bulk  is  accompanied  by  a  diminution  or  increase  of  tempera- 
ture. As  the  application  of  heat  from  some  foreign  source  produces  an  increase 
of  dimensions,  so,  if  the  dimensions  be  increased  from  any  other  cause,  a  cor- 
responding portion  of  the  heat  which  the  body  had  before  the  enlargement  will 
be  absorbed  in  the  process,  and  the  temperature  will  be  thereby  diminished. 
In  the  same  way,  since  the  abstraction  of  heat  causes  a  diminution  of  volume, 
so,  if  that  diminution  be  caused  by  any  other  means,  the  body  will  give  out  the 
heat  which  in  the  other  case  was  abstracted,  and  will  rise  in  its  temperature. 

Numerous  and  well-known  facts  illustrate  these  observations.  A  smith,  by 
hammering  a  piece  of  bar  iron,  and  thereby  compressing  it,  will  render  it  red 
hot.  When  air  is  violently  compressed,  it  becomes  so  hot  as  to  ignite  cotton 
and  other  substances.  An  ingenious  instrument  for  producing  a  light  for  do- 
mestic uses  has  been  constructed,  consisting  of  a  small  cylinder,  in  which  a 
solid  piston  moves  air  tight ;  a  little  tinder,  or  dry  sponge,  is  attached  to  the 
bottom  of  the  piston,  which  is  thea  violently  forced  into  the  cylinder.  The  air 
between  the  bottom  of  the  cylinder  and  the  piston  becomes  intensely  com- 
pressed, and  evolves  so  much  heat  as  to  light  the  tinder. 

In  all  the  cases  where  friction  or  percussion  produces  heat  or  fire,  it  is  be- 
cause they  are  means  of  compression.  The  effects  of  flints — of  pieces  of  wood 
rubbed  together — the  warmth  produced  by  friction  on  the  flesh — are  all  to  be 
attributed  to  the  same  cause. 


The  quality  of  matter  which  is  of  all  others  the  most  important  in  mechani- 
cal investigations,  is  that  which  has  been  called  inertia. 

Matter  is  incapable  of  spontaneous  change.  This  is  one  of  the  earliest  and 
most  universal  results  of  human  observation  ;  it  is  equivalent  to  stating  that 
mere  matter  is  deprived  of  life  ;  for  spontaneous  action  is  the  only  test  of  the 
presence  of  the  living  principle.  If  we  see  a  mass  of  matter  undergo  any 
change,  we  never  seek  for  the  cause  of  that  change  in  the  body  itself ;  we  look 
for  some  external  cause  producing  it.  This  inability  for  voluntary  change  of 
state  or  qualities  is  a  more  general  principle  than  inertia.  At  any  given  mo- 
ment of  time,  a  body  must  be  in  one  or  other  of  two  states,  rest  or  motion.  In- 
ertia, or  inactivity,  signifies  the  total  absence  of  power  to  change  this  state.  A 
body  endued  with  inertia  cannot  of  itself,  and  independent  of  all  external  influ- 
ence, commence  to  move  from  a  state  of  rest ;  neither  can  it,  when  moving, 
arrest  its  progress,  and  become  quiescent. 

The  same  property  by  which  a  body  is  unable  by  any  power  of  its  own  to 
pass  from  a  state  of  rest  to  one  of  motion,  or  vice  versa,  also  renders  it  inca- 
pable of  increasing  or  diminishing  any  motion  which  it  may  have  received  from 
an  external  cause.  If  a  body  be  moving  in  a  certain  direction  at  the  rate  of 
ten  miles  per  hour,  it  cannot,  by  any  energy  of  its  own,  change  its  rate  of  mo- 
tion to  eleven  or  nine  miles  an  hour.  This  is  a  direct  consequence  of  that 
manifestation  of  inertia  which  has  just  been  explained.     For  the  same  power 

VOL..  II.— 3 


34 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


which  would  cause  a  body  moA'ing  at  ten  miles  an  hour  to  increase  its  rate  to 
eleven  miles,  would  also  cause  the  same  body  at  rest  to  commence  moving  at 
the  rate  of  one  mile  an  hour  ;  and  the  same  power  which  would  cause  a  body 
moving  at  the  rate  of  ten  miles  an  hour  to  move  at  the  rate  of  nine  miles  in  the 
hour,  would  cause  the  same  body  moving  at  the  rate  of  one  mile  an  hour  to 
become  quiescent.  It  therefore  appears  that  to  increase  or  diminish  the  mo- 
tion of  a  body  is  an  effect  of  the  same  kind  as  to  change  the  state  of  rest  into 
that  of  motion,  or  vice  versa. 

The  effects  and  phenomena  which  hourly  fall  under  our  observation  afford 
unnumbered  examples  of  the  inability  of  lifeless  matter  to  put  itself  into  motion, 
or  to  increase  any  motion  which  may  have  been  communicated  to  it.  But  it 
does  not  happen  that  we  have  the  same  direct  and  frequent  evidence  of  its  ina- 
bility to  destroy  or  diminish  any  motion  which  it  may  have  received.  And 
hence  it  arises,  that,  while  no  one  will  deny  to  matter  the  former  effect  of  in- 
ertia, few  will  at  first  acknowledge  the  latter.  Indeed,  even  so  late  as  the  time 
of  Kepler,  philosophers  themselves  held  it  as  a  maxim,  that  "  matter  is  more 
inclined  to  rest  than  to  motion  ;"  we  ought  not,  therefore,  to  be  surprised  if, 
in  the  present  day,  those  who  have  not  been  conversant  with  physical  science 
are  slow  to  believe  that  a  body  once  put  in  motion  would  continue  for  ever  to 
move  with  the  same  velocity,  if  it  were  not  stopped  by  some  external  cause. 

Reason,  assisted  by  observation,  will,  however,  soon  dispel  this  illusion. 
Experience  shows  us  in  various  ways  that  the  same  causes  which  destroy 
motion  in  one  direction  are  capable  of  producing  as  much  motion  in  the  oppo- 
site direction.  Thus,  if  a  wheel,  spinning  on  its  axis  with  a  certain  velocity, 
be  stopped  by  a  hand  seizing  one  of  the  spokes,  the  effort  which  accomplishes 
this  is  exactly  the  same  as,  had  the  wheel  been  previously  at  rest,  would  have 
put  it  in  motion  in  the  opposite  direction  with  the  same  velocity.  If  a  carriage 
drawn  by  horses  be  in  motion,  the  same  exertion  of  power  in  the  horses  is 
necessary  to  stop  it,  as  would  be  necessary  to  backit,  if  it  were  at  rest.  Now, 
if  this  be  admitted  as  a  general  principle,  it  must  be  evident  that  a  body  which 
can  destroy  or  diminish  its  own  motion  must  also  be  capable  of  putting  itself 
into  motion  from  a  state  of  rest,  or  of  increasing  any  motion  which  it  has  re- 
ceived. But  this  latter  is  contrary  to  all  experience,  and  therefore  we  are 
compelled  to  admit  that  a  body  cannot  diminish  or  destroy  any  motion  which 
it  has  received. 

Let  us  inquire  why  we  are  more  disposed  to  admit  the  inability  of  matter  to 
produce  than  to  destroy  motion  in  itself.  We  see  most  of  those  motions  which 
take  place  around  us  on  the  surface  of  the  earth  subject  to  gradual  decay,  and 
if  not  renewed  from  time  to  time,  they  at  length  cease.  A  stone  rolled  along 
the  ground,  a  wheel  revolving  on  its  axis,  the  heaving  of  the  deep  after  a  storm, 
and  all  other  motions  produced  in  bodies  by  external  causes,  decay,  when  the 
exciting  cause  is  suspended  ;  and  if  that  cause  do  not  renew  its  action,  they 
ultimately  cease. 

But  is  there  no  exciting  cause,  on  the  other  hand,  which  thus  gradually  de- 
prives those  bodies  of  their  motion  ? — and  if  that  cause  were  removed,  or  its 
intensity  diminished,  would  not  the  motion  continue,  or  be  more  slowly  re- 
tarded ?  When  a  stone  is  rolled  along  the  ground,  the  inequalities  of  its  shape, 
as  well  as  those  of  the  ground,  are  impediments  which  retard  and  soon  de- 
stroy its  motion.  Render  the  stone  round,  and  the  ground  level,  and  the  mo- 
tion will  be  considerably  prolonged.  But  still  small  asperities  will  remain  on 
the  stone,  and  on  the  surface  over  which  it  rolls  :  substitute  for  it  a  ball  of 
highly  polished  steel,  moving  on  a  highly  polished  steel  plane,  truly  level,  and 
the  motion  will  continue  without  sensible  diminution  for  a  very  long  period 
but  even  here,  and  in  every  instance  of  motions  produced  by  art,  minute  asper- 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


ities  must  exist  on  the  surfaces  which  move  in  contact  with  each  other,  which 
must  resist,  gradually  diminish,  and  ultimately  destroy,  the  motion. 

Independently  of  the  obstructions  to  the  continuation  of  motion  arising  from 
friction,  there  is  another  impediment  to  which  all  motions  on  the  surface  of 
the  earth  are  liable — the  resistance  of  the  air.  How  much  this  may  affect  the 
continuation  of  motion,  appears  by  many  familiar  effects.  On  a  calm  day,  carry 
an  open  umbrella  with  its  concave  side  presented  in  the  direction  in  which  you 
are  moving,  and  a  powerful  resistance  will  be  opposed  to  your  progress,  which 
will  increase  with  every  increase  of  the  speed  with  which  you  move. 

We  are  not,  however,  without  direct  experience  to  prove  that  motions  when 
unresisted  will  for  ever  continue.  In  the  heavens  we  find  an  apparatus,  which 
furnishes  a  sublime  verification  of  this  principle.  There,  removed  from  all 
casual  obstructions  and  resistances,  the  vast  bodies  of  the  universe  roll  on  in 
their  appointed  paths,  with  unerring  regularity,  preserving  without  diminution 
all  that  motion  which  they  received  at  their  creation  from  the  Hand  which 
launched  them  into  space.  This  alone,  unsupported  by  other  reasons,  would 
be  sufficient  to  establish  the  quality  of  inertia  ;  but  viewed  in  connexion  with 
the  other  circumstances  previously  mentioned,  no  doubt  can  remain  that  this  is 
a  universal  law  of  nature. 

Organized  bodies  endued  with  the  living  principle,  seem  to  be  the  only  ex- 
ceptions to  this  law.  But  even  in  these  their  members  and  all  their  compo- 
nent parts,  separately  considered,  are  inert,  and  are  subject  to  the  same  laws 
as  all  other  forms  of  matter.  The  quality  of  animation,  from  which  they  derive 
the  power  of  spontaneous  action  or  voluntary  motion,  does  not  belong  to  the 
parts,  but  to  the  whole,  and  not  to  the  whole  by  any  obvious  or  necessary  con- 
nexion, because  it  is  absent  in  sleep,  and  totally  removed  by  death,  even  while 
the  organization  of  every  part  remains,  to  all  appearance,  without  derangement. 
Seeing,  then,  the  whole  visible  material  universe  partaking  in  the  common 
quality  of  inertia,  unable  to  trace  the  conditions  of  life  to  any  material  phenom- 
ena, it  is  impossible  not  to  conclude  that  the  will  of  animated  beings  is  the  re- 
sult of  an  immaterial  principle,  which,  during  the  period  of  life,  governs  their 
organized  bodies.  In  what  this  principle  consists,  what  is  its  seat,  or  by 
what  modes  of  action  it  moves  the  body,  we  are  wholly  unable  to  decide.  But 
the  same  principle — analogy — which  guides  our  investigations  in  every  other 
part  of  physical  science,  ought  to  govern  us  in  this  ;  and  by  that  principle,  the 
spontaneous  motion  found  in  animated  beings,  but  which  in  no  instance  is  mani- 
fested by  mere  matter,  must  be  attributed,  not  to  the  matter  which  composes 
the  bodily  forms  of  these  beings,  but  to  something  of  altogether  a  different 
nature 

Independently  of  this,  which  may  be  considered  as  the  reasoning  proper  to 
physical  science,  philosophers  have  given  another  reason  for  assigning  anima- 
tion to  an  immaterial  principle.  The  will,  from  the  very  nattire  of  its  acts, 
must  belong  to  a  simple,  uncompounded,  and  indivisible  being,  and  conse- 
quently can  never  be  an  attribute  of  a  thing  which  in  its  essence  is  the  very 
reverse  of  this. 

It  has  been  proved  that  an  inability  to  change  the  quantity  of  motion  is  a 
consequence  of  inertia.  The  inability  to  change  the  direction  of  motion  is 
another  consequence  of  this  quality.  The  same  cause  v/hich  increases  or  di- 
minishes motion,  would  also  give  motion  to  a  body  at  rest ;  and  therefore  we 
inferred  that  the  same  inability  which  prevents  a  body  from  moving  itself,  will 
also  prevent  it  from  increasing  or  diminishing  any  motion  which  it  has  re- 
ceived. In  the  same  manner  we  can  show  that  any  cause  which  changes  the 
direction  of  motion  would  also  give  motion  to  a  body  at  rest ;  and  therefore  if  a 
body  change  the  direction  of  its  own  motion,  the  same  body  might  move  itself 


36 


MATTER  AND  ITS  PHYSICAL  PROPERTIES. 


from  a  state  of  rest ;  and  therefore  the  power  of  changing  the  direction  of  any 
motion  which  it  may  have  received  is  inconsistent  with  the  quahty  of  inertia. 
If  a  body  moving  from  A  to  B,  receive  at  B  a  blow  in  the  direction  C  B  E, 


it  will  immediately  change  its  direction  to  that  of  another  line  B  D.  The 
cause  which  produces  this  change  of  direction  would  have  put  the  body  in 
motion  in  the  direction  B  E,  had  it  been  quiescent  at  B  when  it  sustained  the 
blow. 

Again,  suppose  G  H  to  be  a  hard  plane  surface  ;  and  let  the  body  be  sup- 
posed to  be  perfectly  inelastic.  When  it  strikes  the  surface  at  B,  it  will  com- 
mence to  move  along  it  in  the  direction  B  H.  This  change  of  direction  is 
produced  by  the  resistance  of  the  surface.  If  the  body,  instead  of  meeting  the 
surface  in  the  direction  A  B,  had  moved  in  the  direction  E  B,  perpendicular  to 
it,  all  motion  would  have  been  destroyed,  and  the  body  reduced  to  a  state  of 
rest. 

By  the  former  example  it  appears  that  the  deflecting  cause  would  have  put 
a  quiescent  body  in  motion,  and  by  the  latter  it  would  have  reduced  a  moving 
body  to  a  state  of  rest.  Hence  the  phenomenon  of  a  change  of  direction  is  to 
be  referred  to  the  same  class  as  the  change  from  rest  to  motion,  or  from  motion 
to  rest.  The  quality  of  inertia  is,  therefore,  inconsistent  with  any  change  in 
the  direction  of  motion  which  does  not  arise  from  an  external  cause. 

From  all  that  has  been  here  stated,  we  may  infer  generally,  that  an  inani- 
mate parcel  of  matter  is  incapable  of  changing  its  state  of  rest  or  motion  ;  that, 
in  whatever  state  it  be,  in  that  state  it  must  for  ever  persevere,  unless  disturbed 
by  some  external  cause  ;  that  if  it  be  in  motion,  that  motion  must  always  be 
uniform,  or  must  proceed  at  the  same  rate,  the  equal  spaces  being  moved  over 
in  the  same  time  ;  any  increase  of  its  rate  must  betray  some  impelling  cause, 
any  diminution  must  proceed  from  an  impeding  cause,  and  neither  of  these 
causes  can  exist  in  the  body  itself;  that  such  motion  must  not  only  be  con- 
stantly of  the  same  uniform  rate,  but  also  must  be  always  in  the  same  direc- 
tion, any  deflection  from  its  course  necessarily  arising  from  some  external 
influence. 

The  language  sometimes  used  to  explain  the  property  of  inertia  in  popular 
works,  is  eminently  calculated  to  mislead  the  student.  The  terms  resistance 
and  stubbornness  to  move  are  faulty  in  this  respect.  Inertia  implies  absolute 
passiveness,  a  perfect  indifference  to  rest  or  motion.  It  implies  as  strongly 
the  absence  of  all  resistance  to  the  reception  of  motion,  as  it  does  the  absence 
of  all  power  to  move  itself.     The  term  vis  inerticB,  ox  force  of  inactivity,  so  fre- 


MATTER,  AND  ITS  PHYSICAL  PROPERTIES. 


37 


quently  used  even  by  authors  pretending  to  scientific  accuracy,  is  still  more 
reprehensible.  It  is  a  contradiction  in  terms  :  the  term  inactivity  imTplying  the 
absence  of  all  force. 

Before  we  close  this  subject,  it  may  be  advantageous  to  point  out  some 
practical  and  familiar  examples  of  the  general  law  of  inertia.  The  student 
must,  however,  recollect  that  the  great  object  of  science  is  generalization,  and 
that  his  mind  is  to  be  elevated  to  the  contemplation  of  the  laws  of  nature,  and 
to  receive  a  habit  the  very  reverse  of  that  which  disposes  us  to  enjoy  the  de- 
scent from  generals  to  particulars.  Instances,  taken  from  the  occurrences  of 
ordinary  life,  may,  however,  be  useful  in  verifying  the  general  law,  and  in  im- 
pressing it  upon  the  memory ;  and,  for  this  reason,  we  shall  occasionally,  in 
the  present  treatise,  refer  to  such  examples  :  always,  however,  keeping  them 
in  subservience  to  the  general  principles  of  which  they  are  manifestations,  and 
on  which  the  attention  of  the  student  should  be  fixed. 

If  a  carriage,  a  horse,  or  a  boat,  moving  with  speed,  be  suddenly  retarded  or 
stopped  by  any  cause  which  does  not  at  the  same  time  affect  passengers,  riders, 
or  any  loose  bodies  which  are  carried,  they  will  be  precipitated  in  the  direc- 
tion of  the  motion  ;  because,  by  reason  of  their  inertia,  they  persevere  in  the 
motion  which  they  share  in  common  with  that  which  transported  them,  and  are 
not  deprived  of  that  motion  by  the  same  cause. 

If  a  passenger  leap  from  a  carriage  in  rapid  motion,  he  will  fall  in  the  di- 
rection in  which  the  carriage  is  moving  at  the  moment  his  feet  meet  the  ground  ; 
because  his  body,  on  quitting  the  vehicle,  retains,  by  its  inertia,  the  motion 
which  it  had  in  common  with  it.  When  he  reaches  the  ground,  this  motion  is 
destroyed  by  the  resistance  of  the  ground  to  the  feet,  but  is  retained  in  the 
upper  and  heavier  part  of  the  body  ;  so  that  the  same  effect  is  produced  as  if 
the  feet  had  been  tripped. 

When  a  carriage  is  once  put  in  motion  with  a  determinate  speed  on  a  level 
road,  the  only  force  necessary  to  sustain  the  motion  is  that  which  is  sufficient 
to  overcome  the  friction  of  the  road  ;  but  at  starting,  a  greater  expenditure  of 
force  is  necessary,  inasmuch  as  not  only  the  friction  is  to  be  overcome,  but  the 
force  with  which  the  vehicle  is  intended  to  move  must  be  communicated  to  it. 
Hence  we  see  that  horses  make  a  much  greater  exertion  at  starting  than 
subsequently,  when  the  carriage  is  in  motion  ;  and  we  may  also  infer  the 
inexpediency  of  attempting  to  start  at  full  speed,  especially  with  heavy  car- 
riages. 

Coursing  owes  all  its  interest  to  the  instinctive  consciousness  of  the  nature 
of  inertia  which  seems  to  govern  the  measures  of  the  hare.  The  greyhound 
is  a  comparatively  heavy  body  moving  at  the  same  or  greater  speed  in  pursuit, 
The  hare  doubles,  that  is,  suddenly  changes  the  direction  of  her  course,  and 
turns  back  at  an  oblique  angle  with  the  direction  in  which  she  had  been  run- 
ning. The  greyhound,  unable  to  resist  the  tendency  of  its  body  to  persevere 
in  the  rapid  motion  it  had  acquired,  is  urged  forward  many  yards  before  it  is 
able  to  check  its  speed  and  return  to  the  pursuit.  Meanwhile  the  hare  is  gain- 
ing ground  in  the  other  direction,  so  that  the  animals  are  at  a  very  considera- 
ble distance  asunder  when  the  pursuit  is  recommenced.  In  this  way,  a  hare, 
though  much  less  fleet  than  a  greyhound,  will  often  escape  it. 

In  racing,  the  horses  shoot  far  beyond  the  winning-post  before  their  course 
can  be  arrested. 

Remarkable  effects  of  the  inertia  of  matter  are  constantly  exhibited  in  the 
accidents  from  collision  which  take  place  on  railways.  In  England,  where 
the  speed  is  much  greater  than  is  customary  in  this  country,  such  instances 
are  more  frequent  and  fatal.  The  evenness  and  perfection  of  the  roads  and 
carriages  conspire  with  the  extraordinary  speed  to   render  it  difficult  to  stop 


ELASTICITY  OF  AIR. 


Exbansting  Syriuge. — Rate  of  Exhaustion. — Impossible  to  produce  a  perfect  Vacaum. —  Mechani- 
cal Defects. — The  Air-Pump. — Barometer  Gauge. — Siphon  Gauge. — Various  Forms  of  Air- 
Pump. — Pump  without  Suction-Valve. — Experiments  with  Air-Purap. — Bladder  burst  by  atmo- 
spheric Pressure. — Bladder  burst  by  Elasticity  of  Air. — Dried  Fruit  inflated  by  fixed  Air. — Flac- 
cid Bladder  swells  by  Expansion. — Water  raised  by  Elastic  Force. — A  Pump  cannot  act  in  the 
Absence  of  atmospheric  Pressure. — Suction  ceases  when  this  Pressure  is  removed. — The  Magde- 
burg Hemisphere. — Guinea  and  Feather  Experiment. — Cupping. — Effervescing  Liquors. — 
Sparkling  of  Champagne,  &c. — Presence  of  Air  necessary  for  the  Transmission  of  Sound. — The 
condensing  Syringe. — The  Condenser. 


ELASTICITY  OF  AIR. 


41 


ELASTICITY  OE  AIR. 


When  a  part  of  the  air  enclosed  in  any  vessel  is  withdrawn,  that  which  re- 
mains expanding  by  its  elastic  property,  fills  the  dimensions  of  the  vessel  as 
effectually  as  before.  Under  these  circumstances,  however,  it  is  obvious  that 
any  given  space  within  the  vessel  contains  a  less  quantity  of  air  than  it  did 
previously,  inasmuch  as  while  the  whole  dimensions  of  a  vessel  remain  the 
same,  the  total  quantity  of  air  diffused  through  them  is  diminished.  When  the 
same  quantity  of  air  in  this  manner  is  caused  to  expand  into  a  greater  space,  it 
is  said  to  be  rarefied. 

But  on  the  other  hand,  when  a  vessel  containing  any  quantity  of  air  is  caused 
to  receive  an  increased  quantity  by  additional  air  being  forced  into  it,  then  any 
given  portion  of  its  dimensions  will  contain  a  proportionally  greater  quantity 
of  air  than  it  did  before  the  additional  air  had  been  forced  in.  Under  these 
circumstances,  the  air  contained  in  the  vessel  is  said  to  be  condensed,  and  it  is 
our  purpose  in  the  present  lecture  to  describe  the  mechanical  instruments  by 
which  these  processes  of  rarefaction  and  condensation  are  practically  effected. 


THE    EXHAUSTING    SYRINGE. 

The  most  simple  form  of  instrument  for  producing  the  rarefaction  of  air,  is 
that  which  is  called  the  exhausting  syringe.  In  order  to  comprehend  the  con- 
struction and  operation  of  this  instrument,  let  us  suppose  A,  B,  fig.  1,  a  cylin- 
der, or  barrel,  furnished  with  a  stop-cock  C,  inserted  in  a  small  aperture  in  the 
bottom.  Let  the  end  of  this  tube  be  screwed  upon  the  vessel  R,  in  which  the 
rarefaction  is  to  be  made. 

From  the  side  of  the  barrel  near  the  bottoan,  let  another  tube,  D,  proceed, 
also  furnished  with  a  stop-cock.  Let  us  suppose  the  piston  P,  at  the  bottom 
of  the  barrel,  both  stop-cocks  being  closed.  Let  the  piston  P  be  now  drawn 
from  the  bottom  to  the  top,  as  represented  in  fig.  2,  this  piston  being  supposed 
to  move  air-tight  in  the  barrel.     A  vacuum  will  remain  between  the  piston  P 


and  the  bottom  B.  If  the  stop-cock  C  be  opened,  the  air  contained  in  the 
vessel  R,  will,  by  its  elastic  force,  rush  through  the  open  stop-cock  C,  and 
expand  so  as  to  fill  the  barrel.  Thus  the  air  which  previously  occupied  the 
dimensions  of  the  vessel  R,  has  now  expanded  through  the  dimensions  of  R 
and  A,  B.  Let  the  stop-cock  C,  be  now  closed,  and  the  stop-cock  D  opened, 
and  let  the  piston  P  be  pressed  to  the  bottom  of  the  barrel.  The  air  contained 
in  the  barrel  will  thus  be  forced  out  at  the  open  stop-cock  D,  and  driven  into 
external  atmosphere.  Let  the  stop-cock  D  be  next  closed,  and  the  piston 
again  elevated,  as  in  fig.  2.  A  vacuum  will  once  more  be  produced  in  the 
barrel,  and  on  opening  the  stop-cock  C,  the  air  in  R  will  again  expand  into 
the  barrel,  occupying  the  extended  dimensions  as  before.  Let  the  stop-cock 
C  be  again  closed,  and  the  stop-cock  D  opened.  If  the  piston  be  pressed 
to  the  bottom  of  the  barrel  as  before,  the  air  contained  in  the  cylinder  will 
again  be  expelled  through  the  stop-cock  D.  By  continuing  this  process, 
alternately  opening  and  closing  the  two  stop-cocks,  and  elevating  and  de- 
pressing the  piston,  a  quantity  or  air  will  rush  from  the  vessel  R,  on  each 
ascent  of  the  piston,  and  the  same  quantity  will  be  expelled  through  the  tube 
D,  on  each  descent  of  the  piston. 

It  is  evident  that  this  process  may  be  continued  so  long  as  the  air  which  re- 
mains in  R,  is  capable  of  expanding,  by  its  elasticity,  through  the  open  tube, 
C,  into  the  barrel  above. 

A  slight  degree  of  attention  only  is  necessary  to  perceive  that  the  quantity 
of  air  expelled  from  R,  at  each  ascent  of  the  piston,  is  continually  diminished  ; 
and  it  will  not  be  difficult  even  to  explain  the  exact  rate  at  which  this  diminu- 
tion proceeds.  Let  us  suppose  the  magnitude  of  the  barrel  A,  B,  to  have  any 
given  proportion  to  the  dimensions  of  the  vessel  R  ;  suppose,  for  example,  that 
the  dimensions  of  the  barrel  are  the  ninth  part  of  those  of  the  vessel..  When  the 
piston  is  first  raised  from  the  bottom  to  the  top,  the  air  which  previously  occu- 
pied the  vessel  expands  so  as  to  occupy  the  dimensions  of  the  vessel  and  bar- 
rel together.  The  barrel,  therefore,  will  contain  a  tenth  part  of  the  whole  of 
the  enclosed  air ;  for,  since  the  vessel  R  contains  nine  times  as  much  as  the 
barrel,  the  vessel  and  barrel  together  contain  ten  times  as  much  as  the  barrel. 
Consequently,  the  air  enclosed  in  the  barrel  will  necessarily  be  a  tenth  of  the 


ELASTICITY  OF  AIR. 


43 


whole.  On  depressing  the  piston,  this  tenth  part  is  expelled  through  the  tube 
D.  On  elevating  the  piston,  the  air  remaining  in  the  vessel  R,  which  is  nine 
tenths  of  the  original  quantity,  now  expands  through  the  vessel  and  barrel,  and, 
for  the  reason  already  assigned,  the  barrel  will  contain  a  tenth  part  of  this  re- 
maining nine  tenths  ;  that  is,  it  will  contain  nine  hundredth  parts  of  the  original 
quantity.  On  the  second  descent  of  the  piston,  this  nine  hundredth  parts  will 
be  expelled.  The  nine  tenths  which  remain  in  the  cylinder  after  the  first 
stroke  of  the  piston,  have  now  lost  nine  hundredth  parts  of  the  whole,  and  since 
nine  tenths  are  the  same  as  ninety  hundredths,  nine  hundredths  being  deducted 
from  that  leave  a  remainder  of  eighty-one  hundredths. 

This,  therefore,  is  the  proportion  of  the  original  quantity  which  now  remains 
in  the  vessel  R.  When  the  piston  is  next  raised,  this  portion  will  expand  as 
before  into  the  enlarged  space,  and  the  tenth  part  of  it  will  rise  into  the  barrel. 
But  a  tenth  part  of  eighty-one  hundredths  is  eighty-one  thousandths.  Accord- 
ingly, on  the  next  descent,  this  eighty-one  thousandths  will  be  expelled.  The 
eighty-one  hundredths  which  remain  in  the  vessel  R  before  this  diminution, 
are  thus  diminished  by  eighty-one  thousandths.  This  eighty-one  hundredths 
are  equivalent  to  eight  hundred  and  ten  thousandths,  and  therefore  the  quantity 
remaining  in  the  vessel  R,  will  be  found  by  subtracting  eighthy-one  thousandths 
from  eight  hundred  and  ten  thousandths. 

The  remainder  will  therefore  be  seven  hundred  and  twenty-nine  thousandths, 
wliich  will  be  the  proportion  of  the  original  quantity  of  air  which  remains  in  the 
vessel  after  the  third  stroke  of  the  piston.  It  will  not  be  difficult  to  continue 
this  reasoning  further,  and  to  discover,  not  only  the  quantity  of  air  expelled  at 
each  successive  stroke,  but  also  the  quantity  remaining  in  the  vessel  R ;  and 
we  may  without  difficulty  compute  the  following  table : — 


o     . 

Proportion  of  the  original 
quantity  of  air  expelled 
at  each  stroke. 

Proportion  of  the  original  quan- 
tity of  air  remaining  after  each 
stroke. 

Total  quantity  of  air 
expelled. 

1 

1 
10 

9_ 

10 

1 
10 

2 

9 

100 

81 
100 

19 
100 

3 

81 
1,000 

729 
1,000 

271 
1,000 

4 

729 
10,000 

6,491 
10,000 

3.509 
10,000 

5 

6.491 
100,000 

58.419 
100,000 

41.581 
100,000 

6 

.58,419 
1,000,000 

.525,771 
1,000,000 

474,229 
1,000,000 

7 

52,771 
10,000,000 

4,731,939 
10,000,000 

5,268.061 
10,000,000 

To  make  this  table  more  intelligible,  let  us  suppose  that  the  vessel,  R,  con- 
tains in  the  first  instance,  ten  million  grains  of  air.  The  first  stroke  of  the 
piston  expels  a  tenth  part  of  this  quantity,  that  is,  one  million  grains.  There 
remain  in  the  vessel,  R,  nine  million  grains.  The  tenth  part  of  this  nine 
million  is  expelled  by  the  second  stroke,  that  is  nine  hundred  thousand  grains 


44 


ELASTICITY  OF  AIR. 


of  air.  There  now  remain  in  the  vessel  eight  million,  one  hundred  thousand 
grains.  Of  this  again  a  tenth  part  is  expelled  by  the  third  stroke,  that  is,  eight 
hundred  and  ten  thousand  grains.  The  quantity  remaining  in  the  receiver  will 
then  be  seven  million,  two  hundred  and  ninety  thousand  grains.  The  tenth 
part  of  this  is  expelled  by  the  fourth  stroke,  that  is,  seven  hundred  and  twenty- 
nine  thousand  grains,  and  there  remain  in  the  vessel  six  million,  four  hundred 
and  ninety-one  thousand  grains.  The  fifth  stroke  expels  a  tenth  part  of  this, 
or  six  hundred  forty-nine  thousand,  one  hundred  grains,  and  there  then  remain 
in  the  vessel  five  million,  eight  hundred  forty-one  thousand,  nine  hundred  grains. 
A  tenth  part  of  this  again  is  expelled  by  the  sixth  stroke,  that  is,  five  hundred 
eighty-four  thousand,  one  hundred  and  ninety  grains,  and  the  remainder  in  the 
vessel  is  five  million,  two  hundred  and  fifty-seven  thousand,  seven  hundred  and 
ten  grains.  A  tenth  of  this  again,  or  five  hundred  twenty-five  thousand,  seven 
hundred  and  seventy-one  grains,  is  expelled  by  the  seventh  stroke.  The  fol- 
lowing table  exhibits  these  results  : — 


Grains  expelled  at  each 
Stroke. 

Grains  remaining  under 
Pressure.           • 

Total  number  of  grains 
Expelled. 

1 

1,000,000 

9,000,000 

1,000,000 

2 

900,000 

8,100,000 

1,900,000 

3 

810,000 

7,290,000 

2,710,000 

4 

729,000 

6,491,000 

3,439,000 

5 

649,100 

5,841,900 

4,158,100 

6 

584,190                '                5,257,710 

4,742,290 

7 

525,771 

4,731,939 

5,268,061 

By  attending  to  the  numbers  in  the  third  column  of  the  above  table,  it  will 
be  perceived  that  each  succeeding  number  is  nine  tenths  of  the  preceding  one. 
It  follows,  therefore,  that  after  each  stroke  of  the  piston,  the  quantity  of  air 
which  remains  in  the  vessel  R,  will  be  nine  tenths  of  the  quantity  which  it 
contained  before  the  stroke.  From  a  due  consideration  of  this  circumstance  it 
■will  be  perceived  that,  however  long  the  process  of  rarefaction  be  continued, 
the  vessel  R,  can  never  be  completely  exhausted  of  air,  for  a  determinate 
quantity  being  contained  in  it,  nine  tenths  of  this  will  remain  after  the  first 
stroke.  After  the  second  stroke,  nine  tenths  of  this  again  will  remain,  and 
however  long  the  operation  be  continued,  still  a  determinate  quantity  will  re- 
main after  every  succeeding  stroke  of  the  piston,  this  quantity  being  nine  tenths 
of  what  the  vessel  R  contained  after  the  preceding  stroke.  But,  although  a 
perfect  exhaustion  can  never  be  attained  by  these  means,  yet  if  the  instrument 
now  described  could  be  constructed  as  perfect  in  practice  as  it  is  in  theory, 
there  would  be  no  limit  whatever  to  the  degree  to  which  the  air  in  the  vessel 
R  might  be  rarefied.  Thus,  by  a  determinate  and  finite  number  of  descents 
of  the  piston,  it  might  be  reduced  in  weight  to  the  millionth  part  of  a  grain,  or 
even  to  a  quantity  millions  of  times  less  than  this.  Still,  however  small  the 
quantity  which  may  remain  in  the  vessel  R,  so  long  as  the  elastic  force  by 
which  the  particles  repel  each  other  exceeds  the  weight  of  the  final  or  ultimate 
particles  of  the  air,  so  long  that  repulsive  energy  will  cause  it  to  expand 
through  the  tube  C,  into  the  cylinder.  A,  B. 


The  exhausting  syringe  used  in  practice  differs  in  some  particulars  frort^s 
that  which  we  have  here  described  with  a  view  to  illustrate  the  principle  of  its 
operation.  The  stop-cocks  C  and  D,  which  would  require  constant  manipula- 
tion while  the  process  of  rarefaction  is  going  forward,  are  dispensed  with  in 
practice,  and  the  elastic  pressure  of  the  air  itself  is  made  to  act  upon  valves 
which  serve  the  purposes  of  these  cocks.  Let  A,  B,  fig.  3,  represent  an  ex- 
hausting syringe,  having  a  tube  and  stop-cock,  C,  proceeding  from  the  lower 
part,  as  already  described.  The  tube  C,  is  screwed  to  a  very  small  aperture 
in  the  bottom  of  the  barrel.  Across  this  aperture  is  stretched  a  small  piece  of 
oiled  silk,  which  is  impervious  to  air.  It  is  extended  across  the  aperture  so 
loosely,  that  a  slight  pressure  from  below  will  produce  an  open  space  between 
it  and  the  surface  of  the  bottom  near  the  aperture  capable  of  admitting  air  from 
below,  and  yet  so  tight,  that  a  pressure  from  above  will  cause  it  to  lie  close 
against  the  bottom  round  the  aperture,  so  as  to  stop  the  passage  of  air  from 
above. 

By  this  arrangement  it  is  possible  for  air  pressed  with  a  sufficient  force  to, 
enter  the  barrel  through  the  valve  V,  when  the  stop-cock  C  is  opened  ;  but  it 
is  impossible,  on  the  other  hand,  for  air  pressing  above  the  valve  to  escape 
through  it,  since  the  pressure  of  the  air  only  serves  to  render  more  close  the 
contact  between  the  valve  and  the  surface  surrounding  the  aperture  which  it 
covers.  A  small  hole  is  pierced  through  the  piston,  extending  from  the  lower 
to  the  upper  surface,  and  this  hole  at  the  upper  surface  is  covered  with  an 
oiled  silk  valve  V,  in  the  same  manner  as  the  aperture  V,  in  the  bottom.  For 
the  reasons  already  assigned,  it  is,  therefore,  possible  for  air  to  pass  up  through 
this  hole  in  the  piston,  and  escape  at  the  upper  surface  ;  but  it  is  impossible 
for  air,  by  any  pressure,  to  pass  in  the  contrary  direction,  since  such  pressure 
only  renders  the  contact  of  the  valve  more  intimate,  and  consequently  causes 
it  to  be  more  impervious  to  air. 

Let  us  suppose  an  instrument  thus  constructed  to  be  attached  to  a  vessel, 
R,  in  which  the  rarefaction  is  to  be  produced,  and  the  stop-cock  C  to  be 
opened.  On  raising  the  piston  P,  a  vacuum  will  be  produced  between  it  and 
the  valve  V.     The  piston-valve  V  will  now  be  pressed  downward  by  the 


46 


ELASTICITY  OF  AIR. 


S  weight  of  the  atmosphere,  and  will  be  subject  to  no  pressure  from  below,  be- 
(  cause  of  the  absence  of  air  beneath  it.  It  will  then  stop  the  admission  of  air 
)  from  above  the  aperture,  and  will  maintain  the  vacuum  below.  The  elastic 
}  force  of  the  air  contained  in  the  vessel  R,  now  acting  upward  against  the  ex- 
S  hausting  valve  V,  will  raise  it,  and  the  air  will  escape  through  the  space  be- 
?  tween  it  and  the  surface  surrounding  the  aperture,  and  will  thus  fill  the  barrel 
)  above ;  but  the  air  having  expanded  into  an  increased  space  will  have  an 
(  elastic  force  less  than  that  of  the  external  air,  and  consequently  the  piston-valve 
)  Y'  will  be  pressed  down  by  a  greater  force  than  it  is  pressed  up,  and  will 
(  therefore  remain  closed.  Let  the  piston  be  now  depressed  ;  as  it  descends, 
)  the  air  enclosed  in  the  cylinder  acquires  increased  elastic  force,  and  pressing 
(  upon  the  exhausting-valve  V,  causes  it  to  close,  so  as  to  intercept  the  air  in 
)  the  cylinder  from  the  vessel  R.     When  the  piston  has  descended  in  the  barrel 

<  through  such  a  space  as  to  condense  the  air  beneath  it,  so  as  to  give  it  a  greater 

;  elastic  force  than  the-  external  atmosphere,  it  will  press  the  piston-valve  V  ( 
I  upward  with  a  greater  force  than  the  external  air  presses  it  downward.     Con- 
)  sequently  the  valve  V  will  be  opened,  and  the  air  confined  beneath  the  piston 
(  will  begin  to  escape  through  it.     When  the  piston  has  arrived  at  the  bottom  of 
(  the  barrel,  the  whole  of  the  air  will  thus  be  expelled.     This  process   is  re- 

<  peated  whenever  the  piston  is  raised  and  depressed,  and  thus  the  valves,  which 
/  in  the  form  adapted  for  explanation,  required  constant  manipulation,  acquire  a 
i  self-acting  property.  This  form  of  the  instrument,  which  is  that  commonly 
/  jjsed,  is  attended  with  an  obvious  limit  to  its  operation,  which  does  not  exist  in 
s  the  theoretical  form  represented  in  fig.  1.  It  is  evident  that  the  operation  of 
)  the  valves  depends  upon  the  presence  of  air  of  a  certain  detenninate  elastic 
)  force,  in  the  vessel  R,  which  elastic  force  it  is  the  purpose  of  the  instrument 
(  to  reduce  indefinitely.  When  the  elastic  force  of  the  air  contained  in  R,  is  so 
s  far  diminished  that  it  is  only  equal  to  the  force  required  to  raise  the  valve  V, 
)  the  action  of  the  machine  must  stop,  for  any  further  diminution  would  render 
i  the  air  confined  in  R  unable  to  open  the  valve,  and  therefore  no  more  air  covdd 
/  pass  into  the  barrel  A,  B.  This  is  a  practical  limit  of  the  power  of  the  ex- 
)  hausting  syringe.  The  degree  of  perfection  of  which  the  instrument  is  sus- 
)  ceptible,  therefore,  depends  upon  making  the  valve  V,  offer  as  little  resistance 
)  to  being  raised  as  is  consistent  with  its  being  perfectly  air-tight  when  closed. 

)       Bui  we  have  another  limit  to  the  operation  of  this  instrument,  arising  from 
S  the  piston-valve  V.     This  valve  is  closed,  not  only  by  its  own  tension,  but 
?  also  by  the  weight  of  the  incumbent  atmosphere  above  it.     When  the  piston  is 
{  depressed,  the  air  included  in  the  barrel  must  first  attain  a  degree  of  elastic 
?  force  by  condensation  equal  to  the  pressure  of  the  atmosphere,  before  it  can 
open  the  valve  v.     But  this  is  not  sufficient :  it  must  acquire  a  further  in- 
creased elastic  force  equal  to  the  tension  of  the  valve  V,  over  the  aperture,  in 
order  to  raise  that  valve  and  escape,  and  therefore  the  perfection  of  this  valve 
also  depends  on  having  as  little  tension  as  is  consistent  with  being  perfectly 
air-tight  from  above. 

The  efficiency  of  the  instrument  will  also  depend  upon  the  accuracy  with 
which  the  piston  fits  the  bottom  and  sides  of  the  barrel.  When  the  piston  is 
depressed  to  the  bottom,  it  is  considered  in  theory  to  be  in  absolute  contact,  so 
as  to  exclude  every  particle  of  air  from  the  space  between  it  and  the  bottom. 
But  in  practice,  this  perfection  can  never  be  obtained.  It  may,  however,  be 
very  accurately  fitted,  and  the  air  retained  between  it  and  the  bottom  may  be 
reduced  almost  without  limit.  The  small  hole  which  passes  from  the  valve 
W  to  the  bottom  of  the  piston,  will  still  remain,  however,  and  will  continue  to 
be  a  receptacle  for  air,  even  when  the  piston  is  in  close  contact  with  the  bot- 
therefore,  produces   a  defect  in  the  machine  which  is  not 


spa( 


ELASTICITY  OF  AIR. 


47 


removed.  If  we  suppose  the  magnitude  of  this  hole,  together  with  whatever 
space  may  remain  unfilled  between  the  lower  surface  of  the  piston  and  the 
bottom  of  the  barrel,  to  be  the  ten  thousandth  part  of  a  solid  inch,  then  the 
valve  V  will  cease  to  act  when  the  air  which  fills  the  barrel,  the  piston  being 
at  the  top,  is  such  that  if  condensed  into  the  ten  thousandth  part  of  an  inch,  its 
elastic  force  will  exceed  the  atmospheric  pressure  by  a  quantity  less  than  the 
force  required  to  open  the  valve  V.  This  source  of  imperfection  will  evidently 
be  diminished  by  diminishing  the  depth  of  the  aperture  below  the  valve  V,  and 
by  increasing  the  size  of  the  cylinder ;  for  if  the  air  in  the  Jbarrel  be  as  many 
times  rarer  than  the  external  atmosphere,  as  the  magnitude  of  the  barrel  is 
greater  than  the  magnitude  of  the  space  below  the  valve  Y',  then  this  air, 
when  condensed  into  that  space,  will  exert  a  pressure  equal  to  that  of  the  at- 
mosphere. Suppose  the  barrel  contains  ten  cubic  inches  of  air,  and  that  the 
magnitude  of  the  hole  is  the  hundredth  part  of  a  cubic  inch,  then  the  magnitude 
of  the  cylinder  will  be  one  thousand  times  the  magnitude  of  the  space  which 
remains  between  the  valve  V^  and  the  bottom  of  the  barrel,  when  the  piston  is 
pressed  to  the  bottom.  Consequently  the  process  of  rarefaction  would  be  de- 
duced, until  the  air  in  the  receiver  would  be  rendered  one  thousand  times  rarer 
than  the  external  atmosphere. 

The  vessel  R,  being  connected  with  a  tube  furnished  with  a  stop-cock  C, 
may  be  detached  from  the  syringe,  together  with  the  stop-cock,  by  unscrewing 
the  tube  C  ;  and  if  the  stop-cock  be  previously  closed,  the  interior  of  the  vessel 
will  continue  to  contain  the  rarefied  air. 

In  various  branches  of  physical  science,  inquiries  continually  arise,  re- 
specting qualities  and  effects  of  material  substances,  which  are  subject  to  con- 
siderable modification  by  the  pressure  or  other  qualities  of  the  air  which  sur- 
rounds them ;  and  it  is  often  necessary  in  such  investigations  to  discover  what 
these  qualities  and  effects  may  be,  if  the  substances  were  not  exposed  to  the 
mechanical  pressure  or  other  effects  consequent  upon  the  presence  of  the  at- 
mosphere. Although  we  do  not  possess  any  means  of  removing  altogether  the 
presence  of  this  fluid,  yet,  from  what  has  been  already  stated,  it  is  plain  that 
it  may  be  so  attenuated  in  an  enclosed  chamber,  such  as  the  vessel  R,  that 
these  effects  may  be  diminished  in  intensity  to  any  degree  which  experimental 
inquiry  may  demand. 

With  these  views  it  is  necessary,  however,  not  only  to  be  able  to  introduce  the 
substances  which  are  submitted  to  experimental  investigation,  into  the  chamber 
in  which  the  rarefaction  has  been  accomplished,  but  also  to  be  able  to  observe 
them  when  so  situated.  The  latter  purpose  could  be  accomplished  by  construct- 
ing the  receptacle  R,  of  glass  ;  but  still  it  would  be  necessary  to  have  access  to 
the  interior,  and  to  construct  it  of  a  convenient  form  to  receive  the  subjects  of  ex- 
periment, and  even  in  many  cases  to  be  able  to  manipulate  or  produce  changes 
of  position  on  the  object  thus  enclosed. 

For  these  purposes,  the  form  of  the  vessel  R,  and  the  mode  of  connecting  it 
with  the  syringe,  must  be  somewhat  changed,  and  the  arrangement  which  is 
given,  in  order  to  adapt  them  thus  to  all  the  exigencies  of  experimental  inves- 
tigation, is  called  the  air-pump,  an  instrument  which  we  will  now  proceed  to 
explain. 

THE    AIR-PUMP. 


L 


The  vessel  in  which  the  rarefaction  is  produced  by  an  air-pump,  is  called  a 
receiver,  and  is  usually  constructed  of  glass  in  a  cylindrical  form,  with  an 
arched  or  round  top,  furnished  with  a  ball  as  a  convenient  handle.  A  section, 
R,  of  this  is  represented  in  fig.  4.     The  mouth,  or  lower  part,  is  open,  and  it 


ELASTICITY  OF  AIR. 


Fig.  4. 


n 


J 


is  ground  to  a  perfectly  smootli  and  flat  edge.  A  circular  brass  plate  is  con- 
structed, also  ground  truly  plane,  and  perfectly  smooth,  and  its  magnitude  is 
accommodated  to  the  size  of  the  largest  receiver  intended  to  be  used ;  a  sec- 
tion of  this  plate  is  represented  at  S,  S. 

When  the  receiver  is  placed  on  the  plate  with  its  mouth  downward,  the  edge 
of  the  mouth  and  the  surface  of  the  plate  should  be  so  truly  plane  and  smooth 
that  they  may  rest  in  air-tight  contact.  This  may  always  be  insured  by 
smearing  the  ground  edge  of  the  receiver  with  a  little  lard  or  unctuous  matter. 
When  the  receiver  is  thus  laid  on  the  plate,  it  becomes  an  enclosed  chamber, 
similar  to  R,  fig.  3,  but  with  this  convenience,  that  any  substance  or  object  to 
be  submitted  to  experiment  may  be  previously  placed  under  it,  and  observed 
through  it  after  the  air  has  been  rarefied.  In  the  centre  of  the  plate  S,  S,  a 
small  aperture  O,  communicates  with  a  tube  T,  analogous  to  the  tube  inserted 
in  the  bottom  of  the  syringe  in  fig.  3.  This  tube  is  furnished  with  a  stop-cock 
at  C,  which,  when  closed,  cuts  off  all  communication  between  the  receiver  and 
the  syringe,  and  when  open  allows  the  syringe  to  act  on  the  receiver  as  al- 
ready described. 

The  syringe  B,  furnished  with  a  piston  P,  is  fixed  on  a  firm  stand,  and  the 
tube  T,  is  carried  in  such  a  direction  as  to  open  a  communication  with  the 
valve  V,  in  the  bottom  of  the  syringe.  To -facilitate  the  operation,  it  is  usual 
to  raise  and  depress  the  piston,  not  by  the  hand  applied  at  the  extremity  of  the 
piston-rod,  as  formerly  described,  but  by  a  winch,  D,  which  turns  a  toothed 
wheel,  W,  working  in  corresponding  teeth,  formed  on  the  edged  of  the  piston- 
rod  E. 

It  is  not  necessary  again  to  describe  the  operation  of  the  syringe,  since  it  is 
exactly  what  has  been  already  explained  with  reference  to  fig.  3.  The  piston 
P,  is  elevated  and  depressed  by  alternately  turning  the  wheel  W,  in  opposite 
directions,  and  the  piston-valve  V,  and  the  exhausting-valve  V,  have  the  prop- 
erty and  work  in  the  manner  already  described.  This  instrument,  and  that 
represented  in  fig.  3,  diflfcr  in  nothing  except  the  length  and  shape  of  the  com- 
municating-tube T,  the  shape  of  the  receiver  R,  and  the  mechanical  method 
of  working  the  piston. 

To  expedite  the  process  of  rarefaction,  it  is  usual  to  provide  two  syringes 
worked  by  the  same  wheel,  as  represented  in  the  figure,  each  being  drawn  up 
while  the  other  is  depressed.  By  these  means  a  given  degree  of  rarefaction 
is  produced  in  half  the  time  which  would  be  required  with  a  single  syringe. 


ELASTICITY  OP  AIR. 


49 


In  using  this  instrument,  it  is  always  desirable  and  frequently  necessary  to 
ascertain  the  degree  of  rarefaction  which  has  been  accomplished  within  the 
receiver.  This  is  indicated  with  great  precision,  by  an  apparatus  called  a 
barometric-gauge,  represented  at  H,  G.  This  consists  of  a  glass  tube  H,  G, 
the  upper  end,  H,  of  which  has  free  communication  with  the  receiver,  or  rather 
with  the  tube  T,  at  some  point  above  the  stop-cock  C.  The  tube  H,  G,  is 
more  than  thirty  inches  in  length,  and  its  lower  extremity  is  plunged  into  a 
small  cistern  of  mercury.  As  the  rarefaction  proceeds  in  the  receiver,  the 
elastic  force  of  the  air  pressing  upon  the  mercury  in  the  tube  H,  G,  is  dimin- 
ished, and  immediately  becomes  less  than  the  pressure  of  the  external  atmo- 
sphere on  the  surface  of  the  mercury  in  the  cistern  M ;  consequently  this  ex- 
ternal pressure  prevails,  and  forces  mercury  up  to  a  certain  height  in  the  tube 
H,  G.  As  the  rarefaction  of  the  air  in  the  receiver  increases,  its  elastic  force 
being  diminished,  the  atmospheric  pressure  will  prevail  with  increased  effect, 
and  will  cause  the  column  sustained  in  the  tube  to  rise.  The  weight  of  this 
column,  combined  with  the  elastic  pressure  of  the  air  remaining  in  the  re- 
ceiver, is  equal  to  the  atmospheric  pressure,  because  they  are  balanced  by  it, 
and  it  is  therefore  apparent  that  the  elastic  pressure  of  the  air  in  the  receiver 
must  be  equal  to  the  excess  of  the  atmospheric  pressure  above  the  weight  of 
the  mercurial  column  in  the  tube.  Let  us  suppose  that  the  common  barometer 
stands  at  thirty  inches,  and  that  the  column  in  the  gauge  measures  twenty-seven 
inches,  the  difference  between  these,  namely,  three  inches  of  mercury,  will 
express  the  elastic  force  of  the  rarefied  air  in  the  receiver,  for  the  column  of 
thirty  inches  in  the  barometer  measures  the  atmospheric  pressure,  and  the 
column  of  twenty-seven  inches  in  the  gauge  must  be  added  to  the  pressure  of 
the  rarefied  air,  in  order  to  obtain  the  force  which  balances  this  pressure ; 
therefore  the  force  of  the  rarefied  air  must  be  equivalent  to  the  pressure  of 
three  inches,  by  which  the  barometric  column  exceeds  the  mercurial  column 
suspended  in  the  gauge. 

In  small  pumps,  which  are  used  on  the  table,  gauges  of  this  form  are  re- 
jected in  consequence  of  their  inconvenient  dimensions.  An  instrument  called 
a  siphon-gauge  is  then  used,  the  principle  of  which  is  easily  understood.     A 

Fiff.  5. 


^  ft 


<jr>^r 


small  glass  tube,  of  eight  or  ten  inches  in  length,  is  bent  into  the  form  A,  B, 
C,  D,  represented  in  fig.  5.  The  extremity  A,  is  closed,  and  the  extremity  D, 
opened,  and  furnished  with  a  screw,  by  which  it  may  be  attached  to  a  tube 
connected  with  the  tube  T,  fig.  4,  above  the  stop-cock  C.  Pure  mercury  is 
poured  into  the  tube  A,  B,  C,  D,  fig.  5,  until  the  leg  A,  B,  is  completely  filled,  and 
the  mercury  rises  to  S,  about  half  an  inch  above  the  inflection  B.  The  pres- 
sure of  the  atmosphere  communicating  freely  with  the  surface  S,  through  D, 
C,  will  maintain  the  mercury  in  the  space  S,  B,  A,  and  will  prevent  the  sur- 
face S,  from  rising  toward  C,  by  the  pressure  of  the  column  B,  A.     When  D 

VOL.  II.— 4 


50 


ELASTICITY  OF  AIR. 


is  screwed  to  the  pump,  and  put  in  communication  with  the  exhausting-tube  T, 
fig.  4,  above  the  stop-cock  C,  then  the  surface  S,  will  be  pressed  by  the  elastic 
force  of  the  air  in  the  receiver  R,  with  which  it  communicates.  So  long  as 
that  elastic  force  is  capable  of  sustaining  the  column  of  mercury  in  the  leg  B, 
above  the  level  of  the  surface  S,  this  instrument  will  give  no  indication  of  the 
degree  of  rarefaction  ;  but  when  by  the  operation  of  the  syringe,  the  air  in  the 
receiver  is  so  far  exhausted  that  its  elastic  force  is  unable  to  sustain  the  mer- 
curial column  in  B,  A,  above  the  level  S,  then  the  mercury  will  begin  to  fall 
in  the  leg  B,  A,  and  the  surface  S  will  rise  in  the  leg  B,  C.  The  column 
suspended  in  the  leg  B,  A,  above  the  level  S,  will  now  be  the  exact  measure 
of  the  elastic  force  of  the  air  in  the  receiver  which  sustains  it.  In  this  respect 
the  siphon-gauge  must  be  regarded  as  a  more  direct  measure  of  the  elastic 
force  of  the  air  in  the  receiver  than  the  barometer-gauge.  The  latter,  in  fact, 
measures  not  the  elastic  force  of  the  air  in  the  receiver,  but  the  difference  be- 
tween that  elastic  force  and  the  pressure  of  the  atmosphere. 

To  obtain  the  elastic  force  of  the  air  in  the  receiver,  it  is  necessary  also  to 
ascertain  the  indications  of  the  barometer.  The  siphon-gauge,  however,  gives 
at  once  the  pressure  of  the  air  in  the  receiver. 

The  air-pump  has  been  constructed  from  time  to  time  in  a  great  variety  of 
forms,  the  details  of  which  it  would  not  be  proper  to  introduce  into  the  present 
treatise.  The  general  principle  in  all  is  the  same  ;  they  differ  from  each  other 
chiefly  in  the  construction  of  the  piston  and  valves. 

In  the  form  which  has  been  above  described,  the  air  effects  its  escape  from 
the  receiver  at  each  stroke  of  the  piston  by  opening  the  suction-valve  V,  fig.  4. 
Now  in  whatever  way  this  valve  is  constructed,  it  must  require  some  deter- 
minate force  to  raise  it,  and  this  force,  in  the  case  already  described,  is  the 
elastic  force  of  the  rarefied  air  remaining  in  the  receiver.  Thus  the  operation 
of  the  machine  is  accomplished  by  the  presence  in  the  receiver  of  the  very 
agent  which  it  is  the  object  of  the  machine  itself  to  remove,  and  from  tlie  very 
construction  of  the  instrument  it  must  cease  to  act  while  yet  air  of  a  determinate 
pressure  remains  in  the  receiver. 

This  defect  has  been  sometimes   attempted  to  be  removed*  by  causing  the 
suction-valve  to  open,  not  by  the  pressure  of  the  rarefied  air,  but  by  some  me- 
chanical means  acted  upon  by  the  piston.     Such  contrivances,  however,  are 
found  to  be  attended  with  peculiar  inconveniences  which  more  than  outweigh 
their  advantages.     Probably  the  most  simple  and  the  best  contrivance  is  one 
in  which  the  suction-valve  is  altogether  dispensed  with  and  the  air  passes 
freely  through  the  open  tubes  from  the  receiver  to  the  pump-barrel.     Let  T,  fig. 
6,  be  the  exhausting-tube  which  is   carried  from  the  receiver,  and  enters  the 
pump-barrel  at  a  point  distant  from  the   bottom  of  the  barrel  by  a  space  equal 
to  the  thickness  of  the  piston.     The  piston  P,  is  a  solid  plug  which  moves  air- 
tight in  the  barrel,  and  is  propelled  by  a  polished  cylindrical  rod  which  slides 
in  an  air-tight  collar  C,  in  the  top  of  the  cylinder,  which  in  this  case  is  closed. 
A  valve  is  placed  in  the  top  of  the  cylinder,  which  opens  outward,  and  which 
may  be  constructed  in  the  same   manner  as  the  silk-valves  already  described.  ) 
when  the  piston  descends   it  leaves  a  vacuum  above  it — the  external  air  not  i 
being  allowed  admission  through  the  valve  at  the  top  ;  and  when  the  piston  \ 
arrives  at  the  bottom  of  the  barrel,  it  has  passed  the  mouth  of  the  exhausting-  < 
tube  T,  and  fills  the  space  below  it.     The  air  in  the  receiver  then  expands  / 
into  the  empty  pump-barrel,  and  when  the  piston  is  raised,  having  passed  the  ^ 
mouth  of  the  tube  T,  the  air  which  has  expanded  into  the  barrel  is   confined 
between  the  piston  and  the  top,  where,  as  the   piston  rises,  it  is   condensed. 
When  in  acquires  sufficient  elastic  force  it  opens  the  valve  at  the  top   and   is 
discharged  into  the  atmosphere. 


Fig.  6. 


The  valve  in  the  top  of  the  barrel  is  in  this  case  continually  under  the  at- 
mospheric pressure,  and  therefore  the  air  confined  in  the  pump  can  never  be 
driven  through  it,  until  it  is  condensed  by  the  piston,  so  that  its  force  shall  be 
greater  than  that  of  the  atmosphere.  From  the  causes  already  explained, 
arising  from  inaccuracy  of  mechanical  construction,  some  small  space  must  in- 
evitably remain  between  the  piston  and  the  top  of  the  barrel,  even  w^hen  the 
piston  is  drawn  upward  as  far  as  possible.  This  small  space  will  contain  con- 
densed air,  and  the  valve  at  C  will  cease  to  act  when  the  air  which  occupies 
this  space  exceeds  the  atmospheric  pressure  by  a  force  less  than  the  tension 
of  the  valve. 

When  the  piston  is  pressed  to  the  bottom,  a  small  space  will  likewise  re- 
main between  the  piston  and  the  bottom,  which  will  be  occupied  by  air,  but  at 
each  ascent  of  the  piston  this  air  expands,  and  is  subject  to  constant  diminu- 
tion as  the  working  of  the  pump  is  continued.  The  principal  source  of  imper- 
fection in  such  an  instrument,  independently  of  that  v,?hich  arises  from  me- 
chanical inaccuracy  of  its  construction,  depends  on  the  tension  of  the  valve  in 
the  top,  and  the  pressure  of  the  atmosphere  upon  it.  To  diminish  this  imper- 
fection, the  valve  in  the  top  is  sometimes  made  to  communicate  by  a  pipe  with 
a  small  subsidiary  exhausting-syringe,  by  which  the  pressure  of  the  atmosphere 
on  the  valve  may  be  partially  withdrawn,  so  that  a  less  force  acting  under  the 
valve  may  open  it. 

A  perspective  view  of  an  air-pump,  v/ith  all  its  accompaniments,  constructed 
upon  this  principle,  is  exhibited  in  fig.  7,  where  the  several  parts  of  the  ma- 
chine are  marked  with  the  same  letters  as  the  corresponding  part  in  the  sec- 
tional diagram,  fig.  4.  The  subsidiary  syringe  just  alluded  to,  is  also  repre- 
sented at  G.     It  is  worked  by  a  handle,  H. 


EXPERIMENTS    WITH    THE    AIR-PUMP. 


The  pressure  and  elasticity  of  air  are  capable  of  being  strikingly  illustrated 
in  various  ways  by  experiments  with  the  air-pump. 

If  a  glass  receiver,  open  at  both  ends,  have  a  strong  bladder  tied  upon  one 
end  so  as  to  be  air-tight,  and  be  placed  upon  the  open  end  on  the  plate  of  an  air- 
pump,  when  the  air  is  exhausted  from  the  receiver,  the  pressure  of  the  external 
atmosphere  on  the  bladder  will  immediately  cause  its  upper  surface  to  be  con- 


52 


ELASTICITY  OF  AIR. 


Fig.  7. 


cave,  and  when  the  air  is  sufficiently  rarefied  within  the  receiver,  the  pressure 
on  the  bladder  will  burst  it,  producing  a  loud  noise  like  the  discharge  of  a 
pistol.  Again,  if  a  large  glass  bowl,  having  a  bladder  tied  firmly  on  its  mouth  so 
as  to  be  perfectly  air-tight,  be  placed  under  the  receiver  of  the  air-pump,  on  with- 
drawing the  air,  the  elastic  force  of  the  air  confined  in  the  bowl  being  still  un- 
diminished, and  being  no  longer  balanced  by  the  atmospheric  pressure  on  the 
outside,  the  bladder  will  be  blown  into  a  convex  form,  and  when  the  air  in  the 
receiver  is  so  rarefied  that  the  elasticity  of  the  air  confined  in  the  bowl  suffers 
little  resistance,  the  bladder  will  burst,  and  the  air  confined  in  the  bowl  will 
expand  through  the  receiver. 

Fruit,  when  dried  and  shrivelled,  contains  within  it  particles  of  air,  which 
are  held  in  its  pores  by  the  pressure  of  the  external  atmosphere.  If,  therefore, 
this  pressure  be  removed,  we  may  expect  that  the  air  thus  confined  will  ex- 
pand, and  if  there  is  no  aperture  in  the  skin  of  the  fruit  for  its  escape,  it  will 
distend  the  skin.  Fruit,  in  this  case,  placed  under  a  receiver,  will  assume  the 
appearance  of  ripeness,  by  exhausting  the  air ;  for  the  expansion  of  the  air 
contained  in  the  fruit,  by  mflating  the  skin,  will  give  it  a  fresh,  ripe  appear- 
ance. Thus  a  shrivelled  apple  will  appear  to  grow  suddenly  ripe  and  fresh, 
and  a  bunch  of  raisins  will  be  converted  into  a  bunch  of  ripe  grapes. 

A  flaccid  bladder  closed  so  as  to  be  air-tight  at  the  mouth,  contains  within  it 
a  small  portion  of  air.  This  air  presses,  by  its  elasticity,  on  the  inner  surface, 
which  is  resisted  by  the  atmospheric  pressure  from  without.  If  such  a  bladder 
be  placed  under  the  receiver  of  a  pump,  and  the  air  exhausted,  the  external 
pressure  being  thus  removed,  the  elasticity  of  the  air  included  will  cause  the 
bladder  to  swell,  and  it  will  take  all  the  appearance  of  being  fully  inflated. 


ELASTICITY  OF  AIR. 


53 


Such  a  bladder  placed  under  several  heavy  vs^eights  will  raise  them  by  the  ex- 
pansion of  the  air. 


Fig.  8. 


A 


K) 

^ 

^^f 

Let  a  close  glass  vessel,  A,  B,  fig.  8,  be  partially  filled  with  water  B,  and 
let  the  tube  C  D  be  inserted  through  its  neck,  the  end  D  being  below  the  sur- 
face of  the  water;  the  air  above  the  surface  will  thus  be  confined.  If  such  a 
vessel  be  placed  under  a  receiver,  and  the  air  be  withdrawn,  the  elastic  force 
of  the  air  confined  in  A,  B,  above  the  surface  of  the  water,  will  press  the  water 
up  in  the  tube  D,  C,  from  which  it  will  issue  in  a  stream  at  C,  when  the  pres- 
sure of  the  atmosphere  is  removed  by  rarefaction. 

By  means  of  an  air-pump,  we  are  enabled  to  demonstrate  that  the  power 
which  causes  water  to  follow  the  piston  in  a  pump  is  the  atmospheric  pressure, 
by  showing  that  the  water  will  not  follow  the  piston  when  that  atmospheric 
pressure  is  removed.  L'et  a  small  exhausting-syringe,  with  its  lower  end  in  a 
vessel  of  water,  be  placed  on  the  plate  of  the  air-pump,  and  let  a  glass  re- 
ceiver, open  at  the  top,  be  placed  over  it.  On  the  top  of  this  receiver  let  a  brass 
cap  fitting  it  air-tight  be  placed,  through  a  hole  in  the  centre  of  which  a  metal 
rod,  terminating  in  a  hook,  passes  air-tight.  Let  the  hook  be  attached  to  the 
end  of  the  piston-rod,  so  that  by  drawing  the  rod  up  through  the  air-tight  col- 
lar, the  piston  may  be  drawn  from  the  bottom  of  the  cylinder  toward  the  top. 
If  this  be  done  before  the  air  has  been  exhausted  from  the  receiver,  the  water 
will  be  found  to  rise  after  the  piston  as  in  the  common  pump ;  but  as  soon  as 
the  air  in  the  receiver  has  been  highly  rarefied,  it  will  be  found  that  although 
the  piston  may  be  drawn  up  in  the  syringe,  the  water  will  not  follow  it.  This 
efl^ect  may  be  rendered  visible  by  constructing  the  barrel  of  the  pump  or  syringe 
of  glass,  through  which  the  water  will  be  seen  to  rise  in  the  one  case  and  not 
in  the  other.  If  an  air-tight  piston  be  placed  in  close  contact  with  the  bottom 
of  a  syringe  not  furnished  with  a  valve,  any  attempt  to  draw  it  up  will  be  re- 
sisted by  the  atmospheric  pressure  ;  and  if  it  be  forced  to  the  top  of  the  cylin- 
der and  there  discharged,  it  will  be  immediately  urged  with  considerable  force, 
to  the  bottom.  The  atmospheric  pressure  above  the  piston,  acting  with  a  force 
of  about  fifteen  pounds  on  the  square  inch,  produces  this  effect,  for  the  space 
between  the  piston  and  the  bottom  of  the  cylinder  not  containing  any  air,  this 
pressure  is  unresisted.  Now  if  this  piston  be  introduced  under  the  receiver 
of  an  air-pump,  and  be  drawn  up  as  already  described,  it  will  be  found  that  in  pro- 
portion as  the  air  is  withdrawn  from  the  receiver,  less  and  less  force  will  be  re 
quired  to  produce  the  effect ;  and  at  length,  the  rarefaction  will  become  so  great, 
that  the  pressure  of  the  remaining  air  is  incapable  of  overcoming  the  friction 


of  the  piston  with  the  cylinder,  and  it  will,  when  drawn  to  the  top  remain  there, 
without  returning  to  the  bottom.  In  this  state  let  the  air  be  readmitted  to  the 
receiver,  the  piston  will  then  be  immediately  pressed  to  the  bottom  of  the 
cylinder. 

The  celebrated  experiment  of  the  Magdeburgh  hemispheres  may  be  per- 
formed by  means  of  an  air-pump.  Two  hollow  hemispheres  constructed  of 
brass,  as  represented  in  fig.  9,  are  so  formed  that  when  placed  mouth  to  mouth 

Pig-.  9.  ' 


they  shall  be  in  air-tight  contact.  They  are  furnished  with  handles,  one  of 
which  may  be  screwed  off.  In  the  neck  to  which  this  handle  is  screwed  is  a 
tube  furnished  with  a  stop-cock.  The  handle  being  screwed  off,  let  the  hem- 
isphere be  screwed  on  the  pump-plate,  and  the  other  hemisphere  being  placed 
over  it,  let  the  stop-cock  be  opened  so  as  to  leave  a  free  communication  be- 
tween the  interior  of  the  sphere  and  the  exhausting-tube  of  the  air-pump. 
The  pump  being  now  worked,  the  interior  of  the  sphere  will  form  the  receiver, 
from  which  all  communication  with  the  external  air  is  cut  off,  and  rarefaction 
will  be  produced  in  it  to  any  degree  which  may  be  desired.  This  being 
effected,  let  the  stop-cock  be  closed ;  and  let  the  sphere  be  detached  from  the 
pump-plate,  and  the  handle  screwed  upon  it.  If  then  the  two  handles  be 
drawn  in  opposite  directions,  so  as  to  pull  the  hemispheres  from  one  another, 
it  will  be  found  that  they  will  resist  with  considerable  force.  If  the  diameter 
of  the  sphere  be  six  inches,  its  section  through  the  centre  will  be  about  twenty- 
eight  square  inches.  The  hemispheres  will  be  pressed  together  by  a  force 
amounting  to  fifteen  pounds  for  every  square  inch  in  the  section.  If  twenty- 
eight  be  multiplied  by  fifteen,  we  shall  obtain  four  hundred  and  twenty-two, 
which  is  the  amount  of  the  force  with  which  the  hemispheres  will  be  held  to- 
gether. If  one  of  the  handles  be  placed  on  a  strong  hook,  and  a  weight  df 
four  hundred  pounds  be  suspended  from  the  other,  the  weight  will  be  supported 
by  the  pressure  of  the  atmosphere. 

This  was  one  of  the  earliest  experiments  in  which  the  effects  of  atmospheric 
pressure  were  exhibited.  Otto  Guericke,  the  inventor  of  the  air-pump,  con- 
structed, in  1654,  a  pair  of  such  hemispheres,  one  foot  in  diameter.  The  sec- 
tion through  the  centre  of  these  was  about  one  hundred  and  thirteen  square- 
inches,  which  multiplied  by  fifteen  gives  a  pressure  amounting  to  about  seven- 
teen hundred  pounds.  If  the  exhaustion  were  complete,  the  hemispheres 
would  be  held  together  by  this  force  ;  but,  even  though  incomplete,  they  were 
still  able  to  resist  a  prodigious  force  tending  to  draw  them  asunder. 

It  is  a  consequence  of  the  general  theory  of  gravitation,  that  under  the  same 
circumstances,  bodies  are  attracted  in  proportion  to  their  mass ;  and  hence  it 


ELASTICITY  OF  AIR. 


55 


would  follow  that  all  bodies,  whatever  be  their  masses,  should  fall  at  the  same 
rate.  Now  the  instances  which  most  commonly  come  under  our  observation 
seem  to  contradict  this  inference,  for  we  find  a  piece  of  metal  and  a  piece 
of  paper  fall  at  very  different  rates,  and  still  more  different  is  the  rate  at 
which  a  piece  of  metal  and  a  feather  would  fall.  The  cause  of  this  circum- 
stance, however,  is  easily  explained.  The  resistance  offered  by  the  air  is 
proportional  to  the  quantity  of  surface  which  the  body  presents  in  the  direction 
of  its  motion.  Now  the  metal  may  present  a  considerably  less  surface  than 
the  feather,  while  the  force  which  it  exerts  to  overcome  the  resistance  is  many 
times  greater,  because  of  its  greater  weight.  Hence,  it  follows  that  the  resist- 
ance of  the  air  produces  a  different  effect  on  the  metal  compared  with  the 
effect  which  it  produces  on  the  feather ;  but  all  doubt  will  be  removed  if  the 
feather  and  the  metal  are  allowed  to  fall  in  a  chamber  from  which  the  air  has 
been  withdrawn.     A  glass  r^eiver  is   represented  in  fig.  10,  which  may  be 

Fis.  10. 


placed  on  the  plate  of  an  air-pump,  and  on  the  top  is  placed  a  brass  cover, 
which  is  air-tight.  Under  this  several  brass  stages  are  attached,  constructed 
in  the  manner  of  trap-doors  on  the  hinges,  and  supported  by  small  pins,  which 
project  from  the  sides  of  a  metal  rod,  passing  through  an  air-tight  collar  in  the 
brass  cover.  By  turning  this  metal  rod,  the  pins  may  be  removed  from  under 
the  trap-doors,  and  they  will  fall,  disengaging  whatever  may  be  placed  upon 
them.  Suppose  a  piece  of  coin  and  a  feather  be  placed  upon  one  of  these 
stages,  supported  by  a  projecting  pin.  This  arrangement  being  made,  let  the 
brass  cover  be  placed  on  the  receiver,  so  as  to  be  air-tight,  and  let  the  receiver 
be  then  exhausted  by  the  pump.  When  a  high  degree  of  rarefaction  has  been 
produced,  let  the  rod  be  turned  by  the  handle  at  the  top,  so  as  to  remove  the 
pin  from  under  the  stage  ;  the  coin  and  the  feather  will  be  immediately  let  fall, 
and  it  will  be  observed  that  they  will  both  descend  at  exactly  the  same  rate, 
and  strike  the  bottom  at  the  same  instant.  This  is  the  experiment  commonly 
known  as  "  the  guinea  and  feather  experiment." 

The  surgical  process  called  cupping,,  consists  in  removing  the  atmospheric 
pressure  from  the  part  of  the  body  submitted  to  the  operation.     A  vessel  with 


56 


ELASTICITY  OF  AIR. 


an  open  mouth  is  connected  with  an  exhausting  syringe.  The  mouth  is  ap- 
plied in  air-tight  contact  with  the  skin,  and  by  working  the  syringe,  a  part  of 
the  air  is  withdrawn  from  the  vessel,  and  consequently  the  skin  within  the 
mouth  of  the  vessel  is  relieved  from  its  pressure.  All  the  other  parts  of  the 
body,  however,  being  still  subject  to  the  atmospheric  pressure,  and  the  elastic 
force  of  the  fluids  contained  in  the  body  having  an  equal  degree  of  tension, 
that  part  of  the  skin  which  is  thus  relieved  fpom  the  pressure  will  be  swelled 
out,  and  will  have  the  appearance  of  being  sucked  into  the  cupping-glass.  If 
the  skin  be  punctured  by  lancets,  the  blood  will  thus  be  drawn  from  it  in  a 
peculiar  manner. 

..   That  the  presence  of  air  is  necessary  for  the  transmission  of  sound,  may  be 
strikingly  illustrated  by  the   air-pump.     A  small  apparatus,  fig.  11,  which,  by 

Fia:.  11. 


being  drawn  upward  and  downward  alternately,  causes  a  bell  to  ring,  is  placed 
on  the  pump-plate,  and  covered  by  a  receiver  with  an  open  top.  A  brass  cover, 
furnished  with  a  sliding  rod,  is  placed  upon  this.  The  sliding  rod  is  termina- 
ted in  a  hook,  which  catches  the  apparatus,  and  by  which  it  may  be  alternately 
raised  and  lowered,  without  allowing  any  air  to  pass  into  the  receiver.  The 
apparatus  being  thus  suspended  in  the  receiver  by  a  silken  thread,  so  that  it 
shall  not  touch  the  bottom  or  sides,  let  the  air  be  exhausted  by  the  pump. 
When  the  rarefaction  has  been  carried  to  a  sufficient  extent,  let  the  rod  be  al- 
ternately raised  and  lowered,  so  that  the  bell  shall  ring.  It  will  be  found  to  be 
inaudible. 

If  the  air  be  now  gradually  admitted,  the  sound  will  at  first  be  barely  audible, 
but  will  become  louder  by  degrees,  until  the  receiver  is  again  filled  with  air,  in 
the  same  state  as  the  external  atmosphere.  In  this  experiment  care  must  be 
taken  not  to  let  the  sounding  apparatus  rest  on  the  pump-plate,  for  it  will  then 
communicate  a  vibration  to  that,  which  will  finally  affect  the  external  air  and 
produce  a  sound. 

THE    CONDENSING    SYRINGE. 


The  condensing  syringe  is  an  instrument  by  which  a  greater  quantity  of  air 
may  be  forced  into  a  vessel  than  that  vessel  contains  when  it  has  a  free  com- 
munication with  the  external  atmosphere. 


ELASTICITY  OF  AIR. 


Let  A,  B,  fig.  12,  be  a  cylinder  furnished  with  a  piston  P,  which  moves  air- 
tight in  it.  Let  C  be  a  tube  proceeding  from  the  bottom,  and  furnished  with  a 
stop-cock.  Let  us  suppose  this  tube  to  communicate  with  the  receiver  or 
vessel  R,  in  which  it  is  intended  to  condense  the  air.  Let  another  tube,  D, 
proceed  from  the  cylinder,  also  furnished  with  a  stop-cock.  Let  the  piston  be 
now  drawn  to  the  top  of  the  cylinder,  both  stop-cocks  being  open.  The  re- 
ceiver R,  being  in  free  communication  with  the  atmosphere,  will  contain  air 
of  the  same  density  and  pressure  as  the  external  atmosphere.  Let  the  stop-cock 
D  be  now  closed,  and  let  the  piston  be  pressed  to  the  bottom  of  the  cylinder ; 
the  air  confined  in  the  cylinder  below  the  piston  will  thus  be  forced  through 
the  tube  C  into  the  vessel  R,  while  the  piston  is  pressed  against  the  bottom  B. 
Let  the  stop-cock  C  be  closed  so  as  to  prevent  the  escape  of  the  air  from  the 

Fiff.  12.  ' 


vessel  R,  and  let  the  stop-cock  D  be  opened,  so  as  to  allow  a  free  communica- 
tion between  the  cylinder  A,  B,  and  the  external  atmosphere.  Let  the  piston 
be  again  drawn  to  the  top  of  the  cylinder.  The  cylinder  will  then  be  filled 
with  atmospheric  air  of  the  same  density  as  the  external  atmosphere.  Let  the 
stop-cock  D  be  closed,  and  C  opened,  and  let  the  piston  be  once  more  forced 
to  the  bottom  of  the  cylinder ;  the  contents  of  the  cylinder  will  be  thus  again 
discharged  and  forced  into  the  receiver  R.  Let  the  stop-cock  C  be  again 
closed,  and  let  the  process  be  repeated.  It  is  evident  that  at  each  stroke  of 
the  piston  a  volume  of  atmospheric  air  will  be  forced  into  the  receiver  equal  to 
the  dimensions  of  the  cylinder  A,  B,  and  there  is  no  limit  to  the  degree  of  con- 
densation, except  that  which  depends  on  the  strength  of  the  receiver  R,  and 
the  cylinder  and  tubes,  and  on  the  power  by  which  the  piston  is  urged. 

After  each  stroke  of  the  piston,  the  density  of  the  air  in  R  is  increased  by 
the  admission  of  as  much  atmospheric  air  as  fills  the  cylinder  A,  B,  and  there- 
fore the  density,  as  the  process  advances,  receives  equal  increments  at  each 
stroke  of  the  piston.  Let  us  suppose  that  the  receiver  R  has  ten  times  the 
capacity  of  the  cylinder  A,  B,  and  let  us  suppose  that  the  elastic  pressure  of 
the  air  in  R,  at  the  commencement  of  the  operation  is  expressed  by  the  number 
10.     After  the  first  stroke  this  pressure  will  be  expressed  by  the  number  11, 


58 


ELASTICITY  OF  AIR. 


inasmuch  as  the  quantity  of  air  in  R  has  been  increased  by  one  tenth  part  of 
its  volume.  After  the  second  stroke  the  pressure  will  be  expressed  by  the 
number  12.     After  the  third  by  the  number  13,  and  so  on. 

In  the  form  given  in  practice  to  the  condensing  syringe,  the  necessity  for 
manipulation  by  the  stop-cocks  here  represented  is  removed.  A  silk-valve, 
such  as  that  described  in  the  exhausting  syringe,  is  placed  in  the  tube  C,  fig. 
13,  but  opening  downward.  The  neck  of  the  receiver  R  is  furnished  with  a 
stop-cock  and  a  tube,  which  terminates  in  a  screw.     This  screw  is  connected 


with  a  corresponding  one  proceeding  from  the  bottom, of  the  syringe.  By  this 
arrangement,  the  air  is  capable  of  passing  through  the  silk-valve  from  the 
syringe  to  the  receiver,  but  not  in  a  contrary  direction.  A  small  hole  is  made 
through  the  piston,  extending  from  the  upper  to  the  lower  surface,  and  the 
)  silk-valve  is  extended  across  this  hole  on  the  lower  surface,  so  that  air  is  ca- 
l  pable  of  passing  through  this  valve  to  the  cylinder  below  it,  but  not  in  a  con- 
/  trary  direction. 

(  Now  let  us  suppose  that  the  air  in  the  receiver  has  the  same  pressure  and 
)  density  as  the  external  atmosphere,  and  let  the  piston  P  be  at  the  top  of  the 
(  cylinder,  the  air  in  the  cylinder  A,  B,  also  having  the  same  pressure  and  den- 
)  sity  as  the  external  air.  By  pressing  the  piston  toward  the  bottom  of  the 
(  cylinder,  the  air  enclosed  will  become  condensed,  and  by  its  increased  pres- 
l  Sure  will  open  the  valve  V,  and  as  the  piston  descends  will  be  forced  into  the 
(  receiver  R.  When  the  piston  has  arrived  at  the  bottom,  all  the  air  contained 
>  in  the  cylinder  will  be  transferred  into  the  receiver.  It  will  be  retained  there, 
I  because  the  valve  V,  opening  downward,  will  not  permit  its  return.  If  the 
;  piston  be  now  drawn  up  it  will  leave  a  vacuum  below  it  when  it  begins  to  as- 
(  cend,  but  the  pressure  of  the  atmosphere  above  will  open  the  valve.  V,  and  the 
;  air  rushing  through  will  fill  the  cylinder  as  the  piston  ascends,  and  when  the 
(  piston  has  arrived  at  the  top  of  the  cylinder,  the  space  below  it  will  again  be 
;  filled  with  atmospheric  air.  By  the  next  descent  of  the  piston  this  air  is  forced 
(  into  the  receiver  R  as  before,  and  so  the  process  is  continued. 
;  It  should  be  observed,  that  when  the  piston  P  is  drawn  to  the  top  of  the 
(  cylinder,  the  air  which  has  passed  into  A  B  has  not  quite  so  great  a  pressure 


ELASTICITY  OF  AIR. 


59 


as  the  external  atmosphere.  This  arises  from  the  valve  V  requiring  some 
definite  force,  however  small,  to  open  it.  When  the  air  which  has  passed  into 
the  chamber  A,  B,  requires  a  pressure  which  is  less  than  the  atmospheric 
pressure,  by  an  amount  equal  to  the  tension  of  the  valve  V,  then  the  excess 
of  the  pressure  of  the  atmosphere  over  the  resistance  of  the  air  contained  in 
A  B  will  be  insufficient  to  open  the  valve  V,  and  no  more  air  can  pass  into 
the  cylinder.  It  should  also  be  observed,  that  the  valve  V  being  pressed  up- 
ward by  the  elastic  force  of  the  air  condensed  in  the  receiver^  requires  a  still 
greater  pressure  than  this  to  open  it,  and  therefore,  before  the  valve  V  can  be 
opened,  the  air  enclosed  below  the  piston  P  must  always  be  condensed  by  the 
pressure  of  the  piston  in  a  higher  degree  than  the  air  is  condensed  in  the  re- 
ceiver. The  observations  which  have  been  made  respecting  the  limit  of  the 
operation  of  the  exhausting  syringe,  arising  from  mechanical  imperfections  and 
other  causes,  will  also  be  applicable  here.  However  nicely  the  piston  P  and 
the  cylinder  in  which  it  plays  may  be  constructed,  there  will  still  be  some 
small  space  remaining  between  it  and  the  silk-valve  V,  when  it  is  pressed  to 
the  bottom  of  the  cylinder.  Into  this  space  the  air  contained  in  the  cylinder 
may  finally  be  condensed,  and  when  the  pressure  of  the  air  contained  in  the  re- 
ceiver becomes  equal  to  the  pressure  of  the  air  condensed  into  the  space  be- 
tween the  piston  at  the  bottom  of  the  cylinder  and  the  silk-valve,  the  operation 
of  the  instrument  must  necessarily  cease  ;  for  then  the  utmost  degree  of  con- 
densation which  can  be  produced  above  the  silk-valve  V  will  be  insufficient 
to  open  the  valve,  and  therefore  the  syringe  cannot  introduce  more  air  into  the 
receiver. 

THE    CONDENSER. 

The  condenser  has  the  same  relation  to  the  apparatus  just  described  as  the 
air-pump  has  to  the  exhausting  syringe.  The  condenser  consists  of  a  receiver 
firmly  and  conveniently  fixed,  communicating  by  a  tube  with  one  or  two  con- 
densing-syringes,  which  may  be  worked  in  the  same  manner  as  the  exhausting- 
syringe  described  in  the  air-pump. 

In  the  use  of  such  an  instrument,  it  is  convenient  to  possess  the  means  of 
indicating  the  degree  of  condensation  which  has  been  effected.  For  this  pur- 
pose a  mercurial  gauge  is  used  analogous  to  that  which  is  applied  to  the  air- 
pump.     A  bent  tube,  A,  B,  C,  fig.  14,  contains  a  small  quantity  of  mercury, 

Fig.   14. 


A 


S,  B,  S',  in  the  curved  part.  When  the  ends  of  the  tube  are  open,  and  m 
free  communication  with  the  atmosphere,  the  surfaces  S,  S',  will  stand"  at  the 
same  level.     The  extremity  C  is  furnished  with  a  stop-cock,  by  which  a  com- 


munication  with  the  atmosphere  may  be  permitted  or  intercepted.  The  ex- 
tremity A  communicates  by  a  tube  with  the  receiver  in  which  the  air  is  to  be 
condensed.  At  the  commencement  of  the  process,  before  any  condensation  has 
taken  place,  the  stop-cock  C  is  closed,  and  the  air  included  between  it  and  the 
surface  S'  has  then  the  same  pressure  as  the  external  atmosphere.  The  air 
in  the  receiver  having  also  that  pressure,  the  two  surfaces  S  and  S'  necessa- 
rily stand  at  the  same  level.  When  the  condensation  of  air  in  the  receiver 
commences,  the  pressure  on  the  surface  S  is  increased,  therefore  that  surface 
falls,  and  the  surface  S'  rises.  The  pressure  of  the  air  condensed  in  the  re- 
ceiver will  thus  be  balanced  by  the  weight  of  the  column  of  mercury  between 
the  levels  S  and  S',  together  with  the  pressure  of  the  air  enclosed  between  S^ 
and  C.  The  pressure  of  the  air  enclosed  in  S'  C  is  increased  in  the  same 
proportion  as  the  space  S'  C  has  been  diminished.  Now,  as  the  original  pres- 
sure of  the  air  contained  in  this  space  was  equal  to  the  pressure  of  the  atmo- 
sphere, it  is  always  easy  to  find  the  pressure  of  the  air  reduced  in  bulk  by  in- 
creasing the  amount  of  atmospheric  pressure  in  the  same  proportion  as  the 
space  S'  C  has  been  diminished.  Thus  if  the  air  enclosed  in  the  tube  be  re- 
duced to  half  its  original  bulk,  then  the  pressure  it  exerts  will  be  double  the 
atmospheric  pressure.  If  it  be  reduced  to  two  thirds  of  its  bulk,  then  the 
pressure  of  the  enclosed  air  will  be  the  atmospheric  pressure  in  the  proportion 
of  three  to  two,  and  so  on.  The  pressure  thus  computed  being  added  to  the 
pressure  arising  from  the  column  of  mercury  between  the  levels  of  the  surfaces 
S  and  S',  will  give  the  whole  pressure  of  the  air  condensed  in  the  receiver. 

Although  the  condenser  is  not  without  its  use  in  experimental  physics,  yet 
it  is  an  instrument  far  less  important  than  the  air-pump,  to  which  it  is  so 
analogous.  The  cases  are  innumerable  in  which  it  is  necessary  to  inquire 
what  effect  would  take  place  in  the  absence  of  the  atmosphere  ;  but  they  are 
comparatively  few  in  which  it  is  necessary  to  investigate  what  effects  would 
be  produced  under  increased  atmospheric  pressure.  We  do  not,  therefore, 
think  it  necessary  to  enter  into  further  details  concerning  the  condenser. 


EFFECTS  OF  LIGHTIOG 


Classification  of  the  Effects  of  Lightning. — The  sulphureous  Odor  developed  hy  Lightning. — Cases 
collected  by  M.  Arago. — Nature  of  the  Odor. — Chemical  Changes  operated  by  Lightning. — Nitric 
Acid  formed  by  the  Electric  Spark ;  also  Ammonia  and  Nitric  Acid  produced  during  Thunder 
Storms. — Fusion  and  Contraction  of  Metals. — Observations  of  the  Ancients. — Franklin's  cold 
Fusion. — Evidence  against  cold  Fusion. — Masses  of  Metal  melted  by  Lightning. —  Vitref actions 
and  Fulgurites. — Heights  at  which  Vitrefactions  have  been  found. — Facts  collected  by  M.  Arago. — 
Fulminary  Tubes,  or  Fulgurites. — Characters  of  Fulgurites. — Variations  dependant  on  the  Na- 
ture of  the  Soil  where  they  are  found. — Four  Hypotheses  to  explain  their  Origin. — Their  Forma- 
tions in  some  Cases  are  recent. — Sand  fused  by  artificial  Heat  into  the  State  of  the  Fulgurites. — 
Artificial  Fulgurites  formed  by  the  Electrical  Battery. — The  further  Condition  essential  to  explain 
the  Origin  of  Fulgurites. — Recent  Formation  of  Fulgurites  observed. — Mechanical  Effects. — In- 
stances of  the  Mechanical  Action  of  Lightning. — The  Action  is  exerted  in  all  Directions.: — Induc- 
tive Action  of  Lightning. — M.  Arago's  Explanation  of  the  Effect  as  due  to  Vaporization. — Objec- 
tions to  the  Explanation. — Decompositions  of  the  natural  Electricities  of  Bodies. — Induction  be- 
tween the  Clouds  and  the  Earth. — Upward  Flashes  and  Mechanical  Effects. — Arago's  Explana- 
tion.— Magnetic  Effects. — To  be  explained  in  Electro  Magnetism. — Effects  of  conducting 
Bodies  on  Lightning. — Conducting  Properties  of  Metallic  Bodies. — Lightning  passing  along  Con- 
ductors in  Preference  to  Non-Conductors. — Protection  afforded  by  conducting  Bodies. — Lightning 
selects  conducting  Bodies  from  among  others. — Lightning  Conductors  should  descend  to  a  humid 
Soil. — Necessity  of  Continuity  in  a  Conductor. — Effects  proceeding  from  the  Surface  of  the  Earth. — 
Ascent  or  Ebulition  of  Water. — Inundations  from  subterranean  Sources. — Mosaic  Account  of  the 
Deluge;  Analogous  natural  Phenomena. — Electrical  State  of  the  Atmosphere  Favorable  to  the 
Process  of  barking  Trees. — Effect  of  Thunder  on  fermented  Liquors,  &c. — Return  Stroke  report- 
ed by  Brydone. — Theory  of  such  Effects. — Flame  appearing  on  the  Ground. — Not  extinguisha- 
ble  by  Water. — Superposed  Clouds  not  necessary  to  its  Appearance.— Stationary  luminous  Ap- 
pearance.— Lightning  rising  from  the  Earth  like  a  Rocket. — Flames  observed  on  exposed  Points. — 
Luminous  Rain. — Cases  collected  by  M.  Arago. — Luminous  Dust. 


THE   EFFECTS  OF  LI&HTIfOG. 


The  effects  which  have  been  observed  to  attend  the  transmission  of  light- 
ning through  bodies  vv^hich  it  strikes  are  so  various,  and  apparently  unconnected, 
that  any  classification  of  them  is  extremely  difficult.  I  shall  here  adopt  that 
which  M.  Arago  has  given.  The  chief  effects  of  lightning  may,  then,  be  enu- 
merated as  follows  : — 

1 .  The  diffusion  of  smoke  occasionally,  and  a  sulphureous  odor  almost  inva- 
riably. 

2.  The  production  of  chemical  changes  in  the  atmosphere  itself,  and  in  sub- 
stances suspended  in  it. 

3.  The  fusion  of  metals,  and  sometimes  the  contraction  of  their  dimensions 
without  fusion. 

4.  The  vitrifications  of  earthy  substances,  and  the  formation  of  fulgurites, 
or  thunder-tubes. 

5.  Mechanical  effects  in  piercing,  splitting,  and  transporting  from  place  to 
place,  the  parts  of  bodies  which  it  strikes. 

6.  The  production  of  magnetic  effects. 

7.  It  passes  along  certain  substances  in  preference  to  others,  and  in  general 
its  effects  are  dependant  on  the  nature  of  the  bodies  it  strikes. 

8.  The  existence  of  a  storm  in  the  atmosphere  is  accompanied  by  a  state  of 
the  surface  of  the  earth  beneath  it  in  which  lightning  issues  upward  from  it, 
and  objects  upon  it  are  struck  from  below. 

9.  Luminous  rain. 

10.  Rain,  snow,  and  hail,  falling  in  a  storm,  sometimes  emit  light  when  the 
drops  strike  each  other,  or  strike  the  earth. 

We  shall  consider  these  classes  of  effects  in  succession. 


I.      THE    SULPHUREOUS    ODOR    DEVELOPED    BY    LIGHTNING. 

The  following  instances  have  been  collected  by  M.  Arago  : — 

In  a  thunder-storm  on  the  isthmus  of  Darien,  Wafer,  a  surgeon,  observed 


that  the  air  was  infected  with  a  sulphureous  odor  so  strong  as  to  check  respi- 
ration, especially  in  the  woods. 

On  another  occasion,  the  same  observer,  crossing  a  hill  after  sunset,  was 

overtaken  by  rain  so  terrible,  that  it  seemed  as  though  heaven  and  earth  were 

coming  together.     There  were  loud  claps  of  thunder,  and  the  lightning  was 

attended  by  an  odor  of  sulphur  so  intense  that  the  travellers  were  nearly  sufFo- 

/•  cated  by  it. 


) 


Boyle,  in  his  memoirs  for  a  general  history  of  the  air,  relates  that  in  a  thun- 
der-storm which  he  encountered  on  the  borders  of  the  lake  of  Geneva,  the 
air  was  impregnated  with  a  sulphureous  odor  so  strong,  that  a  sentinel  sta- 
tioned near  the  lake  was  nearly  suffocated. 

Legentil  witnessed  a  storm  in  the  Isle  of  France,  in  February,  1771,  in 
which  a  strong  sulphureous  odor  was  perceived. 

On  the  4th  of  November,  1749,  in  north  latitude  forty-two  degrees  and  forty- 
eight  minutes,  and  west  longitude  three  degrees,  the  ship  Montague  was  struck 
by  lightning.     It  seemed  as  if  the  vessel  was  filled  with  burning  sulphur. 

On  the  19th  of  April,  1827,  the  packet-ship  New  York,  in  north  latitude 
thirty-eight  degrees,  and  west  longitude  fifty-three  degrees,  was  twice  struck 
by  lightning,  being  nearly  five  hundred  miles  from  land.  When  first  struck, 
the  paratonnerre  was  not  put  up  ;  yet  the  lightning,  finding  metallic  bodies  in 
its  route,  was  conducted  to  the  water,  having  done  much  injury  to  the  vessel. 
The  cabins  were  filled  with  a  thick  sulphureous  smoke.  When  she  was 
struck  the  second  time,  the  paratonnerre  was  in  its  place,  and  no  damage  was 
done  ;  nevertheless,  various  parts  of  the  ship,  and  the  ladies'  cabin  in  particu- 
lar, was  filled  with  sulphureous  vapor  so  thick  that  objects  could  not  be  seen 
through  it. 

On  the  31st  of  December,  1778,  at  three  o'clock,  P.  M.,  the  India  Company's 
ship  Atlas,  lying  in  the  Thames,  was  struck  by  lightning,  and  a  sailor  was 
killed  in  the  rigging.  The  ship  for  a  moment  seemed  to  be  on  fire,  but  in  fact 
suffered  no  damage  ;  a  strong  sulphureous  odor  was,  however,  diffused  through 
it,  which  continued  during  the  day  and  ensuing  night. 

On  the  ISih  of  July,  1707,  lightning  passed  down  the  flues  of  six  chimneys 
of  a  house  in  the  Rue  Plu?7ict  in  Paris.  A  suffocating  odor  was  diffused 
through  the  house. 

On  the  18th  of  February,  1770,  the  church  of  St.  Kevern,  Cornwall,  was 
struck  with  lightning  during  Divine  service,  when  the  whole  congregation 
were  struck  senseless.  The  church  was  filled  with  a  suffocating  sulphureous 
odor. 

On  the  11th  of  July,  1819,  the  church  at  Chateauneuf-les-Monstiers  (Basses 
Alfes)  being  struck  by  lightning,  was  filled  with  a  dense  black  smoke,  which 
rendered  it  so  dark  that  one  could  walk  in  it  only  by  groping. 

That  the  sulphureous  odor  developed  by  lightning  arises  from  the  actual 
presence  of  some  vaporous  matter,  seems  to  be  demonstrated  by  those  observa- 
tions in  which  an  opaque  cloudy  vapor  filled  the  rooms.  Whether  the  matter 
difi'used  through  the  air  is  transported  from  the  upper  regions  of  the  atmosphere 
by  the  lightning,  or  is  developed  by  the  action  of  the  lightning  on  the  bodies 
which  it  strikes,  is  still  undecided.  The  possibility  of  matter  being  brought 
by  the  lightning  from  the  clouds  is  countenanced  by  the  phenomena  of  ball- 
lightning,  and  by  the  results  of  the  investigations  of  M.  Fusinieri.  Although 
the  odor  diffused  by  lightning  has  been  generally  compared  to  that  produced 
by  the  combustion  of  sulphur,  some  observers  have  assimilated  it  to  phospho- 
rus, and  others  to  nitrous  gas.  If  the  last  were  its  true  description,  an  easy 
exi)lanation  of  it  would  be  obtained  by  considering  the  effects  of  electricity  on 
the  constituents  of  the  atmosphere. 


The  experiment  formerly  alluded  to,  in  which,  by  transmitting  the  electric 
spark  through  atmospheric  air  confined  in  a  glass  tube,  a  combination  took 
place  between  a  portion  of  its  constituents  and  liquid  nitric  acid  was  formed, 
was  due  to  the  celebrated  Cavendish.  After  the  identity  of  lightning  and  elec- 
tricity was  established,  no  doubt  was  entertained  that  the  same  process  took 
place  in  the  atmosphere  whenever  lightning  was  transmitted  through  it.  The 
direct  demonstration  of  this  important  fact  was  made  by  Professor  Liebig  in 
1827. 

That  philosopher  submitted  seventy-seven  samples  of  rain-water,  collected 
on  ditlerent  occasions,  to  the  process  of  slow  distillation.  Of  these  samples, 
seventeen  were  collected  during  or  immediately  after  thunder-storms.  In  the 
residue  obtained  from  these  seventeen,  nitric  acid  was  found  in  greater  w  less 
quantities,  in  combination  with  lime,  or  with  ammonia.  In  fifty-eight  of  the 
other  samples,  these  substances  were  not  found  ;  and  in  the  remaining  two, 
mere  traces  of  nitric  acid  were  just  discoverable. 

The  formation  of  nitric  acid  in  the  atmosphere  during  thunder-storms  sug- 
gests to  philosophical  observers  various  important  objects  of  attention  and 
inquiry.  Under  what  circumstances  of  season,  locality,  height,  and  tempera- 
ture, of  the  clouds,  does  the  quantity  of  nitric  acid  thus  formed  vary?  In  tropi- 
cal regions,  where  thunder-storms  are  phenomena  of  daily  occurrence  for  entire 
months,  is  the  quantity  of  nitric  acid  generated  in  the  air  sufficient  to  feed  the 
natural  veins  of  nitre  found  in  certain  localities  where  the  absence  of  animal 
matter  has  rendered  such  formations  a  matter  of  great  theoretical  difficulty  ? 
The  researches  may  also  lead  to  the  solution  of  the  origin  of  the  other  sub- 
stances, such  as  lime  and  ammonia,  detected  by  Liebig  in  the  pluvial  waters 
falling  from  stormy  clouds,  and  possibly  for  the  sulphureous  gas,  of  which  the 
odor  is  so  remarkable  in  places  where  lightning  penetrates. 

It  would  be  a  curious  and  interesting  result  of  scientific  iuA^estigation  to 
demonstrate  that  the  thunder  of  heaven  elaborates  in  the  clouds  the  chief  in- 
gredient of  the  counterfeit  thunder  which  man  has  invented  for  the  destruction 
of  his  fellows. 

in.     THE    FUSION    AND    CONTRACTION    OF    METALS. 


The  power  of  lightning  to  effect  the  fusion  of  metals  was  observed  by  the 
ancients.  Aristotle,  Lucretius,  Seneca,  and  Pliny,  mention  this  property,  but 
in  a  manner  and  attended  by  circumstances  which,  in  the  judgment  of  many, 
cast  doubts  on  the  truth  of  their  statements.  Aristotle  mentions  the  copper  on 
a  shield  being  fused  by  lightning,  while  the  wood  which  it  covered  was  unin- 
jured. Seneca  states  that  the  coin  contained  in  a  purse  was  fused,  while  the 
purse  was  unchanged  ;  that  a  sword  vi^as  liquefied,  while  the  scabbard  in  which 
it  lay  was  untouched  ;  and  that  the  iron  points  of  spears  being  melted,  flowed 
along  the  wood  to  which  they  were  attached  without  burning  it.  Pliny  relates 
that  coins  of  gold,  silver,  and  copper,  sealed  up  in  a  bag,  were  melted  by  light- 
ning, the  bag  not  being  burnt,  nor  the  wax  which  sealed  it  softened. 

If  the  fusion  or  liquefaction  here  referred  to  were  understood  to  mean  the 
co?ripIete  fusion  of  the  various  pieces  of  metal  mentioned  by  these  several  wri- 
ters, there  would  be  undoubtedly  great  difficulty  in  reconciling  their  statements 
with  the  known  properties  of  matter.  But  if,  on  the  other  hand,  partial  or  su- 
perficial fusion  be  meant,  the  well-ascertained  results  of  modern  observation 
corroborate  this  ancient  evidence. 

In  1781,  M.  D'Aussac  and  the  horse  on  which  he  was  mounted  were  killed 

VOL.  11,-5 


by  lightning  in  the  neighborhood  of  Castres.  The  blade  of  the  sword  which 
he  wore  was  fused  upon  its  surface  at  several  places,  while  the  scabbard  con- 
taining it  was  not  burned.  This  circumstance  is  not  inconsistent  with  the 
known  properties  of  bodies.  The  part  of  the  blade  not  fused  being  a  good 
conductor  of  heat,  abstracted  the  heat  from  the  fused  part  before  it  had  time  to 
burn  the  scabbard. 

The  statements  of  the  ancient  writers  above  quoted  being  taken  literally,  led 
Franklin  to  adopt  the  hypothesis  of  cold  fusion.  To  admit  the  possibility  of  a 
wooden  scabbard  containing  the  heavy  mass  of  incandescent  liquid  metal  which 
must  have  resulted  from  the  fusion  of  a  Roman  sword  without  being  burnt, 
was  impossible.  He  therefore  proposed  to  remove  the  difficulty  by  admitting 
that  lightning  possesses  the  property  of  melting  metals  without  heating  them. 
This  affords  one  of  the  many  instances  of  the  errors  which  arise  from  framing 
hypotheses  to  explain  phenomena,  the  existence  and  nature  of  which  are  not 
accurately  ascertained.  The  strict  rules  of  philosophical  reasoning  required 
that  Franklin  should  demonstrate  as  a  matter  of  fact  that  the  metal  liquefied  by 
lightning  is  actually  cold  while  in  the  state  of  fusion. 

That  lightning  fuses  metals  by  raising  their  temperature  to  the  point  of  fu- 
sion, is  proved  by  the  fact  that  metal  fused  by  lightning  falling  in  liquid  drops 
on  a  wooden  floor,  or  on  the  deck  of  a  vessel,  has  burnt  holes  in  the  wood. 

The  fusion  eff"ected  by  lightning  is  not  confined  to  that  of  thin  wire  or  to 
the  slight  superficial  fusion  above  mentioned.  Considerable  masses  of  metal 
have  been  on  various  occasions  melted.  When  the  power  has  not  sufficient 
energy  to  produce  fusion,  the  iron  is  often  rendered  incandescent  and  soft,  and 
reduced  to  the  state  necessary  for  welding  it.  With  a  still  more  feeble  power, 
it  is  only  raised  to  a  temperature  more  or  less  elevated.  The  following  facts 
are  collected  by  M .  Arago  in  illustration  of  these  principles  : — 

On  the  20th  of  April,  1807,  at  Great  Mouton,  in  Lancashire,  a  windmill  was 
struck  with  lightning,  which,  having  passed  along  a  large  iron  chain,  softened 
the  links,  so  that  by  their  own  weight  they  were  welded  together,  and  the 
chain  was  converted  into  a  rod  of  iron. 

In  June,  1829,  the  sam.e  occurrence  took  place  in  a  windmill  at  Lothill,  in 
Essex. 

On  the  5th  of  April,  1807,  at  Vezinet,  near  Paris,  lightning  struck  a  key, 
and  softened  it  so  that,  by  its  weight,  it  was  welded  to  its  ring. 

In  March,  1772,  lightning  struck  a  bar  of  iron  inserted  at  the  most  elevated 
part  of  the  dome  of  St.  Paul's  cathedral,  which  was  intended  by  the  architect 
to  be  in  metallic  connexion  with  the  pipe  by  which  the  water  is  conducted 
from  the  roof  to  the  ground.  This  connexion  was  accidentally  interrupted  at 
a  certain  point,  and  there  it  was  found  that  the  bar  had  been  rendered  red  hot. 
This  bar  was  four  inches  broad,  and  half  an  inch  thick. 

In  August,  1777,  the  weathercock  of  a  tower  in  Cremona  was  struck  by 
lightning,  and  the  marble  stones  of  the  tower  broken  and  scattered.  The 
thunder  was  the  most  violent  ever  heard  in  that  place.  The  iron  rod  of  the 
weathercock,  which  was  half  an  inch  in  diameter,  was  broken,  but  showed  no 
mark  of  fusion. 

On  the  12th  of  July,  1770,  lightning  struck  the  house  of  Mr.  J.  Moulde,  in 
Philadelphia,  and  fused  a  rod  of  copper  six  inches  long,  but  of  unascertained 
diameter. 

In  1754,  the  steeple  at  Newbur}',  in  the  United  States,  was  struck  by 
lightning,  after  which  it  was  examined  by  Franklin,  who  found  that  the  light- 
ning had  passed  along  an  iron  wire  twenty  feet  long,  and  about  the  thickness 
of  a  knitting-needle,  which  it  reduced  to  smoke.  The  course  of  the  wire 
along  the  walls  and  floors  was  marked  by  a  black  line,  like  that  left  by  a  train 


of  gunpowder  which  has  been  fired.  In  this  case  the  wire  was  probably  burned. 
Another  wire,  in  the  same  tower,  of  the  thickness  of  a  goosequill,  transmitted 
the  lightning  without  being  fused. 

When  Captain  Cook  was  anchored  in  the  roadstead  of  Batavia,  his  ship  was 
struck  by  lightning,  which  produced  a  shock  like  that  of  an  earthquake.  An 
iron  wire,  a  quarter  of  an  inch  in  diameter,  extending  from  the  mast-top  to  the 
water,  appeared  for  a  moment  to  be  on  fire.     No  damage  was  sustained. 

On  the  18th  of  June,  1782,  lightning  struck  the  house  of  Mr.  Parker,  at 
Stoke-Newington,  near  London,  and  having  passed  down  one  of  the  pipes,  pro- 
vided to  conduct  the  fluvial  waters  from  the  roof,  from  that  it  passed  into  a  bed- 
chamber, where  it  followed  the  course  of  a  wire  which  connected  a  cord  at  the 
bedside  with  a  night-bolt  at  the  dour,  by  which  a  person  could  bolt  or  unbolt 
the  door  without  leaving  the  bed.  Such  a  bolt  passes  through  two  rings  at- 
tached to  the  doorframe,  which,  in  this  case,  served  as  a  gauge  for  the  length 
of  the  connecting  wire.  After  the  liglitning  had  passed  along  it,  the  wire  was 
found  so  much  shortened  that  the  bolt  could  not  be  let  fall. 

Wire  extended  between  two  fixed  points  is  often  broken  by  lightning,  which 
may  be  explained  by  the  contraction  just  mentioned,  and  the  fixed  points  not 
allowing  the  wire  to  yield. 


IV.    OF    VITRIFICATIONS    AND    FULGURITES. 

As  evidence  of  the  heights  at  which  the  presence  of  lightning  has  been  man- 
ifested, the  vitrifications  observed  in  certain  places  have  been  already  mention- 
ed. Saussure,  in  1787,  observed  these  effects  on  the  Dome  de  Goute,  one  of 
the  summits  of  Mont-Blanc.  Ramond  observed  them  on  several  summits  of  the 
Pyrenees,  especially  the  Pic  du  Midi  and  Mont-Perdu,  and  on  the  rock  Sana- 
doire,  in  the  Puy-de-D6me.  Humboldt  and  Bonpland  found  similar  appearan- 
ces on  the  rock  El  Frayle,  at  the  top  of  Teluca,  one  of  the  Cordilleras,  near 
the  city  of  Mexico. 

These  several  observers  merely  saw  the  vitrifications  ;  they  inferred  their 
cause  by  the  form  of  reasoning  called,  in  logic,  a  disjunctive  syllogism  ;  that  is, 
by  severally  rejecting  every  other  possible  cause,  they  concluded  that  lightning 
must  have  been  the  true  one.  That  a  question  so  important  may  not  rest  solely 
on  such  negative  proof,  M.  Arago  has  collected  the  following  facts  in  support 
of  it  : —  ^ 

On  the  3d  of  July,  1725,  at  Mixbury,  in  Northamptonshire,  lightning  struck 
on  an  open  field,  and  killed  a  shepherd  and  five  sheep.  Close  to  the  body  of 
the  man  were  found  two  holes,  five  inches  in  diameter  and  forty  inches  deep. 
Near  the  bottom  of  one  of  them  was  found  a  very  hard  stone,  measuring  ten 
inches  long,  six  inches  broad,  and  four  inches  thick,  with  its  surface  vitrified. 

In  the  year  1750  lightning  struck  the  tower  of  Asinelli,  at  Bologna,  and  did 
some  injury  to  it.  Beccaria,  who  examined  it,  found  the  bricks  at  the  place 
where  the  lightning  struck  vitrified. 

On  the  3d  of  September,  1789,  lightning  struck  an  oak  in  the  park  of  Lord 
Aylesford,  and  killed  a  man  who  sought  shelter  under  it.  This  person  carried 
a  walking-stick,  which  apparently  conducted  the  Hghtning  to  the  ground,  for  at 
its  point  was  found  a  hole  five  inches  in  depth  and  tvi^o  inches  and  a  half  in  di- 
ameter ;  and  below  this,  to  a  depth  of  twelve  inches,  were  found  marks  of  A'it- 
rification. 

The  fact  last  mentioned  leads  to  the  consideration  offulminary  tubes,  or  ful- 
gurites, of  which  it  may  almost  be  regarded  as  an  example. 

The  tubes  were  first  discovered  in  1711,  by  Pieman,  a  shepherd,  at  Massel, 
m  Silesia.     Specimens  of  them  were  sent  to  the  mineralogical  museum  at 


68 


THE  EFFECTS  OF  LIGHTNING. 


Dresden,  and  are  still  preserved  there.  Nearly  a  century  elapsed  before  they 
were  seen  again,  when,  in  1805,  Dr.  Hentzen  found  them  in  Paderhorn,  com- 
monly called  La  Senne.  This  philosopher  first  assigned  their  origin.  They 
have  been  since  found  in  great  numbers  at  Pillau,  near  Konigsberg  ;  at 
Nietleben,  near  Halle  upon  Saale  ;  at  Drigg,  in  Cumberland  ;  in  the  sandy 
country  at  the  foot  of  Regenstein,  near  Blankenburg ;  and  in  the  sands  of  Ea- 
hia,  in  Brazil. 

At  Drigg  the  fulgurites  are  found  in  hillocks  of  moveable  sand,  about  forty 
feet  high,  close  to  the  sea.  At  La  Senne  they  are  usually  discovered  at  the 
brow  of  hills  of  sand  about  the  same  height ;  sometimes  also  in  a  cavity,  form- 
ed like  a  basin,  one  hundred  feet  in  circumference,  and  fifteen  feet  deep. 

Fulgurites  are  usually  hollow  tubes.  At  Drigg  their  diameter  is  generally 
two  and  one  fourth  inches.  Those  at  La  Senne  vary  from  one  fiftieth  of  an  inch 
to  half  an  inch  in  diameter,  and  contract  as  they  descend,  terminating  frequent- 
ly in  a  point.  The  thickness  of  their  sides  varies  from  the  fiftieth  of  an  inch 
to  an  inch.  These  tubes  usually  descend  in  the  vertical  direction,  being  occa- 
sionally, however,  inclined  at  an  angle  of  40°  to  the  horizon.  Their  total 
length  sometimes  amounts  to  above  thirty  feet.  Numerous  transversal  fissures 
divide  them  into  fragments,  the  lengths  of  which  vary  from  half  an  inch  to  six 
inches.  The  sand  by  which  they  are  surrounded  dries  and  falls  off  after  a  lapse 
of  time,  and  these  fragments  are  then  seen  on  the  surface  of  the  ground,  the 
sport  of  the  wind. 

Most  commonly,  in  clearing  away  the  surrounding  sand,  the  fulgurite  is  found 
to  consist  of  a  single  tube.  On  following  it  to  a  certain  depth,  this  is  divided 
into  two  or  three  branches,  each  of  which  again  divides  into  small  lateral  rami- 
fications, varying  from  one  inch  to  twelve  inches  in  length.  These  final  rami- 
fications are  conical,  and  terminate  in  points,  which  are  gradually  inclined 
downward. 

The  interior  surface  of  the  tubes  is  coated  with  a  perfect, and  very  brilliant 
glass,  resembling  vitreous  opale,  or  hyalite.  It  cuts  glass  and  strikes  fire  with 
steel.  Whatever  be  the  form  of  these  tubes,  they  are  always  surrounded  by  a 
crust  composed  of  grains  of  quartz  agglutinated  together.  This  crust  is  some- 
times round  ;  it  is  oftenest  like  the  bark  of  a  stump  of  an  old  birch-tree.  The 
interior  and  exterior  surfaces  correspond  in  form,  as  if  the  tube  were  soft  and 
flexible,  and  acquired  hardness  after  being  bent. 

When  examined  with  a  microscope,  the  exterior  crust  presents  marks  of  fu- 
sion. At  a  certain  distance  from  the  centre  of  the  tube  the  grains  or  globules 
acquire  a  reddish  tint.  The  color  of  the  material  of  the  tube,  and  especially  of 
the  exterior  parts,  depends  on  the  nature  of  the  sandy  soil  in  which  it  has  been 
formed.  In  the  superior  strata,  which  consists  of  common  soil,  the  exterior  of 
the  tube  is  usually  black;  deeper,  it  is  a  yellowish  gray;  and  deeper  still,  a 
grayish  white.  Finally,  where  the  sand  is  pure  and  white,  the  tube  exhibits 
nearly  perfect  whiteness. 

Such  being  the  appearances  presented  hy  fulgurites,  the  question  is  present- 
ed :  Whence  do  they  originate,  and  by  what  natural  process  have  they  been 
formed?  Four  hypotheses  were  proposed  to  explain  them:  1.  They  might 
have  been  incrustations  formed  round  roots,  which  disappeared  after  the  opera- 
tion ;  2.  They  might  be  stalactites  or  other  mineral  formations  ;  3.  They  might 
be  cells  belonging  to  ancient  marine  animals  of  the  worm  species ;  4.  They 
might  be  produced  by  lightning  penetrating  the  ground. 

The  first  three  of  these  hypotheses  include,  as  a  necessary  condition,  the 
formation  of  the  fulgurites  at  an  epoch  more  or  less  remote  from  the  present 
time.  If  it  can  be  shown,  then,  that,  whatever  be  their  origin,  it  must,  in  some 
cases  at  least,  be  recent,  these  hypotheses  must  be  severally  rejected.     The 


phenomena  at  Drigg  are  conclusive  as  to  the  recency  of  the  formation  of  the 
fulgurites,  and  are  therefore  fatal  to  these  hypotheses. 

The  hillocks  of  sand  in  which  the  fulgurites  at  Drigg  are  formed  are  shifting, 
being  subject  to  constant  change  by  the  wind.  The  tubes  in  them  must,  there- 
fore, be  of  recent  formation. 

But  it  is  necessary  to  show  that  the  state  in  which  the  sand  is  found  in  the 
internal  and  external  coating  of  the  tube,  as  well  as  in  every  part  of  its  thick- 
ness, can  he  produced  by  intense  heat. 

This  has  accordingly  been  done.  The  sand  in  which  the  tubes  have  been 
formed  has  been  exposed  to  the  action  of  various  degrees  of  heat  by  means  of 
the  blowpipe,  and  efiects  have  been  produced  which  correspond  with  the  state 
of  the  tubes,  and  prove  that  intense  heat  can  produce  the  observed  effects. 

Since  we  have  in  the  electricity  of  the  machines  another  lightning  infinitely 
less  in  its  degree,  but  still  the  same  in  kind,  a  further  corroboration  of  this  hy- 
pothesis would  be  obtained,  if  by  means  of  this  artificial  lightning  artificial  ful- 
gurites could  be  formed.  MM.  Savart,  Hachette,  and  Beudant,  transmitted  the 
charge  of  a  powerful  electrical  battery  through  a  mass  of  glass  reduced  to  pow- 
der, and  obtained  fulgurites  an  inch  in  length,  and  having  an  external  diameter 
varying  from  an  eighth  to  a  tenth  of  an  inch,  with  an  internal  diameter  of  about 
the  twenty-fifth  of  an  inch. 

One  step  more  is  necessary  to  establish  the  origin  of  fulgurites.  This  step 
would  consist  in  producing  an  example  of  the  lightning  being  actually  seen  to 
strike  the  ground  where  a  fulgurite  was  afterward  found,  none  having  been 
there  before.     This  step  is  not  wanting. 

Dr.  Fiedler,  who  has  published  a  work  in  German  on  fulgurites,  supplies  the 
following  facts  : —  , 

An  apothecary  of  Frederichdorf  was  brought  to  two  men  who  had  been 
struck  with  lightning.  He  found  in  the  ground  where  they  lay  two  fulgurites, 
like  those  of  La  Senne. 

On  the  confines  of  Holland,  in  a  sandy  country,  a  shepherd,  after  having 
seen  the  lightning  strike  a  hillock  of  sand,  found  in  the  very  point  where  it 
struck  a  fulgurite. 

On  the  13th  of  July,  1823,  lightning  struck  a  birch-tree  near  the  village  of 
Rauschen,  in  the  province  of  Saralande,  on  the  shores  of  the  Baltic,  and  at  the 
same  time  set  fire  to  a  juniper-bush.  The  inhabitants  ran  to  the  spot,  and  found 
near  the  tree  two  narrow  and  deep  holes.  One  of  them,  notwithstanding  the 
cooling  eff'ect  of  the  rain  which  was  falling,  was  hot  to  the  touch.  Professor 
Hagen,  of  Konigsberg,  examined  these  holes,  and  found  them,  after  excavation, 
to  have  all  the  usual  characters  of  fulgurites. 

The  origin  of  fulgurites  may  then  be  considered  as  demonstrated. 

V.    MECHANICAL    EFFECTS. 


The  mechanical  effects  of  lightning,  seen  in  piercing  solid  bodies  with  holes, 
in  splitting  them  in  pieces,  and  in  projecting  their  fragments  (sometimes  of 
enormous  weight)  to  great  distances,  are  so  well  known,  and  so  generally  ad- 
mitted, that  it  will  be  needless  to  multiply  instances  in  proof  of  it  ;  but  a  cir- 
cumstantial statement  of  some  remarkable  cases  of  this  kind  may  throw  light 
upon  the  manner  in  which  the  electric  fluid  acts. 

In  the  autumn  of  1778  lightning  struck  the  house  of  Casselli,  an  engineer,  at 
Alexandria.  It  did  no  damage,  but  pierced  the  panes  of  glass  in  the  windows 
with  several  small  holes  about  the  sixth  of  an  inch  in  diameter.  Small  cracks 
in  the  glass  diverged  from  these  holes  as  centres. 

In  August,  1777,  lightning  struck  the  steeple  of  the  parish  church  of  St. 


Sepulchre  at  Creraona,  broke  the  iron  cross  which  surmounted  the  tower,  and 
projected  to  a  distance  the  Aveathercock,  which  revolved  under  the  cross,  and 
which  was  made  of  copper,  tinned,  and  coated  with  oil-paint. 

This  weathercock  was  found  to  have  been  pierced  with  eighteen  holes,  nine 
of  which  were  very  prominent  on  one  side,  and  the  other  nine  on  the  other. 
As  there  was  no  appearance  of  more  than  one  stroke  of  lightning,  all  these 
holes  must  be  supposed  to  have  been  pierced  at  once.  The  position  of  the 
holes  are  such  as  would  have  been  produced  by  blows  imparted  simultaneously 
in  opposite  directions  on  parts  of  the  metal  nearly  contiguous,  and  the  inclina- 
tion of  the  beards  or  projecting  edges  of  the  holes  on  one  side  correspond  ex- 
actly with  those  on  the  other,  the  directions  of  all  the  eighteen  beards  being 
parallel. 

On  the  3d  of  July,  1821,  lightning  struck  a  house  at  Geneva,  and  pierced 
the  tin  which  covered  a  part  of  the  roof  with  several  holes,  leaving  evident 
marks  of  fusion.  One  piece  of  tin  in  particular,  which  covered  the  angle  made 
by  a  chimney  with  the  surface  of  the  roof  near  it,  was  pierced  with  three  near- 
ly circular  holes,  about  an  inch  and  three  quarters  in  diameter,  and  about  five 
inches  apart,  measured  from  centre  to  centre.  The  metal  at  the  edges  of  these 
holes  was  bent,  as  it  would  have  been  by  a  force  bursting  through  it  in  one 
direction  or  the  other.  The  edges  of  the  two  holes  were  bent  on  contrary 
sides. 

On  the  night  between  the  14th  and  15th  of  April,  1718,  the  church  of  Goues- 
non,  near  Brest,  was  struck  by  lightning  with  such  force  that  it  shook  as  if  by 
an  earthquake.  The  stones  of  the  walls  were  projected  in  all  directions  to  a 
distance  of  from  fifty  to  sixty  yards. 

The  lightning  which  formerly  struck  the  chateau  of  Clermont,  in  Beauvoisis, 
made  a  hole  twenty-six  inches  wide  and  the  same  depth  in  the  wall ;  the  date 
of  the  building  of  which  was  so  far  back  as  the  time  of  Caesar,  and  which  was 
so  hard  that  a  pickaxe  could  with  difficulty  make  any  impression  upon  it. 

On  the  night  between  the  21st  and  22d  of  June,  1723,  lightning  struck  a  tree 
in  the  forest  of  Nemours.  The  trunk  was  split  into  two  fragments,  one  seven- 
teen and  the  other  twenty-two  feet  long.  These  fragments,  so  heavy,  that  one 
of  them  would  require  the  combined  strength  of  four  men,  and  the  other  that  of 
eight  men,  to  lift  it,  were,  nevertheless,  projected  by  the  lightning  to  the  dis- 
tance of  about  seventeen  yards. 

In  January,  1762,  lightning  struck  the  church  of  Breag,  in  Cornwall,  the 
southwest  pinnacle  of  the  tower  of  which  it  destroyed.  A  stone,  weighing  one 
hundred  and  seventy  pounds,  was  projected  from  the  roof  of  the  church  to  a 
distance  of  sixty  yards  in  the  direction  of  the  south.  Another  fragment  of  stone 
was  projected  to  the  north  to  a  distance  of  four  hundred  yards.  A  third  was 
projected  to  the  southwest. 

About  the  middle  of  the  last  century,  a  rock  of  micaceous  schist,  measuring 
105  feet  long,  10  feet  wide,  and  about  4  feet  thick,  was  struck  by  lightning  at 
Funzie  in  Fetlar,  in  Scotland,  and  was  broken  into  three  principal  fragments, 
not  counting  smaller  pieces.  One  of  these  fragments  twenty-six  feet  long,  ten 
feet  wide,  and  four  feet  thick,  had  been  merely  inverted  in  its  position.  An- 
other twenty-eight  feet  long,  seven  feet  wide,  and  five  feet  thick,  was  projected 
over  the  hill  to  a  distance  of  fifty  yards.  The  remaining  piece,  forty  feet  long, 
was  projected  in  the  same  direction,  with  still  greater  force,  and  fell  in  the  sea. 

On  the  6th  of  August,  1809,  at  Swinton,  about  five  miles  from  Manchester, 
lightning  struck  the  house  of  Mr.  Chadwick,  at  2,  P.  M.  A  sulphureous  vapor 
immediately  filled  the  house.  The  external  wall  of  a  building  erected  against 
the  house  as  a  coal-shed,  was  torn  from  its  foundations,  and  raised  in  a  mass. 
It  was  transported,  maintaining  its  vertical  position,  to  some  distance  from  its 


THE  EFFECTS  OF  LIGHTNING. 


original  place  ;  one  of  its  ends  was  transported  nine  and  the  other  four  feet. 
This  wall  thus  raised  and  transported,  was  composed  of  seven  thousand  bricks, 
which,  independent  of  the  mortar  by  which  they  were  cemented  together, 
would  have  weighed  about  twenty-six  tons.  This  wall  was  eleven  feet  high 
and  three  feet  thick,  and  its  foundation  was  about  a  foot  below  the  level  of  the 
ground.  Above  this  coal-shed  was  a  cistern,  which,  at  the  time  of  the  phe- 
nomenon, contained  a  quantity  of  water,  and  the  shed  contained  about  a  ton  of 
coals. 

If  these  mechanical  effects  could  be  explained  by  supposing  them  to  be  pro- 
duced by  the  moving  force  of  the  electric  fluid  itself  impinging  on  the  bodies 
which  are  struck,  no  difficulty  would  arise  from  the  extreme  lightness  and 
tenuity  of  the  electric  fluid,  for  the  momentum  of  a  body  depends  as  much  on 
iis  velocity  as  on  its  weight,  and  however  subtle  the  electric  fluid  may  be,  it  is 
possible  to  imagine  a  velocity  by  which  it  may  acquire  any  proposed  moving 
force.  There  are,  however,  circumstances  among  the  observed  effects,  which 
cannot  be  explained  by  the  mere  impact  of  any  fluid  upon  the  bodies  struck. 
One  of  those  is,  that  the  fragments  of  bodies  struck  by  lightning  are  usually 
dispersed  in  all  directions,  and  this  is  the  case  even  when  the  fragments  are 
large  and  heavy  masses.  If  the  pinnacle  of  the  church  at  Breag  had  been 
struck  by  the  mechanical  force  of  a  body  moving  in  a  determinate  direction,  it 
could  not  have  happened  that  two  large  and  heavy  masses  of  stone  would  be 
driven,  one  to  a  distance  of  sixty  yards  south,  and  the  other  four  hundred  yards 
north.  If  the  circumstances  attending  bodies  struck  by  lightning  be  attentively 
considered,  it  will  be  apparent  that  they  are  such  as  would  be  produced  by  a 
force  suddenly  called  into  action,  and  directed  outward  from  the  internal  di- 
mensions of  the  body,  so  as  to  burst  it  in  pieces.  If  the  approach  of  lightning 
could  be  shown  to  be  capable  of  producing,  instantaneously,  within  a  body,  a 
highly  elastic  fluid,  such  a  fluid,  in  exerting  an  outward  pressure,  would  burst 
the  body,  exactly  as  the  explosion  of  gunpowder  forces  out  the  ball,  or  failing 
to  do  so,  bursts  the  gun. 

From  what  has  been  established  respecting  the  action  of  free  electricity,  it 
is  evident  that  lightning  will  decompose  the  natural  electricities  of  any  bodies 
which  it  approaches,  drawing  toward  itself  the  fluid  of  its  own  name,  and  re- 
pelling to  the  more  remote  parts  the  contrary  fluid.  If  the  body  be  a  conduc- 
tor, this  decomposition  will  take  place,  and  the  free  electricities  of  opposite 
names  will  be  accumulated  on  opposite  sides  of  it,  and  when  their  tensions  ex- 
ceed that  of  the  atmosphere,  they  will  escape.  If  it  be  not  a  conductor,  then 
the  natural  electricities,  being  forced  asunder  by  the  inductive  action  of  the 
lightning,  may  produce  the  effect  of  a  confined  elastic  fluid,  and  a  separation 
of  the  parts  of  the  body  will  be  the  consequence. 

The  hypothesis  proposed  by  M.  Arago,  to  explain  the  mechanical  effects 
of  lightning  refers  their  origin  to  the  water,  or  other  fluids  contained  in  the 
pores  of  the  body  on  which  the  lightning  acts.  .  Lightning  is  proved  by  obser- 
vation to  evolve  heat  sufficiently  intense  to  reduce  metallic  wires  suddenly  to  a 
state  of  incandescence.  M.  Arago  argues,  that  it  may  therefore  be  reasonably 
inferred  that  it  may  also  produce  a  like  effect  on  the  minute  threads  of  water 
which  pervade  the  interstices  of  certain  bodies.  By  the  experiments  of  MM. 
Dulong  and  Arago,  the  elasticity  of  steam  at  the  temperature  of  500  degrees 
Fahr.  amounts  to  45  atmospheres.  But  this  temperature  is  much  less  than 
that  of  red-hot  iron.  It  may  therefore  be  inferred  that  any  small  portions  of 
water  contained  in  the  pores  of  bodies,  which  suddenly  acquire  as  much  heat 
as  would  render  iron  red-hot,  must  acquire  an  elastic  force  so  enormous  as  to 
be  capable  of  producing  any  of  the  mechanical  effects  which  have  ensued  from 
lightning.     In  foundries,  where  a  small  quantity  of  water  has  accidentally  been 


THE  EFFECTS  OF  LIGHTNING. 


deposited  in  the  mould  in  which  the  hquid  metal  is  poured,  the  most  terrible  < 
explosions  have  taken  place  at  the  moment  the  metal  comes  in  contact  with  | 
the  water.  Admit  that  humidity  is  found  in  the  fissures  and  cells  of  the  blocks  < 
of  stone  which  form  a  building,  and  if  the  thunder  strikes  this  stone,  the  sud-  j 
den  production  of  vapor  within  it  would  break  it,  and  its  fragments  would  be  < 
projected  in  all  directions.  In  like  manner,  the  sudden  formation  of  vapor  in 
the  ground  beneath  the  foundations  of  the  walls  of  a  house  would  be  sufficient  < 
to  raise  the  walls  in  a  mass,  and  transport  them  to  a  distance.  The  circum-  , 
stances  attending  the  action  of  lightning  on  trees  are  still  more  easily  expiica-  < 
ble  by  M.  Arago's  hypothesis,  since  the  sap  and  vegetable  juices,  being  placed  ] 
in  lines  parallel  to  the  direction  of  the  fibres,  the  vapor  which  would  be  formed  ' 
would  split  them  in  pieces  exactly  in  the  manner  in  which  trees  are  observed  [ 
to  be  split  by  lightning. 

This  explanation,  ingenious  as  it  is,  is  not  free  from  objection.  That  water 
may  be  auddenly  and  strongly  heated  by  lightning  when  the  body  which  con- 
tains it  is  a  conductor  of  heat,  may  be  admitted.  But  when  lightning  strikes 
a  large  block  of  stone,  the  heat  must  penetrate  its  dimensions  before  it  can 
reach  the  water  which  may  be  contained  within  them  ;  but  stone  being  almost 
a  non-conductor  of  heat,  its  surface  might  be  fused,  while  its  internal  dimen- 
sions would  not  suffer  a  sensible  elevation  of  temperature,  especially  when  the 
stone  is  exposed  to  the  source  of  heat  only  for  an  instant.  Wood  is  also  a  bad 
conductor  of  heat,  yet  M.  Arago's  hypothesis  seems  to  require  the  admission, 
that  a  tree  struck  by  lightning  is  heated  sufficiently  to  produce  aqueous  vapor 
of  enormous  elasticity,  without  producing  the  combustion,  or  even  the  carbon- 
ization of  the  wood  itself.  The  soil,  or  earthy  matter  at  the  surface  of  the 
ground,  is  also  almost  a  non-conductor  of  heat,  yet  M.  Arago  requires  the 
admission,  that  the  lightning  acting  on  it  produces  a  vapor  from  water  below  it 
of  sufficient  pressure  to  lift  the  wall  of  a  house  and  project  it  to  a  distance. 

None  of  these  difficulties  appear  to  attend  the  supposition  that  the  natural 
electricities  of  non-conducting  bodies,  being  forcibly  decomposed  by  the  prox- 
imity of  the  electric  fluid  which  forms  the  lightning,  and  which  may  he  con- 
ceived to  have  an  almost  infinite  intensity,  their  violent  separation  resisted,  as 
it  would  be,  by  the  non-conducting  quality  of  the  bodies  themselves,  would  be 
attended  with  all  the  effects  which  M.  Arago  ascribes  to  the  sudden  formation 
of  vapor,  without  any  of  the  difficulties  or  objects  which  are  involved  in  that 
supposition. 

If  the  electricity  projected  from  the  thunder-cloud  be  supposed  to  be  positive, 
that  of  the  ground  which  it  approaches  will  necessarily  be  negative,  and  more 
intensely  negative  the  more  intensely  positive  is  the  electricity  coming  from 
the  cloud  and  the  more  nearly  it  approaches  the  ground. 

Whatever  hypothesis  may  be  adopted  to  explain  the  facts,  the  terms  ascend- 
ing and  descending  lightning  may  be  allowed,  if  they  be  understood  to  refer  to 
the  direction  in  which  the  electricity  is  propagated,  as  manifested  by  its  effects. 
Facts  are  not  wanting  to  indicate  the  progress  of  the  electric  influence  upward. 

On  the  24th  of  February,  1774,  lightning  struck  the  steeple  of  the  village  of 
Rouvroi,  to  the  northwest  of  Arras.  A  pavement  composed  of  large  blue 
stones,  which  was  laid  under  the  steeple,  was  violently  raised  upward. 

In  the  summer  of  1787,  lightning  struck  two  persons  who  took  refuge  under 
a  tree  near  the  village  of  Tacon  in  Beaujolois.  Their  hair  was  driven  upivard 
and  found  upon  the  top  of  the  tree.  A  ring  of  iron  which  was  upon  the  shoe  of 
one  of  these  persons  was  found  afterward  suspended  on  one  of  the  upper  branches. 

On  the  29th  of  August,  1808,  lightning  struck  a  small  building  near  the  hos- 
pital of  Salpetriere  in  Paris.  A  laborer  who  was  in  it  was  killed,  and,  after 
the  event,  the  pieces  of  his  hat  were  found  incrusted  on  the  ceiling  of  the  room. 


THE  EFFECTS  OF  LIGHTNING. 


When  trees  have  been  barked  by  lightning,  it  frequently  happens  that  the 
bark  is  stripped  from  the  base  of  the  trunk  upward  to  a  certain  height,  and  the 
upper  part  of  the  tree  is  untouched.  This  occurred  with  several  trees  in  the 
Champs  Elysees  at  Paris,  in  a  storm  which  took  place  in  June,  1778. 

The  leaves  of  trees  which  have  been  struck  by  lightning  often  exhibit  the 
effects  of  heat  on  their  lower  surfaces,  but  not  at  all  on  the  superior  surfaces. 

All  these  effects  M.  Arago  thinks  are  capable  of  being  explained  by  the 
vapor  of  water  issuing  upward  after  being  evolved  by  the  lightning  acting  on 
water  contained  in  the  ground. 

They  are  also  capable  of  explanation  by  the  escape  of  negative  electricity 
from  the  ground  upward. 


VI. MAGNETIC    EFFECTS. 


This  class  of  effects  is  so  well  known,  and  so  perfectly  explained  by  the 
principles  established  in  electro-magnetism,  that  it  will  not  be  necessary  to 
devote  any  space  here  to  the  enumeration  of  instances  of  them. 


VII. — EFFECTS    OF    CONDUCTING    BODIES    ON    LIGHTNING. 

Although  the  properties  of  metallic  substances,  and  other  conductors,  io  ref- 
erence to  lightning,  are  capable  of  being  inferred  by  analogy  from  the  princi- 
ples of  common  electricity,  yet  the  difference  of  the  intensity  of  the  atmo- 
spheric electricity  in  storms,  and  the  artificial  electricity  of  the  machines,  is  so 
enormous  that  it  cannot  be  without  great  utility  to  record  the  circumstantial 
statements  of  those  effects  of  lightning  which  illustrate  the  influence  of  conduc- 
tors when  affected  by  electricity  of  a  tension  so  much  greater  than  any  which 
can  be  obtained  in  ordinary  experiments. 

The  unvarj'ing  preference  which  electricity  gives  to  conductors  over  non- 
conductors in  the  selection  of  its  route,  is  strikingly  illustrated  in  the  following 
narrative,  addressed  to  the  abbe  Nollet,  soon  after  the  discovery  of  the  virtue 
of  conductors  by  the  count  Latour  Landry. 

On  the  29th  of  June,  1763,  in  a  violent  thunder-storm,  lightning  struck  the 
steeple  of  the  church  of  Antrasme,  near  Laval.  It  entered  the  church  and 
fused  or  blackened  the  gilding  of  the  frames  and  borders  of  particular  niches. 
It  blackened  and  scorched  [demi-grillee)  the  cruets  [burettes),  which  lay  in  a 
small  cupboard,  and,  finally,  it  pierced  two  deep  regular  holes  like  those  of  an 
auger  in  a  marble  closet  where  the  church  plate  was  kept,  and  which  was 
placed  in  a  niche  formed  in  a  wall  of  sandstone. 

These  damages  were  repaired ;  the  gilding  was  restored,  the  holes  stopped, 
and  the  painting  renewed.  On  the  20th  of  June,  1764,  lightning  again  struck 
the  steeple.  It  entered  the  church  at  the  same  place  ;  blackened  the  gilding 
which  it  had  blackened  before  ;  fused  that  which  it  fused  before  ;  extended  its 
damage  to  precisely  the  same  limits,  without  exceeding  them  ;  blackened  and 
scorched  [grillee)  the  cruets ;  and,  finally,  reopened  the  two  holes  in  the  mar- 
ble closet. 

The  protection  afforded  by  conductors  to  surrounding  non-conductors,  and 
the  damage  done  by  lightning  in  forcing  its  way  to  the  former,  and  escaping 
from  them  through  the  latter,  is  proved  by  the  following  instances  : — 

When  lightning  struck  the  tower  oi  Newbury,  in  1754,  on  the  occasion  for- 
merly mentioned,  it  first  destroyed  the  superior  part,  which  consisted  of  a  pyr- 
amid of  carpentry  about  seventy  feet  high.  Having  scattered  this  mass  of 
woodwork  it  encountered  a  metaUic  wire  which  descended  through  the  tower 
to  a  point  about  twenty  feet  lower.     It  fused  this  wire  in  several  places,  but 


74 


THE  EFFECTS  OF  LIGHTNING. 


the  carpentry  surrounding  it  suffered  no  damage,  although  the  flash  had  by  no  < 
means  expended  its  force,  as  was  proved  by  its  effects  in  descending  lower.       * 

Arriving  at  the  lower  extremity  of  this  Avire  the  lightning  again  passed  ( 
through  the  carpentry,  which  it  damaged  considerably  ;  and  such  was  its  in-  ' 
tensity,  that  when  it  reached  the  ground  it  tore  up  several  of  the  foundation  ( 
stones  of  the  building,  and  projected  them  to  a  considerable  distance.  ' 

The  power  of  metals  and  similar  conductors  to  give  a  free  passage  to  the  ( 
electric  fluid,  is  not  the  only  quality  from  which  they  derive  importance  in  ref-  ' 
erence  to  atmospheric  electricity.     When  lightning  comes  into  the  neighbor-  , 
hood  of  masses  of  metal,  whether  they  be  exposed  or  covered  by  non-con-  ' 
ductors,  the  lightning  will  force  its  way  to  them,  bursting  through  any  inter-  ! 
vening  non-conducting  bodies,  and  fracturing  or  otherwise   damaging  them. 
This  may  be  easily  explained  by  the  known  effects  of  induction.     The  in- 
ductive action  of  the  lightning,  decomposing  the  natural  electricities  of  the 
m«tal,  attracts  the  fluid  of  the  same  name  to  the  end  nearest  to  it,  and  is  recip- 
rocally attracted  by  it.     The  energy  of  this  attraction  may  be  sufficient  to  pro- 
duce the   effects  Avhich  are  observed.     Lightning  will  also  desert  a  smaller 
metallic  conductor  and  rush  to  a  larger  one,  breaking  its  way  through  interve- 
ning non-conductors.     The  principle  of  induction  is  equally  applicable  to  the 
explication  of  this  effect. 

Lightning  having  struck  a  large  rod  of  iron  placed  on  the  roof  of  the  house  of 
Mr.  Raven,  in  Carolina,  U.  S.,  passed  along  a  brass  wire  which  was  carried 
down  the  external  surface  of  the  wall,  and  connected  with  a  bar  of  metal 
which  was  sunk  in  the  ground.  In  its  descent  the  lightning  fused  all  that 
part  of  the  wire  extending  from  the  roof  to  the  first  floor  above  the  level  of  the 
ground,  without  damaging  the  wall  against  which  the  wire  was  attached.  At  the 
height  of  the  first  floor  it  took  another  course,  deserting  the  wire,  bursting 
through  the  wall,  in  which  it  made  a  large  aperture,  and  entered  the  kitchen. 
The  cause  of  this  singular  deviation  at  right  angles  to  its  former  course  be- 
came manifest,  when  it  was  found  that  a  gun  standing  on  its  stock  rested  with 
its  barrel  against  the  kitchen  wall,  exactly  at  the  place  where  the  lightning  forced 
its  way  through  it.  The  lightning  passed  along  the  barrel  of  the  gun  without 
injuring  it,  breaking,  however,  the  stock,  and  damaging  the  hearthstone  near  it. 

In  the  night  between  the  17th  and  18th  of  July,  1767,  lightning  struck  a 
house  in  the  Rue  Plu7nmet,  in  Paris.  Several  frames  were  suspended  in  one 
of  the  rooms,  one  of  which  only  was  gilt;  this  one  it  attacked,  neglecting  all 
the  others.  A  tin  lantern,  and  two  thin  glass  bottles,  lay  upon  the  table  ;  it 
demolished  the  lantern,  but  spared  the  bottles.  In  another  room  was  placed 
an  iron  stove ;  this  was  destroyed,  while  everything  else  in  the  room  was  un- 
injured. In  another  room  was  a  wooden  chest  containing  several  articles  made 
of  iron  ;  the  chest  was  broken,  and  the  iron  articles  presented  evident  marks  of 
fusion,  yet  half  a  pound  of  gunpowder,  which  was  contained  in  an  open  pow- 
derhorn  which  lay  among  these  articles,  was  not  fired. 

On  the  15th  of  March,  1773,  hghtning  struck  the  liouse  of  Lord  Tilney,  at 
Naples.  A  large  assembly,  consisting  of  not  less  than  five  hundred  persons, 
happened  to  be  in  the  house  at  the  time,  among  whom  were  Saussure  and 
Sir  William  Hamilton.  Almost  all  the  gildings  of  the  rooms,  the  cor- 
nices of  the  ceilings,  the  rods  supporting  the  drapery  of  the  furniture,  the 
gilding  of  chairs  and  sofas,  the  gilded  frames  of  the  doors,  and  the  bell-cords, 
were  fused,  blackened,  or  scaled  off  As  usual,  the  greatest  effects  were  pro- 
duced wherever  the  continuity  of  the  conducting  matter  was  interrupted.  It 
is  certain  that  lightning  sufficiently  powerful  to  fuse  wire  would  kill  a  man.  In 
this  case,  therefore,  lightning  sufficiently  intense  to  produce  death  traversed 
nine  rooms,  containing  five  hundred  persons,  without  injuring  any  one,  its 


THE  EFFECTS  OF  LIGHTNING. 


75 


course  being  confined  to  a  series  of  accidental  conductors  supplied  by  the 
walls  and  furniture. 

In  1759  the  detachment  of  French  soldiers  which  conducted  Captain  Dibden 
a  prisoner  of  war  at  Martinique,  took  shelter  from  rain  under  the  wall  of  a 
small  church  Avhich  had  neither  tower  nor  steeple.  Lightning  struck  the 
building,  killed  two  of  the  soldiers  leaning  against  the  wall,  and  made  a  breach 
in  the  wall,  immediately  behind  them,  four  feet  high  and  three  feet  wide.  On 
examining  the  place,  it  was  found  that  within  the  chapel,  at  the  place  of  the 
breach,  a  collection  of  massive  bars  of  iron  were  placed,  intended  to  support 
a  monument.  Those  soldiers  who  were  not  placed  opposite  to  the  iron  were 
uninjured. 

On  the  10th  of  June,  1764,  lightning  struck  the  steeple  of  St.  Bride's  chmch, 
in  Fleet  street,  London,  and  did  great  damage.  The  weathercock  was  first 
struck  ;  from  that  the  lightning  descended  along  a  bar  of  iron  buried  among 
the  massive  stones  of  which  the  steeple  is  built.  This  bar  was  two  inches  in 
diameter,  and  twenty  feet  long,  and  its  lower  end  was  let  into  a  cavity  five 
inches  deep  in  a  stone,  and  secured  there  by  lead.  The  gilding  on  the  cross 
and  weathercock  was  partly  destroyed,  and  all  that  remained  was  blackened. 
The  soldering  in  several  places  was  fused.  Along  the  descending  bar  no  trace 
of  the  fluid  was  discoverable ;  but  at  its  lower  extremity,  where  the  continuity 
of  the  metal  was  broken,  were  marks  of  violent  effects.  The  stone  in  which 
the  end  of  the  bar  was  inserted  was  broken  in  pieces  :  a  large  breach  was 
made  at  the  same  place  in  the  side  of  the  steeple.  The  lightning  thence 
seemed  to  have  descended  by  leaps  from  one  iron  cramp  to  another  immedi- 
ately below  it.  It  did  not,  however,  confine  its  path  merely  to  the  descending 
direction :  wherever  iron  cramps  were  inserted  within  the  masonry,  to  bind 
the  blocks  of  stone  together,  the  fulminating  fluid  penetrated  and  left  its  marks. 
In  fine,  the  stones  were  split,  broken,  pulverized,  displaced,  and  launched  to  a 
distance  like  projectiles,  in  the  neighborhood  of  the  extremities  of  all  the  bars 
of  iron  used  in  the  construction  of  the  building. 

In  the  case  of  the  house  struck  in  1767,  in  the  Rue  Plummet,  in  Paris,  already 
mentioned,  a  remarkable  example  of  the  influence  of  a  hidden  mass  of  iron  was 
oflfered.  The  only  injury  done  to  the  exterior  of  the  building  was  the  entire 
demolition  of  the  entablature,  behind  which  was  disclosed  a  number  of  large 
pieces  of  iron  used  in  its  construction. 

It  is  evident  from  these  instances,  that  so  long  as  a  continuity  of  metal  is 
aflbrded,  no  damage  is  done  by  lightning.  But  a  continuity  of  any  conducting 
matter  ought  to  produce  the  same  effect. 

If  the  metal  be  continued  to  the  ground,  and  the  ground  be  sufficiently  hu- 
mid to  afford  a  free  passage  to  the  electricity,  no  injurious  effects  ensue,  and 
the  lightning  passes  quietly  into  the  crust  of  the  globe.  But  if  the  ground  be 
dry,  it  becomes  a  non-conductor,  and  the  electricity  escapes  with  an  explosion. 

On  the  28th  of  August,  1760,  lightning  struck  a  bar  of  iron  erected  on  the 
roof  of  the  house  of  Mr.  Maine,  in  the  United  States,  and  partially  fused  it. 
This  bar  descended  to  the  ground,  which  it  penetrated  to  some  depth,  but  the 
soil  not  being  sufficiently  humid,  the  lightning  produced  an  explosion,  broke 
up  the  ground,  and  damaged  the  foundations  of  the  house. 

On  the  5th  of  September,  1779,  at  Manhei?n,  on  the  Rhine,  lightning  struck 
an  iron  bar  raised  on  the  roof  of  the  hotel  of  the  ambassador  of  Saxony,  by 
which  it  was  conducted  along  the  roof  and  walls  of  the  building  to  the  ground. 
The  ground  being  dry,  it  quitted  the  bar  with  an  explosion  which  produced  a 
vortex  of  sand  which  was  witnessed  by  several  persons,  and  of  which  evident 
traces  remained. 

When  the  continuity  of  the  conductor  is  broken,  and  the  lightning  escapes 


THE  EFFECTS  OF  LIGHTNING. 


by  an  explosion,  the  whole  conductor  is  rendered  luminous,  which  never  hap- 
pens when  the  conductor  is  uninterrupted. 

Lightning  struck  the  conductor  on  the  house  of  Mr.  West,  in  Philadelphia, 
and  the  place  where  its  lower  extremity  met  the  ground,  at  about  five  feet  be- 
low the  surface,  being  dry,  the  lightning  escaped  by  explosion.  A  heavy 
shower  fell  at  the  moment,  which  having  moistened  the  pavement,  the  whole  sur- 
face of  the  ground  for  several  yards  around  the  conductor  seemed  to  be  on  fire. 


VIII. EFFECTS    PROCEEDING    FROM    THE    SURFACE    OF    THE    EARTH. 

The  class  of  appearances  now  to  be  noticed  require  the  more  detailed  and 
especial  description,  inasmuch  as  they  are  more  rarely  subjects  of  observation, 
and  many  of  them  are  difficult  to  be  connected  with  the  known  principles  of 
electricity. 

When  storms  are  breaking  in  the  heavens,  and  sometimes  long  before  their 
commencement,  and  when  their  approach  has  not  yet  been  manifested  by  any 
appearances  in  the  firmament,  phenomena  are  observed,  apparently  sympathetic, 
proceeding  from  the  deep  recesses  of  the  earth,  and  exhibited  under  very  various 
forms  at  its  surface.     Instead  of  recounting  this  extraordinary  class  of  physi-  ; 
cal  facts  in  general  terms,  which  from  their  nature  must  want  that  precision  so  I 
desirable  in  such  descriptions,  and  which  are  always  liable  to  inaccuracy  when  ) 
a  legitimate  theory  of  the  phenomena  is  wanting,  we  shall  here  state  the  par-  \ 
ticular  facts  collected  by  the  active  zeal  of  M.  Arago  on  this  interesting  sub- 
ject. 

Davini  wrote  to  Vallisneri  that  he  had  observed,  near  Modena,  a  fountain 
whose  waters  were  clear  or  turbid  according  as  the  sky  was  clear  or  clouded. 
Vallisneri  himself  states  that  he  observed  that  the  salt  marshes  of  Zibia,  Que- 
^eola,  Cassola,  and  also  in  the  duchy  of  Modrma,  and  the  sulphur  springs,  an- 
nounce an  approaching  storm  before  there  is  any  appearance  of  it  in  the  heavens, 
by  a  sort  of  ebullition,  and  by  subterranean  noises  like  that  of  thunder,  and 
sometimes  even  by  actual  thunder. 

ToALDO  relates  that  in  the  hills  of  Vicentino,  at  a  little  distance  from  the 
parish  church  of  Molvena,  there  is  a  fountain  called  by  the  people  of  the  place 
Bifoccio,  because  it  has  two  sources.  When  a  storm  is  approaching,  this  foun- 
tain, even  after  a  long  drought  and  at  times  when  it  is  completely  dry,  gushes 
out  suddenly  and  fills  a  large  canal  with  turbid  water,  which  spreads  over  the 
adjacent  valleys. 

At  two  miles  from  the  source  of  this  fountain,  near  the  parish  church  of 
Villa-raspn,  in  the  court-yard  of  M.  Joseph  Pigati,  of  Vicenza,  is  a  deep  well 
which,  on  the  approach  of  a  storm,  boils  with  such  violence  as  to  terrify  the 
inhabitants  of  the  place. 

It  is  stated  in  the  journal  of  Brugnatelli,  that,  on  the  19th  of  July,  1824,  im- 
mediately after  a  storm,  the  waters  of  the  lake  Massaciuccoli,  in  the  duchy  of 
Lucca,  became  as  white  as  if  a  quantity  of  soap  had  been  dissolved  in  them. 
This  appearance  continued  during  the  following  day,  and  on  the  next  day  mul- 
titudes of  fish  of  every  size  were  found  dead  upon  its  banks. 

No  one  who  has  witnessed  the  local  floods  which  take  place  in  storms  of 
thunder  and  rain  can  fail  to  be  struck  with  the  inadequacy  of  the  quantity  of 
rain,  however  highly  estimated,  which  can  fall  within  given  limits,  to  account 
for  the  enormous  quantity  of  water  discharged  over  plains  and  through  valleys 
from  the  higher  jregions.  Direct  evidence  is  not,  however,  wanting  to  prove, 
that  in  such  cases  the  internal  waters  of  the  earth  are  often  discharged  through 
temporary  fissures,  which  break  open  in  the  sides  of  hills  and  other  places. 
An  occurrence  of  this  kind   took  place  in  Yorkshire,  in   the   month  of  June, 


THE  EFFECTS  OF  LIGHTNING. 


77 


1686,  when  two  villages  were  entirely  destroyed  by  the  flood.  During  a  storm 
an  immense  chasm  was  opened  in  the  side  of  a  hill,  and  a  mass  of  water  issu- 
ing from  it  contributed  much  more  than  the  rain  to  the  flood  which  ensued. 

In  October,  1755,  a  sudden  inundation  produced  immense  ravages  in  Piedmont; 
the  Po  overflowed  its  banks.  This  disaster  was  preceded  by  horrible  thunder  ; 
and  the  unanimous  opinion  of  all  who  witnessed  the  occurrence,  including  the 
celebrated  Beccaria,  who  left  the  record  of  it,  Avas,  that  its  chief  cause  was  an 
immense  volume  of  subterranean  water,  which,  during  the  storm,  suddenly 
issued  from  openings  which  it  made  for  itself  in  the  bosom  of  the  hills. 

It  is  impossible  to  contemplate  these  phenomena  without  calling  to  mind  the 
Mosaic  record  of  the  flood.  In  that  record,  the  source  of  the  waters  by  which 
the  earth  was  submerged  is  stated  not  to  arise  solely  from  the  rain  which  fell 
from  the  clouds — 

"  In  the  six  hundredth  year  of  Noah's  life,  in  the  second  month,  the  seven- 
teenth day  of  the  month,  the  same  day  were  all  the  fountains  of  the  great  deep 
broken  up,  and  the  windows  of  heaven  were  opened." — Gen.  vii.  11. 

The  breaking  up  of  the  fountains  of  the  great  deep,  as  distinguished  from 
the  opening  of  the  windows  of  heaven,  either  has  no  meaning,  or  must  be  taken 
to  express  the  breaking  out  of  the  subterraneous  waters  by  clefts  and  fissures 
in  the  crust  of  the  earth.  That  the  expressions  are  not  accidental  tautology  or 
pleonasm,  is  proved  by  their  repetition  in  the  next  chapter,  where  the  termina- 
tion of  the  flood  is  described  : — ■ 

"  And  God  remembered  Noah,  and  every  living  thing,  and  all  the  cattle  that 
were  with  him  in  the  ark ;  and  God  made  a  wind  to  pass  over  the  earth,  and 
the  waters  assuaged.  The  fountains,  also,  of  the  deep,  and  the  windows  of 
heaven,  were  stopped,  and  the  rain  from  heaven  was  restrained." — Gen.  viii.  12. 

The  rupture  of  the  crust  of  the  globe  by  the  influence  of  the  electricity  of 
the  atmosphere,  exerted,  upon  large  masses  of  subterraneous  water,  would  not 
be  inexplicable,  if  it  could  be  shown  as  a  matter  of  fact  that  the  same  influence 
is  capable  of  producing  a  swelling  and  heaving  upward  of  the  unconfined  wa- 
ters of  the  ocean.     Incontestable  and  recent  evidence  of  this  fact  is  not  want- 


In  April,  1827,  the  packet-ship  New  York,  between  that  port  and  Liverpool, 
was  assailed  by  a  violent  storm,  in  which  the  sea  appeared  to  boil  as  if  a  thou- 
sand submarine  volcanoes  were  in  a  state  of  eruption  at  its  bottom.  Three 
columns  of  water  were  seen  which  arose  toward  the  clouds,  falling  back  in 
foam,  then  rising  anew  to  fall  back  again. 

On  the  Mont  d'Or,  in  Auvergne,  is  an  ancient  building  in  the  middle  of  whicli 
is  a  cistern  hewn  out  of  a  single  block  of  stone  called  Gsesar's  cistern.  In  the 
bottom  of  this  are  two  holes  communicating  with  a  spring  through  which  wa- 
ter rises  with  a  motion  and  noise  like  that  of  ebullition.  Frequent  observations 
have  been  made  on  this  spring  by  Dr.  Bertrand,  who  states  that  it  increases  con- 
siderably when  the  weather  is  stormy.  The  increase  of  noise  which  attends 
it  is  known  among  the  inhabitants  of  the  valley  as  a  presage  of  coming  storms  ; 
it  is  a  sign  which  they  say  never  deceives  them. 

The  celebrated  Duhamel  du  Monceau  states  that  silent  lightnings,  unaccom- 
panied by  wind  or  rain,  called  heat-lightnings,  have  the  property  of  breaking 
the  ears  of  corn.  Farmers  are  well  acquainted  with  this  fact.  On  the  3d 
September,  1771,  Duhamel  himself  witnessed  this  fact;  on  the  morning  of  that 
day  there  was  much  lightning,  and  he  afterward  found  that  all  the  ears  of  corn 
which  were  ripe  were  broken  off"  at  the  nearest  knot.  The  only  ears  which 
remained  standing  were  the  green  ones. 

These  and  similar  effects  indicate  an  influence  emanating  from  the  ground. 
Such  effects  are  not  confined  to  corn,  but  probably  extend  to  all  vegetable  sub- 


stances.     The  following  fact,  as  stated  in  the  Bibliotheque  Britannique  o(  Gene-  < 
va,  for  the  year  1796,  supplies  an  example  of  this  : —  ; 

A  wood  of  oak  situated  on  an  eminence  two  leagues  from  Geneva,  was  < 
barked  in  May,  1795.  This  operation  can  only  be  effected  in  the  season  of  the  i 
year  when  the  sap,  moving  between  the  wood  and  the  bark,  diminishes  suffi-  < 
ciently  the  adherence  of  the  latter  to  enable  it  to  be  separated  with  facility  from  { 
the  tree.  The  workmen  remark,  also,  that  the  state  of  the  atmosphere  produces  ( 
an  evident  influence  on  the  process.  < 

One  day  the  wind  was  blowing  from  the  north  and  the  sky  was  unclouded —  < 
the  bark  was  removed  with  more  than  usual  difficulty.  In  the  afternoon  clouds  < 
rose  in  the  west,  thunder  rolled,  and  at  the  same  instant  the  bark,  to  the  great  ' 
astonishment  of  the  workmen,  fell  spontaneously  from  the  trees.  They  soon  , 
had  reason  to  ascribe  this  to  the  state  of  the  atmosphere,  since  the  effects  ceased  ' 
when  the  storm  passed  away.  , 

There  are  a  multitude  of  popular  impressions  respecting  the  effects  of  thun- 
der, which  have  been  generally  regarded  as  destitute  of  foundation,  and  not  [ 
even  worthy  of  serious  attention.  Such  are  the  received  opinions  that  thunder 
curdles  milk,  renders  wine,  beer,  and  other  fermented  liquors,  sour,  and  taints 
fresh  meat.  After  the  facts,  however,  which  have  been  stated  above,  it  would 
be  rash  to  pronounce  assertions  so  unanimous  of  cooks,  brewers,  winemakers, 
butchers,  &c.,  to  be  false.  Instead  of  being  regarded  as  subjects  of  ridicule 
and  contempt,  such  questions  should  be  submitted  to  serious  experimental  in- 
quiry. 

Among  the  numerous  manifestations  of  the  discharge  of  electric  matter  from 
the  surface  of  the  earth  produced  by  the  influence  of  the  electricity  of  the  air, 
one  of  the  most  circumstantial  and  authentic  is  due  to  Brydone,  who,  being  on 
the  spot  where  the  occurrences  took  place,  was  in  part  witness  to  them,  and 
collected  the  particulars  from  other  eye-witnesses  with  scrupulous  care. 

On  the  10th  July,  1785,  a  storm  broke  out  between  noon  and  one  o'clock,  in 
the  neighborhood  of  CoW-stream.  During  its  continuance,  there  occurred  in 
the  surrounding  country  several  remarkable  accidents. 

A  woman  who  was  cutting  grass  on  the  banks  of  the  Tweed,  was  suddenly 
thrown  down  without  any  apparent  cause.  She  called  her  companions  imme- 
diately to  her  aid,  and  told  them  that  she  received  a  sudden  and  violent  blow 
on  the  soles  of  her  feet,  but  whence  it  proceeded  she  could  not  tell.  At  the 
moment  this  happened  there  was  neither  thunder  nor  lightning. 

A  shepherd  attached  to  a  farm  called  Lennel  Hill,  saw  a  sheep  suddenly  fall 
which  the  moment  before  appeared  in  perfect  health.  He  ran  to  raise  it  from 
the  ground  and  found  it  stiff  dead.  The  storm  was  then  approaching,  but  dis- 
tant. 

Two  coal-wagons,  driven  by  two  boys,  seated  on  the  benches  in  front  of  them, 

had  just  crossed  the  Tweed,  and  were  in  the  act  of  ascending  a  hill  on  the  banks 

of  the  river,  when  a  loud  explosion  was  heard  like  the  report  of  several  guns 

fired  nearly  together,  and  unattended  by  any  rolling  or  continued  sound  like 

that  which  usually  accompanies  thunder.     At  the  moment  of  this  explosion, 

the  boy  who  drove  the  second  wagon  saw  the  foremost  wagon  with  the  two 

'  horses  and  driver  suddenly  fall  to  the  ground,  the  coal  being  scattered  about  in 

I  all  directions.     On  examination,  the  driver  and  horses  were  found  to  be  stiff 

'  dead.     The  coal  which  was  dispersed  had  the  appearance  of  having  been  for 

I  some  time  in  the  fire.     At  the  points  where  the  tires  of  the  wheels  rested  at 

'  the  time  of  the  explosion,  the  ground  was  found  to  be  pierced  by  two  circular 

I  holes,  which  being  examined  by  Brydone,  half  an  hour  after  the  occurrence, 

'  emitted  a  strong  odor  resembling  that  of  ether.     The  tires   of  the  wheels 

)  showed  evident  marks  of  fusion  at  the  points  which  were  in  contact  with  the 


THE  EFFECTS  OF  LIGHTNING. 


79 


road  at  the  moment  of  ihe  explosion,  and  at  no  other  part.  The  hair  was 
singed  on  the  legs  and  under  the  bellies  of  the  horses,  and  by  a  careful  exam- 
ination af  the  marks  left  in  the  dust  of  the  road  where  they  fell,  it  was  appa- 
rent that  they  must  have  been  struck  suddenly  stone  dead,  so  that  no  life  re- 
mained when  they  touched  the  ground.  Had  there  been  any  convulsive  strug- 
gle, the  marks  would  have  been  visible.  The  body  of  the  driver  was  scorched 
in  different  places,  and  his  dress,  shirt,  and  particularly  his  hat,  were  reduced 
to  rags.     A  strong  odor  proceeded  from  them. 

All  the  witnesses  of  this  occurrence  agreed,  that  no  luminous  appearance 
whatever  attended  it.  The  driver  of  the  second  wagon  was  conversing  with 
his  comrade,  and  was  looking  toward  him  at  the  moment  he  was  struck  down, 
being  at  about  twenty  yards  behind  him,  but  saw  no  light.  A  shepherd  stand- 
ing in  an  adjacent  field,  told  Mr.  Brydone  that  he  had  his  eye  on  the  wagon 
at  the  very  instant  of  the  explosion,  but  he  saw  no  light.  He  saw  a  vortex  of 
dust  arise  at  the  place  of  the  explosion,  but  unaccompanied  by  any  luminous 
appearance.  Finally,  Mr.  Brydone  himself  at  the  moment  of  the  ev&nt  was 
standing  at  an  open  window,  with  a  watch  in  his  hand,  explaining  to  the  per- 
sons around  him  the  method  of  calculating  the  distance  of  the  lightning,  by  ob- 
serving the  interval  between  the  flash  and  the  thunder,  and  he  heard  the  ex- 
plosion, but  perceived  no  light. 

The  explanation  of  these  effects  which  naturally  presents  itself  to  a  mind 
conversant  with  the  laws  established  by  experiment  on  artificial  electricity  is 
that  the  natural  electricities  of  some  subterraneous  conductors  are  decomposed 
by  the  inductive  action  of  the  atmosphere,  or  by  other  causes,  and  that  the  flu- 
id thus  liberated  and  accumulated  immediately  under  the  non-conducting  crust 
which  forms  the  surface  breaks  through  that  crust,  and  passes  to  the  nearest 
external  conductor.  Hence  the  fusion  of  the  tires  of  the  wheels  by  electricity 
issuing  from  holes  immediately  under  them. 

The  absence  of  light  in  the  electric  emanations  which  proceed  from  the 
ground  is  not  general.  The  following  statements  coming  from  an  authority  not 
to  be  questioned  will  illustrate  this  : — 

On  the  10th  of  September,  1713,  Maffei  relates,  that  having  been  delayed 
for  some  time  near  the  chateau  of  Fosdinovo,  in  the  territory  of  Massacanara, 
he  took  shelter  from  a  storm  in  the  chateau,  where,  with  the  Marquis  de  Malas- 
pina,  he  was  received  by  the  mistress  of  the  house  in  a  room  situate  on  the 
ground  floor.  There  they  saw  suddenly  appear  on  the  surface  of  the  ground 
a  vivid  flame,  having  a  light  partly  white  and  partly  azure.  This  flame  was 
much  agitated,  but  had  no  progressive  motion.  After  gradually  acquiring  a 
considerable  volume,  it  suddenly  disappeared!'  At  the  instant  of  its  disappear- 
ance Maffei  felt  in  his  shoulder,  proceeding  from  his  back  upward,  a  peculiar 
tickling  sensation  (un  chatouillement  particulier)  ;  plaster  detached  from  the 
ceiling  of  the  room  fell  upon  his  head,  and  in  fine,  he  heard  an  explosion  dif- 
ferent, however,  from  the  sound  of  thunder. 

In  a  letter  addressed  to  Apostolo  Zeno,  Maffei  states  that,  on  the  26tli  of 
July,  1731,  lightning  struck  at  Casalaone,  accompanied  by  thunder  as  loud  as 
a  cannonade,  the  principal  tower,  tore  away  the  escutcheon  bearing  the 
arms  of  the  town,  destroying  the  stone  mouldings,  and  did  other  damage.  This 
occurrence  was  preceded  by  the  appearance  of  a  great  flame  at  a  little  distance 
from  the  ground. 

The  following  statement  is  on  the  authority  of  the  abbe  Richaud : — 

"  On  the  2d  July,  1750,  at  3  o'clock  in  the  afternoon,  being  in  the  church  of 
St.  Michel,  at  Dijon,  during  a  storm  I  saw  appear  suddenly  between  the  first 
two  pillars  of  the  principal  nave  a  red  flame,  which  was  suspended  in  the  air 
at  the  height  of  three  feet  from  the  floor.     This  flame  then  gradually  augment- 


ed  its  volume  until  it  attained  the  height  of  from  twelve  to  fifteen  feet.  After 
having  risen  through  several  fathoms  in  a  diagonal  direction  nearly  to  the  height 
of  the  organ  gallery,  it  disappeared  with  an  explosion  like  the  report  of  a  cannon 
discharged  in  the  church." 

The  fire  evolved  from  the  earth  by  the  influence  of  atmospheric  causes,  is 
not  extinguished  by  passing  through  water. 

On  the  night  between  the  4th  and  5th  of  September,  ]  767,  during  a  violent 
storm,  the  keeper  of  a  fish-pond  near  Parthenai,  in  Poitou,  saw  the  entire  pond 
covered  with  a  flame  so  dense  as  to  prevent  him  from  seeing  the  surface  of  the 
water.     The  next  day  dead  fish  floated  on  the  pond. 

The  existence  of  a  storm  in  the  air  is  not  a  necessary  condition  in  the  causes 
which  govern  the  evolution  of  these  terrestrial  fires. 

On  the  4th  of  November,  1749,  in  latitude  N.  42°  48',  longitude  W.  2^,  a 
few  minutes  before  noon,,  the  sky  being  unclouded,  a  globe  of  bluish  fire,  having 
the  appearance  of  a  mill-stone,  rolled  rapidly  along  the  surface  of  the  sea 
toward  the  British  ship  Montague.  At  a  little  distance  from  the  vessel  it  rose 
vertically  from  the  water  and  struck  the  masts  with  an  explosion  like  that  of 
several  hundred  pieces  of  artillery,  committing  much  damage  to  the  masts  and 
rigging.  Five  sailors  were  laid  senseless  on  the  deck,  one  of  whom  was  se- 
verely burned.  The  usual  effect  of  lightning  were  observed.  A  sulphureous 
odor  was  diffused  through  the  ship,  and  large  iron  nails,  torn  from  various  parts 
of  the  vessel,  were  projected  on  the  deck  with  such  force  that  strong  pincers 
were  necessary  to  draw  them  out. 

Sometimes  luminous  emanations  assume  the  appearance  of  a  cloud  of  light, 
maintaining  a  stationary  position. 

Major  Sabine  and  Captain  James  Ross,  in  their  first  northern  expedition,  being 
in  the  Greenland  seas,  during  one  of  the  dark  nights  of  these  regions,  were 
calkd  up  by  the  officer  of  the  deck  to  observe  an  extraordinary  appearance. 
Ahead  of  the  vessel,  and  lying  precisely  in  her  course,  appeared  a  stationary 
light,  resting  on  the  water  and  rising  to  a  considerable  elevation — every  other 
part  of  the  heavens  and  the  horizon,  all  around  the  ship,  being  as  black  as 
pitch.  As  there  was  no  known  danger  in  this  phenomenon,  the  course  of  the 
vessel  was  not  changed.  When  the  ship  entered  the  region  of  this  light,  tlie 
officers  and  crew  looking  on  with  the  liveliest  interest,  the  whole  vessel  was 
illuminated,  the  most  elevated  parts  of  the  masts  and  sails,  and  the  minutest  parts 
of  the  rigging,  became  visible.  The  extent  of  this  luminous  atmosphere  might 
have  been  about  450  yards.  When  the  bow  of  the  ship  emerged  from  it,  it  seemed 
as  if  the  vessel  were  suddenly  plunged  in  darkness.  There  was  no  gradual  de- 
crease of  illumination.  The  ship  was  already  at  a  considerable  distance  from 
the  luminous  region,  when  it  was  again  visible,  as  a  stationary  light  astern. 

This  narrative  was  addressed  to  M.  Arago  in  a  letter  from  Dr.  Robinson,  of 
Armagh,  who  received  it  from  MM.  Sabine  and  Ross.  "  The  cause  of  these 
phenomena,"  says  M.  Arago,  "  to  use  the  beautiful  expression  of  Pliny,  is  still 
hidden  in  the  majesty  of  nature." 

Besides  these  unusual  luminous  phenomena,  many  philosophers,  among 
whom  are  MafFei  and  Chappe,  have  maintained  that  storms  are  almost  always  at- 
tended by  common  lightning,  which  issues  from  the  earth  and  strikes  the  clouds. 
Nor  are  such  statements  made  in  a  general  and  vague  form,  but  the  partisans 
of  this  doctrine  declare  that  they  have,  themselves,  distinctly  seen  such  light- 
ning rise  like  a  rocket.  If  such  statements  be  correct,  it  must  be  assumed 
that  the  speed  of  this  ascending  lightning  is  infinitely  less  than  that  of  the 
cuspidated  lightning,  since  the  progressive  motion  of  the  latter  cannot  be  ob- 
served. The  ascending  lightning,  if  the  accoimts  of  it  be  correct,  must  be 
analogous  in  its  motion  to  ball-lightning. 


Of  the  flames  which  issue  from  the  earth  and  form  objects  upon  it,  the  most 
common  and  most  frequently  observed  are  those  which  have  appeared  on  the 
points  of  spears,  and  more  frequently  still  on  the  extremities  of  the  masts  and 
yards  of  ships.  These  were  observed  by  and  known  to  the  ancients  long  before 
electricity  assumed  its  place  among  the  sciences.  When  they  appear  in  two 
flames  on  the  masts  and  rigging  of  A'essels  seamen  call  them  Castor  and  Pol- 
lux, when  as  a  single  flame,  Helen.  The  latter  is  regarded  as  an  evil  omen, 
the  former  a  presage  of  a  favorable  voyage.  Passing  over  many  examples  of 
these  phenomena  of  remote  date,  and  which  might  be  considered  of  doubtful 
accuracy,  we  shall  here  state  a  iew  of -the  more  recent  instances  of  them. 

On  the  25th  of  January,  1822,  during  a  heavy  shower  of  snow,  M.  de  Thi- 
elavv,  on  his  route  to  Freyburg,  observed  the  branches  of  the  trees  in  a  heavy 
shower  of  snow,  to  emit  a  bluish  light. 

On  the  14th  of  January,  1824,  immediately  after  a  storm,  a  large  black  cloud 
overspreading  the  sky,  M.  Maxadorf  saw  a  wagon  on  which  a  load  of  straw 
was  transported  into  the  middle  of  a  field,  near  Cathen,  and  observed  that  the 
blades  of  straw  stood  on  end,  and  seemed  to  be  on  fire,  a  vivid  flame  also  is- 
sued from  the  whip  of  the  driver.  This  appearance  lasted  about  ten  minutes 
and  ceased  when  the  wind  had  dispersed  the  cloud. 

On  the  8th  of  May,  1831,  some  officers  of  the  French  engineers  and  artil- 
lery were  walking  after  sunset,  with  their  heads  uncovered,  on  the  terrace 
of  Bab-Azoun,  at  Algiers.  Each  looking  at  the  others  observed  with  unquali-  I 
fied  astonishment,  that  the  hairs  of  his  companions  stood  on  end,  and  little  jets 
of  flame  issued  from  them.  When  the  officers  raised  their  hands,  similar  jets 
issued  from  their  fingers. 

Similar  phenomena  are  seen  to  issue  from  the  pointed  extremities  of  steeples 
and  other  elevated  structures. 

IX.    LUMINOUS    RAIN. 


The  following  are  the  proofs  and  examples  of  the  occurrence  of  this  class 
of  phenomena  collected  by  M.  Arago  : — 

On  the  3d  June,  1731,  Hallai,  prior  of  the  Benedictines  of  Lessay,  near 
Constance,  states  that  he  saw  in  the  evening,  during  a  thunder-storm,  rain  fall 
like  drops  of  red-hot  liquid  metal. 

In  1761,  Bergman  wrote  to  the  Royal  Society  of  London  that  he  observed 
on  two  occasions,  toward  evening,  and  when  no  thunder  was  heard,  rain  which 
sparkled  as  it  struck  the  ground,  which  seemed  to  be  covered  with  waves  of 
fire. 

On  the  morning  of  the  22d  of  September,  1773,  in  the  district  of  Skara,  in 
East  Gothia,  in  Sweden,  a  thunder-storm  broke,  attended  by  very  violent  rain. 
The  rain  commenced  at  six  o'clock  in  the  evening.  All  the  accounts  agree 
in  stating  that  the  drops  struck  fire  and  scintillated   on  touching  the  ground. 

On  the  3d  of  May,  1768,  near  La  Canche,  about  two  leagues  from  Arnay- 
le-Duc,  M.  Pasumot  was  caught  on  an  open  plain  by  a  violent  storm.  The 
rain-water  collected  abundantly  on  the  leaf  of  his  hat,  and  when  he  stooped  his 
head  to  let  it  flow  off",  he  observed  that  in  its  fall,  encountering  that  which  fell 
from  the  clouds  at  about  twenty  inches  from  the  ground,  sparks  were  emitted 
between  the  two  portions  of  liquid. 

On  the  28th  of  October,  1772,  on  his  way  from  Brignai  to  Lyons,  the  abbe 
Bertholon  was  caught  in  a  storm  at  five  o'clock  in  the  morning.  Rain  and  hail 
fell  heavily.  The  drops  of  rain  and  the  hail-stones  which  struck  the  metallic 
parts  of  the  mounting  of  his  horse's  trappings,  emitted  jets  of  light. 

A  friend  of  Howard,  the  meteorologist,  on  his  way  from  London  to  Bow,  on 


vol..  II. 


THE  EFFECTS  OF  LIGHTNING. 


the  19tli  of  May,  1809,  during  a  violent  storm,  saw  distinctly  the  drops  of  rain 
emit  light  when  they  struck  the  ground. 

On  the  25th  of  January,  1822,  the  miners  of  Freyburg  informed  Lampadius 
that  the  sleet  which  fell  during  a  storm,  emitted  light  when  it  struck  the 
ground. 

This  emission  of  light  is  not  peculiar  to  water,  whether  in  a  liquid  or  frozen 
state. 

During  the  eruption  of  Vesuvius,  which  took  place  in  1794,  a  shower  of  dust 
as  fine  as  snuff  fell  in  Naples  and  its  environs.  This  dust  emitted  light,  which, 
though  pale,  was  distinctly  visible  at  night.  Mr.  James,  an  English  gentleman, 
who  happened  at  the  time  to  be  in  a  boat  near  Terra  del  Greco,  observed  that 
his  hat  and  those  of  the  boatmen  and  the  parts  of  the  sails  where  the  dust 
lodged,  shed  around  a  sensible  light. 

These  several  phenomena  seem  capable  of  easy  explanation,  by-  admitting 
the  rain,  hail,  or  snow,  coming  from  the  clouds,  and  the  surface  of  the  earth 
and  objects  upon  it,  to  be  in  opposite  electrical  states. 


POPULAR   PALLACIES. 


Fallacious  Indications  of  Senses. — Errors  of  the  Sense  of  Feeling. — Erroneous  Impressions  of  Heat 
and  Cold. — Explanation  of  these  by  the  Principle  of  Conduction. — "Why  a  Fan  is  cooling. — Feats 
of  the  Fire-King  explained. — Horizontal  Appearance  of  the  Sun  and  Moon. — Deceptive  Oval 
Disk  in  the  Horizon. — Deceptions  of  Vision — of  Taste — of  Smelling. 


POPULAR  FALLACIES. 


85 


POPULAR    FALLACIES. 


Of  all  the  means  of  estimating  physical  effects,  the  most  obvious,  and  those 
upon  which  mankind  place  the  strongest  confidence,  are  the  senses.  The  eye, 
the  ear,  and  the  touch,  are  appealed  to  by  the  whole  world  as  the  unerring  wit- 
nesses of  the  presence  or  absence,  the  qualities  and  degrees,  of  light  and  color, 
sound  and  heat.  But  these  witnesses,  when  submitted  to  the  scrutiny  of  rea- 
son, and  cross-examined,  so  to  speak,  become  involved  in  inexplicable  perplex- 
ity and  contradiction,  and  speedily  stand  self-convicted  of  palpable  falsehood. 
Not  only  are  our  organs  of  sensation  not  the  best  witnesses  to  which  we  can 
appeal  for  exact  information  of  the  qualities  of  the  objects  which  surround  us, 
but  they  are  the  most  fallible  guides  which  can  be  selected.  Not  only  do  they 
fail  in  declaring  the  qualities  or  degrees  of  the  physical  principles  to  which 
they  are  by  nature  severally  adapted,  but  they  often  actually  inform  us  of  the 
presence  of  a  quality  which  is  absent,  and  of  the  absence  of  a  quality  which 
is  present. 

The  organs  of  sense  were  never,  in  fact,  designed  by  nature  as  instruments 
of  scientific  inquiry ;  and  had  they  been  so  constituted,  they  would  probably 
have  been  unfit  for  the  ordinary  purposes  of  life.  It  is  well  observed  by  Locke, 
that  an  eye  adapted  to  discover  the  intimate  constitution  of  the  atoms  which 
form  the  hand  of  a  clock,  might  be,  from  the  very  nature  of  its  mechanism,  in- 
capable of  informing  its  owner  of  the  hour  indicated  by  the  same  hand.  It 
may  be  added,  that  a  pair  of  telescopic  eyes,  which  would  discover  the  mole- 
cules and  population  of  a  distant  planet,  would  ill  requite  the  spectator  for  the 
loss  of  that  ruder  power  of  vision  necessary  to  guide  his  steps  through  the  city 
he  inhabits,  and  to  recognise  the  friends  which  surround  him.  The  compari- 
son of  instruments  adapted  for  the  use  of  commerce  and  domestic  economy, 
and  those  designed  for  domestic  purposes,  furnishes  a  not  less  appropriate 
illustration  of  the  same  fact.  The  highly  delicate  balance  used  by  the  philoso- 
pher in  his  inquiries  respecting  the  relative  weights  and  proportions  of  the  con- 
stituent elements  of  bodies,  would,  by  reason  of  its  very  perfection  and  sensi- 


86 


POPULAR  FALLACIES. 


bility,  be  utterly  useless  in  the  hands  of  the  merchant  or  the  housewife.  Each  ( 
class  of  instruments  has,  however,  its  peculiar  uses ;  and  is  adapted  to  give  ] 
indications  with  that  degree  of  accuracy  which  is  necessary  and  sufficient  for  i 
the  purpose  to  which  it  is  applied.  ] 

The  term  heat  in  its  ordinary  acceptation,  is  used  to  express  a  feeling  or  ' 
sensation  which  is  produced  in  us  when  we  touch  a  hot  body.  We  say  that  [ 
the  heat  of  a  body  is  more  or  less  intense,  according  to  the  degree  in  which  ' 
the  feeling  or  sensation  is  produced  in  us.  The  term  is  often,  however, 
used  in  a  somewhat  different  sense.  It  is  here  applied  to  express  a  cer- 
tain state  of  body,  which  is  attended  with  certain  distinct  mechanical  effects, 
many  of  which  are  capable  of  being  actually  measured,  and  one  of  which  only 
is  the  effect  produced  on  our  organs,  and  through  them,  on  the  mind,  to  which 
alone,  in  the  popular  sense,  the  term  heat  is  applied.  This  distinction  in  the 
use  of  the  term  has  induced  some  philosophers  to  adopt  another  word,  caloric, 
to  express  the  physical  effect,  while  the  common  term,  heat,  has  been  retained 
to  express  the  sensation.  It  does  not  appear  to  us  to  be  necessary  to  adopt  this 
term,  because  it  never  happens  that  any  confusion  arises  from  the  two  senses 
of  the  term  heat ;  and,  besides,  the  use  of  the  term  caloric  is  apt  to  lead  the 
mind  to  the  assumption  of  an  hypothesis,  or  theory,  concerning  the  nature  of 
heat,  the  consequences  of  which  are  apt  to  be  mixed  with  that  investigation 
which  should  be  founded  on  the  results  of  experiment  alone. 

The  touch,  by  which  we  acquire  the  perception  of  heat,  like  the  eye,  ear, 
and  other  organs,  is  endowed  with  a  sensibility  confined  within  certain  limits  ; 
and  even  within  these  we  do  not  possess  any  exact  power  of  perceiving  or 
measuring  the  degree  of  the  quality  by  which  the  sense  is  affected.  If  we 
take  two  heavy  bodies  in  the  hand,  we  shall  in  many  cases  be  able  to  declare 
that  one  is  heavier  than  the  other ;  but  if  we  are  asked  whether  one  be  exactly 
twice  as  heavy,  or  thrice  as  heavy  as  the  other,  we  shall  be  utterly  unable  to 
decide.  In  like  manner,  if  the  weights  be  nearly  equal,  we  shall  be  unable  to 
declare  whether  they  are  exactly  equal  or  not.  If  we  look  at  two  objects,  differ- 
ently illuminated,  we  shall  in  the  same  way  be  in  some  cases  able  to  declare 
which  is  the  more  splendid ;  but  if  their  splendor  be  nearly  equal,  the  eye 
will  be  incapable  of  determining  whether  the  equality  of  illumination  be  exact 
or  not.  It  is  the  same  with  heat.  If  two  bodies  be  very  different  in  tempera- 
ture, the  touch  will  sometimes  inform  us  which  is  the  hotter ;  but  if  they  be 
nearly  equal,  we  shall  be  unable  to  decide  which  has  the  greater  or  which 
the  less  temperature. 

But  even  this  information,  rude  and  unsatisfactory  as  it  is,  is  more  full  than 
that  which  the  evidence  of  the  touch  frequently  furnishes. 

After  what  has  been  explained  in  the  preceding  part  of  this  treatise,  the 
reader  will  have  no  difficulty  in  perceiving  that  feeling  can  never  inform  us  of 
the  quantity  of  heat  which  a  body  contains,  much  less  of  the  relative  quantities 
contained  in  two  bodies.  In  the  first  place,  the  touch  can  never  be  affected  by 
heat  which  exists  in  the  latent  state.  Ice-cold  water,  and  ice  itself,  feel  to  have 
the  same  temperature,  and  to  contain  the  same  quantity  of  heat ;  and  yet  it 
is  proved  that  ice-cold  water  contains  a  great  deal  more  heat  than  ice  ;  nay, 
that  it  can  be  compelled  to  part  with  its  redundant  heat,  and  to  become  ice  ;  and 
that  this  redundant  heat,  when  so  dismissed,  may  be  made  to  boil  a  considera- 
ble quantity  of  water.  But  it  is  not  only  in  the  case  of  latent  heat,  which  can- 
not be  felt  at  all,  that  the  touch  fails  to  inform  us  of  the  quantities  of  heat  in  a 
body.  It  has  been  shown  that  different  bodies  are  raised  to  the  same  tempera- 
ture by  very  different  quantities  of  heat.  If  water  and  mercury,  both  at  the 
temperature  of  32°,  be  touched,  they  will  be  felt  to  be  both  equally  cold ;  and 
if  they  be  both  raised  to  100°  and  then  touched,  they  will  be  felt  to  be  both 


POPULAR  FALLACIES. 


I  equally  warm;  and  the  inference  would  be,  that  equal  quantities  of  heat  must 
)  have  been  in  the  meanwhile  communicated  to  them.  Now,  on  the  contrary,  it 
s  has  been  proved  that,  in  this  case,  the  quantity  of  heat  which  has  been  com- 
;  municated  to  the  water  is  not  less  than  thirty  times  the  quantity  which  has  been 
(  imparted  to  the  mercury.  In  fact,  to  cause  the  same  change  of  temperature, 
)  and,  therefore,  the  same  feeling  of  heat,  in  different  bodies,  requires  very  differ- 
(  ent  quanthies  of  heat  to  be  imparted  to  them.  It  is  plain,  therefore,  that  the 
)  sense  of  touch  totally  fails  in  the  discovery  of  the  quantities  of  heat  which 
s  must  be  added  to  different  bodies,  in  order  to  produce  in  them  the  same  change 
)  of  temperature. 

)       But  it  may  be  said  that  the  thermometer  itself  is  here  in  the  same  predica- 
)  ment  as  the  touch,  and  that  this  scientific  measure  of  heat  likewise  fails  to  in- 
s  dicate  the  quantity  of  that  principle  which  has  been  added  or  subtracted.     Set- 
)  ting  aside,  however,  the  estimation  of  quantities  of  heat,  the  sense  of  touch  is 
S  not  less  fallacious  in  the  indications  which  it  gives  of  temperature  itself ;  and 
)  here,  indeed,  the  error  and  confusion  into  which  it  is  apt  to  lead,  when  unaided 
)  by  the  results  of  science,  are  very  conspicuous.     If  we  hold  the  hand  in  wa- 
)  ter  which  has  a  temperature  of  about  90°,  after  the  agitation  of  the  liquid  has 
S  ceased  we  shall  become  wholly  insensible  of  its  presence,  and  will  be  uncon- 
/  scious  that  the  hand  is  in  contact  with  any  body  whatever.     We  shall,  of  course, 
)  be  altogether  unconscious  of  the  temperature  of  the  water.     Having  held  both 
^  hands  in  this  water,  let  us  now  remove   the  one  to  water  at  a  temperature  of 
)  200°,  and  the  other  to  water   at  the  temperature  of  32°.     After  holding  the 
'  hands  for  sometime  in  this  manner,  let  them  be  both  removed,  and  again  im- 
mersed in  the  water  at  90°  ;  immediately  we  shall  become  sensible  of  warmth 
in  the  one  hand,  and  cold  in  the  other.     To  the  hand  which  had  been  immersed 
in  the  cold  water,  the  water  at  90°  will  feel   hot,  and  to  the  hand  which  had 
been  immersed  in  the  water  at  200°,  the  water  at  90°  will  feel  cold.    If,  there- 
fore, the  touch  be  in  this  case  taken  as  the  evidence  of  temperature,  the  same 
water  will  be  judged  to  be  hot  and  cold  at  the  same  time. 

If,  in  the  heat  of  summer,  we  descend  into  a  cave,  we  become  sensible  that 
we  are  surrounded  by  a  cold  atmosphere ;  but  if,  in  the  rigor  of  a  frosty  win- 
ter, we  descend  into  the  same  cave,  we  are  conscious  of  the  presence  of  a 
warm  atmosphere.  Now  a  thermometer  suspended  in  the  cave  on  each  of  these 
occasions,  will  show  exactly  the  same  temperature,  and,  in  fact,  the  air  of  the 
cave  maintains  the  same  temperature  at  all  seasons  of  the  year.  The  body,  how- 
ever, being  in  the  one  case  removed  from  a  warm  atmosphere  into  a  colder  one, 
and  in  the  other  case,  from  a  very  cold  atmosphere  into  one  of  a  higher  tempera- 
ture, becomes  in  the  latter  case  sensible  of  warmth,  and  in  the  former,  of  cold. 
Thus  we  see  that  the  sensation  of  heat  depends  as  much  on  the  state  of  our 
own  bodies,  as  that  of  the  external  bodies  which  excite  the  sensation  ;  the 
same  body  at  the  same  temperature  producing  different  sensations  of  heat  and 
cold,  according  to  the  previous  state  of  our  bodies  when  exposed  to  it. 

But  even  when  the  state  of  our  bodies  is  the  same,  and  the  temperature  of 
external  objects  the  same,  different  objects  will  feel  to  us  to  have  different  de- 
grees of  heat.  If  we  immerse  the  naked  body  in  a  bath  of  water  at  the  tem- 
perature of  120°,  and,  after  remaining  for  some  time  immersed,  pass  into  a  room 
in  which  the  air  and  every  object  is  raised  to  the  same  temperature,  we  shall 
experience,  in  passing  from  the  water  into  the  air,  a  sensation  of  coldness.  If 
we  touch  different  objects  in  the  room,  all  of  which  are  at  the  temperature  of 
120°,  we  shall  nevertheless  acquire  very  different  perceptions  of  heat.  When 
the  naked  foot  rests  on  a  mat  or  carpel,  a  sense  of  gentle  warmth  is  felt ;  but 
if  it  be  removed  to  the  tiles  of  the  floor,  heat  is  felt  sufficient  to  produce  incon- 
venience.    If  the  hand  be  laid  on  a  marble  chimney-piece,  a  strong  heat  is 


likewise  felt,  and  a  still  greater  heat  on  any  metallic  object  in  the  room.  Walls 
and  woodwork  will  be  felt  warmer  than  the  matting,  or  the  clothes  which  are 
put  on  the  person.  Now,  all  these  objects  are,  nevertheless,  at  the  same  tem- 
perature. From  this  chamber  let  us  suppose  that  we  pass  into  one  at  a  low 
temperature  ;  the  relative  heats  of  all  the  objects  will  now  be  found  to  be  re- 
versed— the  matting,  carpeting,  and  woollen  objects,  will  feel  the  most  warm  ; 
the  woodwork  and  furniture  will  feel  colder  ;  the  marble  colder  still ;  and  metal- 
lic objects  the  coldest  of  all.  Nevertheless  here,  again,  all  the  objects  are 
exactly  at  the  same  temperature,  as  may  be  in  like  manner  ascertained  by  the 
thermometer. 

In  the  ordinary  state  of  an  apartment,  at  any  season  of  the  year,  the  objects 
which  are  in  it  all  have  the  same  temperature,  and  yet  to  the  touch  they  will 
feel  warm  or  cold  in  different  degrees :  the  metallic  objects  will  be  coldest ; 
stone  and  marble  less  so ;  wood  still  less  so ;  and  carpeting  and  woollen  ob- 
jects will  feel  warm. 

When  we  bathe  in  the  sea,  or  in  a  cold  bath,  we  are  accustomed  to  consider 
the  water  as  colder  than  the  air,  and  the  air  colder  than  the  clothes  which  sur- 
round us.  Now  all  these  objects  are,  in  fact,  at  the  same  temperature.  A 
thermometer,  surrounded  by  the  cloth  of  our  coat,  or  suspended  in  the  atmo- 
sphere, or  immersed  in  the  sea,  will  stand  at  the  same  temperature. 

A  linen  shirt  when  first  put  on  will  feel  colder  than  a  cotton  one,  and  a  flan- 
nel shirt  will  actually  feel  warm  ;  yet  all  these  have  the  same  temperature. 

The  sheets  of  the  bed  feel  cold  and  blankets  warm  ;  the  blankets  and  sheets, 
however,  are  equally  warm.  A  still,  calm  atmosphere,  in  summer,  feels  warm  ; 
but  if  a  wind  arises  the  same  atmosphere  feels  cold.  Now  a  thermometer,  sus- 
pended under  shelter,  and  in  a  calm  place,  will  indicate  exactly  the  same  tem- 
perature as  a  thermometer  on  which  the  wind  blows. 

These  circumstances  may  be  satisfactorily  explained,  when  it  is  considered 
that  the  human  body  maintains  itself  almost  invariably,  in  all  situations,  and  at 
all  parts  of  the  globe,  at  the  temperature  of  96°  ;  that  a  sensation  of  cold  is 
produced  when  heat  is  withdrawn  from  any  part  of  the  body  faster  than  it  is 
generated  in  the  animal  system  ;  and,  on  the  other  hand,  warmth  is  felt  when 
either  the  natural  escape  of  the  heat  generated  is  intercepted,  or  when  some 
object  is  placed  in  contact  with  the  body  which  has  a  higher  temperature  than 
that  of  the  body,  and  consequently  imparts  heat  to  it.  The  transition  of  heat 
from  the  body  to  any  object  when  that  object  has  a  lower  temperature,  or  from 
the  object  to  the  body  when  it  has  a  higher  temperature,  depends,  in  a  certain 
degree,  on  the  conducting  power  of  the  objects  severally,  and  the  transition 
will  be  slow  or  rapid,  according  to  that  conducting  power.  An  object,  there- 
fore, which  is  a  good  conductor  of  heat,  if  it  has  a  lower  temperature  than  the 
body,  carries  off  heat  quickly,  and  feels  cold ;  if  it  has  a  higher  temperature 
than  the  body,  it  communicates  heat  quickly,  and  feels  hot. 

A  bad  conductor,  on  the  other  hand,  carries  off  and  communicates  heat  very 
slowly,  and  therefore,  though  at  a  lower  temperature  than  the  body,  is  not  felt 
to  be  colder,  and,  though  at  a  higher  temperature,  not  felt  to  be  warm. 

Most  of  the  apparent  contradictions  which  have  been  already  adduced  in  the 
results  of  sensation,  compared  with  thermometric  indications,  may  be  easily 
understood  by  these  principles. 

When  we  pass  from  a  hot  bath  into  a  room  of  the  same  temperature,  the  air, 
though  at  a  higher  temperature  than  our  body,  communicates  heat  to  it  more 
slowly  than  the  water,  because,  being  a  more  rare  and  attenuated  substance,  a 
less  number  of  its  particles  are  in  actual  contact  with  the  body ;  and  also  such 
particles  as  are  in  contact  with  the  body  take  almost  the  same  temperature  as 
the  body,  and  adhere  to  it,  forming  a  sort  of  coating  or  shield,  by  which  the 


POPULAR  FALLACIES. 


89 


)  body  is  defended  from  the  effects  of  the  hotter  part  of  the  surrounding  atmo- 
)  sphere.  A  carpet,  being  a  bad  conductor  of  heat,  fails  to  transmit  heat  to  the 
/  foot,  and  therefore,  though  at  a  higher  temperature  than  the  body,  creates  no 
s  sensation  of  warmth.  The  tiles  and  marble,  being  better  conductors  of  heat, 
/  and  at  a  higher  temperature  than  the  body,  transmit  heat  readily,  and  metallic 
(  objects  still  more  so :  these,  therefore,  feel  hot.  On  passing  into  a  cold  room, 
/  the  very  contrary  effects  ensue.  Here  all  the  objects  have  a  temperature  be- 
(  low  that  of  the  body ;  the  carpet  and  other  bad  conductors,  not  being  capable 
I  of  receiving  heat  when  touched,  produce  no  sensation  of  cold  ;  wood,  being  a 
(  better  conductor,  feels  cooler ;  marble,  being  a  better  conductor,  gives  a  still 
)  stronger  sensation  of  cold  ;  and  metal,  the  best  of  all  conductors,  produces  that 
S  sensation  in  a  still  greater  degree. 

\  In  cold  temperatures,  the  particles  of  water  which  carry  off  the  heat  from 
s  the  body  are  far  more  numerous  than  those  of  air,  and  therefore  carry  the  heat 
(  off  more  rapidly ;  and,  besides,  they  are  constantly  changing  their  position  ; 
(  the  particles  warmed  by  the  body  immediately  ascend  by  their  levity,  and  cold 
/  particles  come  into  contact  with  the  skin.  Thus  water,  although  a  bad  conductor 
(  of  heat,  has  the  same  effect  as  a  good  conductor,  by  the  effect  of  its  currents. 
(  Sheets  feel  colder  than  the  blankets,  because  they  are  better  conductors  of 
)  heat,  and  carry  off  the  heat  more  rapidly  from  the  body  ;  but  when,  by  the  con- 
^  tinuance  of  the  body  between  them,  they  acquire  the  same  temperature,  they 
\  will  then  feel  even  warmer  than  the  blanket  itself.  Hence  it  may  be  under- 
/  stood  why  flannel,  worn  next  the  skin,  forms  a  warm  clothing  in  cold  climates, 
s  and  a  cool  covering  in  hot  climates. 

)       To  explain  the  apparent  contradiction  implied  in  the  fact  that  the  use  of  a 
)  fan  produces  a  sensation  of  coldness,  even  though  the  air  which  it  agitates  is 
)  not  in  any  degree  altered  in  temperature,  it  is  necessary  to  consider  that  the 
)  air  which  surrounds  us  is  generally  at  a  lower  temperature  than  that  of  the 
body.     If  the  air  be  calm  and  still,  the  particles  which  are  in  immediate  con- 
tact with  the  skin  acquire  the  temperature  of  the  skin  itself,  and,  having  a  sort 
of  molecular  attraction,  they  adhere  to  the  skin  in  the  same  manner  as  particles 
of  air  are  found  to  adhere  to  the  surface  of  glass  in  philosophical  experiments. 
Thus  sticking  to  the  skin,  they  form  a  sort  of  warm  covering  for  it,  and  speed- 
ily acquire  its  temperature.     The  fan,  however,  by  the  agitation  which  it  pro- 
duces, continually  expels  the  particles  thus  in  contact  with  the  skin,  and  brings 
new  particles  into  that  situation.     Each  particle  of  air,  as  it  strikes  the  skin,, 
takes  heat  from  it  by  contact,  and,  being  driven  off,  carries  that  heat  with  it,, 
thus  producing  a  constant  sensation  of  refreshing  coolness. 

Now  from  this  reasoning  it  would  follow  that,  if  we  were  placed  in  a  room 
in  which  the  atmosphere  has  a  higher  temperature  than  96°,  the  use  of  a  fan 
would  have  exactly  opposite  effects,  and,  instead  of  cooling,  would  aggravate 
the  effects  of  heat ;  and  such  would,  in  fact,  take  place.     A  succession  of  hot  ' 
particles  would,  therefore,  be  driven  against  the  skin,  while  the  particles  which  ( 
would  be  cooled  by  the  skin  itself  would  be  constantly  removed.  ' 

It  may  be  objected  to  some  of  the  preceding  reasonings,  that  glass  and  por-  ( 
celain,  though  among  the  worst  conductors  of  heat,  generally  feel  cold.  This,  J 
however,  is  easily  explained.  When  the  surface  of  glass  is  first  touched,  in  < 
consequence  of  its  density  and  extreme  smoothness,  a  great  number  of  particles  > 
come  into  contact  with  the  skin  ;  each  of  these  particles,  having  a  tendency  to  < 
an  equilibrium  of  temperature,  takes  heat  from  the  skin,  until  they  acquire  the  ; 
same  temperature  as  the  body  which  is  in  contact  with  them.  When  the  sur-  ( 
face  of  the  glass,  or  perhaps  the  particles  to  some  very  small  depth  within  it,  ; 
have  acquired  the  temperature  of  the  skin,  then  the  glass  will  cease  to  feel  < 
cold,  because  its  bad-conducting  power  does  not  enable  it  to  attract  more  heat  ) 


POPULAR  FALLACIES. 


!  ffom  the  body.  In  fact,  the  glass  will  only  feel  cold  to  the  touch  for  a  short 
space  of  time  after  it  is  first  touched.  The  same  observation  will  apply  to  por- 
celain and  other  bodies  which  are  bad  conductors,  and  yet  which  are  dense 
and  smooth.  On  the  other  hand,  a  mass  of  metal,  when  touched,  will  continue 
to  be  felt  cold  for  any  length  of  time,  and  the  hand  will  be  incapable  of  warm- 
ing it,  as  was  the  case  with  the  glass. 

A  silver  or  metallic  teapot  is  never  constructed  with  a  handle  of  the  same 
metal,  while  a  porcelain  teapot  always  has  a  porcelain  handle.  The  reason  of 
this  is,  that  metal  being  a  good  conductor  of  heat,  the  handle  of  the  silver  or 
other  metallic  teapot  would  speedily  acquire  the  same  temperature  as  the  water 
which  the  vessel  contains,  and  it  would  be  impossible  to  apply  the  hand  to  it 
without  pain.  On  the  other  hand,  it  is  usual  to  place  a  wooden  or  ivory  han- 
dle on  a  metal  teapot.  These  substances  being  bad  conductors  of  heat,  the 
handle  will  be  slow  to  take  the  temperature  of  the  metal,  and  even  if  it  does 
take  it,  will  not  produce  the  same  sensation  of  heat  in  the  hand.  A  handle, 
apparently  silver,  is  sometimes  put  on  a  silver  teapot,  but,  if  examined,  it  will 
be  found  that  the  covering  only  is  silver  ;  and  that  at  the  points  where  the  han- 
dle joins  the  vessel,  there  is  a  small  interruption  between  the  metallic  covering 
and  the  metal  of  the  teapot  itself,  which  space  is  sufficient  to  interrupt  the 
communication  of  heat  to  the  silver  which  covers  the  handle.  In  a  porcelain 
teapot,  the  heat  is  slowly  transmitted  from  the  vessel  to  its  handle  ;  and  even 
when  it  is  transmitted,  the  handle,  being  a  bad  conductor,  may  be  touched  with- 
out inconA^enience. 

A  kettle  which  has  a  metal  handle  cannot  be  touched,  when  filled  with  boil- 
ing water,  without  a  covering  of  some  non-conducting  substance,  such  as  cloth, 
or  paper,  while  one  with  a  wooden  handle  may  be  touched  without  inconve- 
nience. 

The  feats  sometimes  performed  by  quacks  and  mountebanks,  in  exposing 
their  bodies  to  fierce  temperatures,  may  be  easily  explained  on  the  principle 
here  laid  down.  When  a  man  goes  into  an  oven,  raised  to  a  very  high  temper- 
ature, he  takes  care  to  have  under  his  feet  a  thick  mat  of  straw,  wool,  or  other 
non-conducting  substance,  upon  which  he  may  stand  with  impunity  at  the  pro- 
posed temperature.  His  body  is  surrounded  with  air,  raised,  it  is  true,  to  a  high 
temperature,  but  the  extreme  tenuity  of  this  fluid  causes  all  that  portion  of  it 
in  contact  with  the  body,  at  any  given  time,  to  produce  but  a  slight  effect  in 
communicating  heat.  The  exhibitor  always  takes  care  to  be  out  of  contact 
with  any  good  conducting  substance  ;  and  when  he  exhibits  the  effect  produ- 
ced by  the  oven  in  which  he  is  enclosed,  upon  other  objects,  he  takes  equal 
care  to  place  them  in  a  condition  very  different  from  that  in  which  he,  himself, 
is  placed  ;  he  exposes  them  to  the  effect  of  metal  or  other  good  conductors. 
Meat  has  been  exhibited,  dressed  in  the  apartment  with  the  exhibitor ;  a  metal 
surface,  is,  in  such  a  case  provided,  and  probably  heated  to  a  much  higher  tem- 
perature than  the  atmosphere  which  surrounds  the  exhibitor. 

But  although  the  sense  of  touch  be,  perhaps,  the  most  exposed  to  have  its 
impressions  misinterpreted,  it  is  not  the  only  sense  which  affords  examples  of 
striking  popular  fallacies.     Abundance  of  these  are  offered  in  the  case  of  the  i 
sense  of  sight. 

Every  one  is  familiar  with  the  appearance  of  the  sun  and  moon  when  rising  , 
and  setting.  The  apparently  large  orb  which  they  present  to  the  senses  is  an  ' 
object  of  familiar  notice.  Is  not  every  one  impressed  with  a  convicfion  that  i 
the  apparent  magnitude  of  the  sun  when  it  rises,  glowing  with  a  redness  ac-  J 
quired  from  the  depth  of  air  through  which  its  rays  then  pass,  is  much  greater  < 
than  the  apparent  magnitude  of  the  same  object  at  noonday  1  and  is  not  the  j 
same  impression  admitted  with  respect  to  the  rising  or  setting  full  moon,  com-  / 


POPULAR  FALLACIES. 


91 


pared  with  the  same  object  seen  on  the  meridian  ?  Yet  nothing  is  more  easy  ) 
than  to  prove,  as  a  matter  of  fact,  that  these  impressions  are  fallacious.  Let  ) 
any  one  adopt  any  convenient  method  which  may  occur  to  him,  to  measure  the  j 
apparent  magnitude  of  the  sun  in  the  horizon,  and  again  in  the  meridian,  and  } 
he  will  find  them  the  same.  This  may  be  accomplished  by  extending  two  ) 
threads  of  fine  silk  parallel  to  each  other  in  a  frame,  and  placing  them  in  such  < 
a  position,  and  at  such  a  distance  from  the  eye,  that  when  presented  to  the  sun  j 
or  moon,  in  the  horizon,  they  will,  exactly,  touch  its  upper  and  lower  limb,  so  ( 
that  their  apparent  distance  asunder  will  be  equal  to  the  apparent  diameter  of  \ 
the  lunar  or  solar  disk.  ' 

If  this  arrangement  be  preserved,  and  the  sun   or  moon  be  viewed  in  the  ' 
same  manner  when  at,  or  near,  the  meridian,  it  will  be   found  that  the  threads 
will  equally  touch  its  upper  and  lower  limbs,  and  that  their  interval  will  still 
measure  its  apparent  diameter. 

In  fact,  all  astronomical  telescopes  are  provided  with  an  apparatus  by 
which  observations  of  this  kind  can  be  made  with  the  greatest  accuracy  and 
facility.  There  is  a  system  of  parallel  fibres  or  wires  extended  across  the  field 
of  view,  which  are  removed  toward  or  from  each  other  by  an  adjusting  screw. 
The  magnitudes  of  the  disks  of  the  sun,  moon,  or  planets,  can  be  ascertained 
by  moving  two  of  these  wires  until  one  of  them  shall  touch  the  upper,  and  the 
other  the  lower  limb  of  the  disk.  By  means  of  such  an  instrument,  the  mag- 
nitude of  the  sun  or  moon  in  the  horizon,  and  in  the  meridian,  may  be  meas- 
ured, and  it  is  found  not  to  be  sensibly  different. 

It  will,  therefore,  be  evident  that  whatever  be  the  cause  of  the  illusion,  the 
apparent  magnitude  of  the  sun  or  moon  is  not  greater  at  rising  or  setting  than 
in  the  meridian.  Whence,  then,  it  may  be  asked,  arises  an  impression  so  uni- 
versally entertained  ?  In  fact,  the  moon  is  4,000  miles  further  from  us  when  it 
sets  or  rises,  than  when  it  south's,  or  passes  the  meridian,  and,  strictly  speaking, 
therefore,  its  apparent  diameter,  instead  of  appearing  larger,  ought  to  appear 
about  a  sixtieth  part  less. 

This  illusion  has  been  attempted  to  be  explained  by  supposing  that,  as  the 
moon  is  less  brilliant  in  the  horizon  than  in  the  zenith,  we  open  the  pupil  of 
the  eye  wider  on  looking  at  it  when  in  the  horizon,  and  it  is  for  this  reason  we 
see  it  larger.  But  this  reasoning  is,  obviously,  invalid,  inasmuch  as  we  know 
from  the  principles  of  optics,  that  the  image  produced  in  the  eye  has  the  same 
magnitude  to  whatever  extent  the  pupil  may  be  dilated  or  contracted. 

The  explanation  of  this  singular  effect,  in  which  all  astronomers  appear  to 

concur,  refers  it  to  mental,  and  not  optical   causes  ;  strictly  speaking,  it  is  not 

an  optical  illusion.     The  organ  of  vision  does  not,  itself,  present  to  us  a  larger 

moon  in  the  horizon  than  in  the  zenith,  as  is   proved    incontestably  by  the  mi- 

trometric  wires.     The  error  is,  then,  one  of  the  mind  and  not  one  of  the  sen- 

\  ses.     The  estimate  which  we  form  of  the  actual  magnitude  of  any  visible  ob- 

'  ject,  depends  on  a  comparison  of  the   apparent  magnitude   which  that  object 

I  presents  to  the  eye,  with  the  distance  at  which  we  imagine  it  to  be.     Thus  if 

[  there  be  two  objects,  buildings,  for  example,  which  have  to  the  eye  the  same 

I  apparent  height,  but  which  we  know  or  believe  to  be  at  different  distances  from 

J  us,  we  instinctively,  and  without  any  operation   of  the  judgment,  of  which  we 

)  are  conscious,  conceive  that  which  is  more  distant  to  be  the   largest ;  and  in 

j  like  manner,  if  two  objects,  which  are  at  different  distances,  appear  to  the  eye 

)  to  be  of  different  heights,  the  more  remote  being  less  than  the  nearer,  we  judge 

I  them,  nevertheless,  to  be  equal  in  size,  ascribing,  by  an  unconscious  action  of 

)  the  mind,  the  difference  of  their  apparent  magnitudes  to  their  difference  of  dis- 

?  tance. 

[       To  apply  this  reasoning  to  the  case  of  the  sun  or  moon,  we  are  to  consider 


92 


POPULAR  FALLACIES. 


that  when  either  of  these  objects  are  in  the  horizon,  a  portion,  at  kast,  of  the 
space  between  the  eye  and  them  is  occupied  by  a  series  of  objects  with  the 
magnitudes  and  relative  positions  of  which  we  are  familiar.  We  are,  there- 
fore, enabled  to  make  some  estimate  of  a  portion  of  the  space  that  intervenes 
between  the  eye  and  the  object.  But  when  the  object  is  in  a  more  elevated 
})Osition  in  the  firmament,  no  part  of  the  intervening  distance  is  thus  spaced 
out,  and  we  are  accustomed  to  consider  the  object  nearer  to  the  eye.  It  is  for 
this  reason  that  the  first  impression  produced  upon  the  mind  by  a  view  of  the 
firmament  is  that  of  a  flat,  spherical  vault,  resting  upon  the  circle  of  the  hori- 
zon, the  higher  parts  being  much  nearer  to  us  than  its  horizontal  boundaries. 
This  universal  impression  will  be  readily  acknowledged  by  every  observer. 
Yet  that  it  is  a  mental  and  not  an  optical  deception,  is  proved  by  showing  that 
the  visual  magnitudes  when  measured  are  the  same  for  every  object  at  all  al- 
titudes. 

Conceding  this,  then,  it  will  be  asked  how  it  explains  the  universal  impres- 
sion of  the  enormously  large  disk  of  the  sun  or  moon  whenjising  or  setting, 
the  answer  is,  that  when  in  or  near  the  horizon  the  mind  is  impressed  with  the 
idea  that  the  distance  of  these  objects  is  much  greater  than  when  in  the 
meridian,  and  that  their  apparent  magnitude  being  the  same,  the  real  magni- 
tude is  judged  to  be  greater  in  the  same  proportion  as  the  distance  is  supposed 
to  be  greater.  Thus,  if  we  are  impressed  with  the  notion  that  the  sun  seen  in 
the  horizon  is  twice  as  distant  as  the  sun  seen  in  the  meridian,  we  shall  infer 
its  diameter  to  be  twice  as  great,  since  it  appears  the  same ;  and  if  its  diame- 
ter is  twice  as  great,  its  apparent  superficial  magnitude  will  be  four  times  as 
great. 

The  operations  of  the  judgment  in  such  cases  are  so  rapid,  and  the  effect 
of  habit  is  such,  that  we  are  altogether  unconscious  of  them.  A  thousand  ex- 
amples might  be  given  of  bodily  actions  and  motions  performed  by  the  dictates 
of  the  will,  of  which  we  retain  no  consciousness.  It  is  difficult  in  the  case  we 
have  just  explained,  for  minds  unaccustomed  to  metaphysical  inquiries,  to  sat- 
isfy themselves  of  the  validity  of  the  explanations  we  have  given.  Yet,  if  it 
be  remembered  that  it  is  capable  of  unequivocal  proof  that  the  illusion  is  not 
optical,  and  that,  in  fact,  the  apparent  magnitude  of  the  moon  in  the  horizon 
and  the  meridian  are  not  different,  it  will  easily  follow  that  the  error  must  be 
mental,  and  the  only  explanation  which  has  ever  been  given  of  it  is  that  which 
we  have  here  offered. 

While  referring  to  the  subject  of  the  appearance  of  the  sun  and  moon  at  ri- 
sing and  setting,  I  may  take  the  opportunity  of  noticing  the  oval  form  which 
they  present,  the  vertical  diameter  being  shorter  than  the  horizontal  diameter. 
This  is  not,  as  in  the  former  case,  an  optical  illusion  ;  it  is  an  effect  produced 
by  the  power  of  the  atmosphere  to  deflect  the  rays  of  light  which  are  transmit- 
ted through  it.  By  this  principle  of  refraction,  all  objects  appear  at  a  greater 
altitude  than  that  which  they  really  have ;  and  this  error  of  position  increases 
as  they  approach  the  horizon.  In  accordance  with  this  principle,  the  upper 
limb  of  the  sun  is  less  elevated  than  the  lower  limb,  and,  consequently,  the 
two  limbs  are  brought  nearer  together  than  they  would  be  if  equally  affected 
by  refraction.  On  the  other  hand,  the  extremities  of  the  horizontal  diameters 
being  equally  affected,  its  length  is  not  altered.  Since,  therefore,  the  vertical 
diameter  is  shortened,  and  the  horizontal  diameter  unaltered,  the  figure  becomes 
an  oval.  Strictly  speaking,  this  is  the  case  with  the  sun  and  moon  in  all  parts 
of  the  heavens  except  in  the  zenith ;  but  the  effect  is  so  slight  that,  except  at 
low  altitudes,  it  is  not  perceptible. 

The  cause  of  the  red  color  which  the  sun  and  moon  have  when  near  the 
horizon  is,  that  the  atmosphere  through  which  the  light  passes,  being  generally 


POPULAR  FALLACIES. 


93 


charged  more  or  less  with  cloudy  matter ;  the  bluish  tints  are  absorbed,  and 
the  predominance  of  the  red  light  transmitted. 

There  is  perhaps  no  sense  which  more  requires  the  vigilant  exercise  of  the 
understanding  to  rectify  its  impressions,  than  that  of  sight.  The  susceptibility 
of  the  organ  of  vision  itself  is  liable  to  frequent  and  rapid  change,  and  the 
same  objects  at  different  times  produce  upon  it  extremely  different  impres- 
sions. A  situation  in  which,  in  one  condition  of  the  eye,  we  shall  appear  to 
be  in  absolute  darkness,  will  present  to  us,  in  another  state  of  the  organ,  sufR- 
cient  light  to  render  visible  the  objects  around  us.  If  we  are  suddenly  de- 
prived of  the  illumination  of  any  strong  artificial  light,  we  appear  to  be  for  the 
moment  in  absolute  darkness  ;  but  when  the  organ  of  vision  has  had  time  to 
recover  itself,  we  often  find  that  there  is  sufEcient  light  to  guide  us. 

"  Thus  when  the  lamp  that  lighted 
The  traveller  at  first  goes  out. 
He  feels  awhile  benighted, 

And  lingers  on  in  fear  and  doubt. 

"  But  soon,  the  prospect  clearing, 
In  cloudless  starlight  on  he  treads, 
And  finds  no  lamp  so  cheering 

As  that  light  which  heaven  sheds." 

Thomas  Moore. 


The  mechanism  which  the  all-wise  Artisan  that  made  the  eye  has  contrived 
to  meet  these  contingencies  is  marked  by  the  same  perfection  that  prevails 
through  all  his  works.  The  opening  in  the.  front  of  the  eye,  called  the  pupil, 
through  which  light  is  admitted  to  produce  vision,  is  surrounded  by  an  elastic 
ring,  called  the  iris,  which  is  capable  of  being  contracted  or  enlarged  by  the 
action  of  certain  muscles  with  which  it  is  connected.  It  is  the  magnitude  of 
this  opening  that  determines  the  quantity  of  light  transmitted  to  the  retina.  If, 
then,  we  are  in  a  room  illuminated  with  a  strong  lamp,  the  muscles  which 
govern  the  opening  of  the  pupil  contract  its  dimensions  until  so  much  light  only 
is  admitted  as  is  consistent  with  the  healthful  condition  of  the  eye.  If  the 
lamp  be  suddenly  extinguished,  and  the  room  be  left  dependant  only  on  the 
light  admitted  by  the  windows,  from  the  nocturnal  firmament,  we  shall  at  first 
appear  to  be  in  profound  darkness,  but  immediately  the  pupil  will  begin  to  ex- 
pand, and  will  presently  become  so  enlarged  that  enough  of  light  will  be  re- 
ceived into  the  eye  to  render  the  objects  around  us  faintly  visible. 

If  in  this  condition  of  the  organ  the  lamp  again  be  suddenly  brought  into  the 
room,  the  eye  will  be  pained  by  its  light,  and  the  eyelid  will  immediately  drop 
to  give  it  relief ;  for  the  enlargement  of  the  pupil  which  has  taken  place  to  ac- 
commodate it  to  the  faint  light  to  which  it  was  previously  exposed,  will  admit 
so  great  a  quantity  of  the  strong  light  of  the  lamp  as  to  hurt  the  retina,  and  the 
contraction  of  the  pupil  cannot  be  effected  with  sufficient  rapidity  to  protect  the 
organ  from  this  injury.  But  the  beneficent  Maker  of  the  eye  has  provided  for 
this  purpose  the  eyelid,  which  is  capable  of  closing  instantaneously,  and  which 
gives  the  pupil  time  to  contract,  and  to  accommodate  its  dimensions  to  the 
new  condition  to  which  it  is  exposed. 

The  perception  we  receive  of  the  color  of  an  object  depends  often  as  much 
on  the  condition  of  the  eye  when  the  object  is  seen  as  upon  the  object  itself. 
By  the  action  of  lights  of  different  colors,  the  sensibility  of  the  retina  may  be 
so  modified  that  the  same  object  will  appear  at  different  times  to  have  different 
colors,  and  unreal  objects  will  often  be  perceived.  These  are  called  spectra. 
If  we  place  on  a  sheet  of  white  paper  a  red  wafer,  and,  illuminating  it  strongly, 
direct  the  eye  steadily  to  it  for  a  short  time,  and  then  look  at  the  paper  close 
beside  it,  we  shall  there  see  a  blue  wafer  of  the  same  size.     This  object  is  an 


94 


POPULAR  FALLACIES. 


optical  spectrum.  The  cause  of  its  appearance  is  easily  explained.  By  the  ac- 
tion of  the  strong  red  light  proceeding  from  the  wafer,  the  retina  is  rendered  for 
the  moment  insensible  to  the  operation  of  a  more  feeble  red  light  upon  it,  for 
the  same  reason  as  the  ear  would  be  insensible  to  the  ticking  of  a  clock  imme- 
diately after  being  affected  by  a  discharge  of  artillery.  Accordingly,  when  the 
eye,  after  viewing  the  red  wafer,  looks  at  a  white  paper  beside  it,  the  action  of 
that  portion  of  the  compound  white  light  reflected  from  the  paper  which  is 
red  fails  to  produce  any  perception,  and  the  remaining  constituents  are  not  per- 
ceived, which  accordingly  present  a  bluish  tint.  To  comprehend  this,  and 
other  similar  illusions,  it  is  very  necessary  to  remember  that  white  light  is  a 
compound  of  reds,  yellows,  and  blues,  and  that  if  we  deprive  it  of  any  one  of 
these  elements  it  will  assume  the  tint  produced  by  the  others.  Thus,  if  the 
eye  be  insensible  to  red  light,  all  white  objects  will  appear  to  it  with  a  tint 
composed  of  yellow  and  blue.  If  it  be  insensible  to  blue  light,  then  white  ob- 
jects will  appear  orange. 

The  eye  may  be,  and  sometimes  is,  either  from  disease,  or  from  original  im- 
perfection of  structure,  either  imperfectly  sensible  or  altogether  insensible  to 
lights  of  particular  colors.  To  such  eyes  all  objects  will  appear  to  have  col- 
ors different  from  those  which  they  present  to  organs  of  vision  in  the  usual 
healthy  state.  We  can  thus  easily  understand  the  condition  of  a  jaundiced  eye. 
Such  an  organ  is  more  or  less  insensible  to  the  blue  and  red  lights,  but  highly 
sensitive  to  the  yellow.  White  objects  to  such  an  eye  will  appear  yellow,  and 
all  other  objects  will  appear  in  tints  different  from  their  proper  colors,  and  par- 
taking more  or  less  the  yellow  hue. 

Instances  have  more  than  once  occurred,  and  are  recorded  in  the  works  on 
optics,  of  individuals  thus  incapable,  from  original  defects  of  vision,  of  perceiv- 
ing particular  colors.  The  late  Doctor  Dalton,  of  Manchester,  was  a  conspic- 
uous example  of  this. 

But,  as  we  have  above  stated,  even  a  healthy  and  perfect  eye  will  be  ren- 
dered temporarily  insensible  to  the  impression  of  particular  colors  by  being 
exposed  for  a  short  time  to  the  strong  action  of  colored  lights.  Optical  illu- 
sions are  produced  in  this  way  in  the  exhibition  of  fireworks.  When  luminous 
balls,  some  red  and  some  white,  are  thrown  up  into  the  air,  the  white  appear 
blue  beside  the  red,  and  are  generally  imagined  to  be  really  blue.  The  effect, 
however,  is  a  visual  illusion,  ascribable  to  the  cause  just  explained. 

In  the  sky  toward  sunset,  when  reddish  clouds  are  arranged  with  openings 
between  them,  the  sky  at  such  openings  appears  green,  although  it  be  really 
blue. 

In  astronomical  observations  on  the  stars  there  is  a  curious  case,  in  which 
it  has  never  been  settled  whether  the  apf>earance  is  real  or  illusive.  Many  of 
the  stars,  which  to  the  eye  appear  individual  objects,  prove  to  be  double  when 
examined  with  powerful  telescopes.  The  two  stars,  thus  composing  a  double 
star,  are  frequently  of  different  colors,  and  it  is  found  that  when  one  is  red  the 
other  is  of  a  bluish  tint.  Now  we  know  that  it  would  appear  of  this  tint,  even 
though  it  were  a  white  object,  by  reason  of  the  presence  of  the  red  star. 
Whether,  in  these  cases  of  double  stars,  the  blue  one  would  be  really  blue,  or 
is  rendered  so  by  the  optical  effect  adverted  to,  has  not  been  decided,  it  being 
impossible  to  view  it  except  in  juxtaposition  with  its  red  companion. 

If  the  eye  be  directed  to  the  sun  for  a  few  seconds,  and  the  eyelids  then  be 
closed,  a  blue  spectrum  of  the  sun  will  be  seen,  and  will  continue  to  be  visible 
until  the  retina  recover  its  state  of  repose. 

If  we  write  a  page  or  two  with  red  ink,  and  then  commence  to  write  with 
black  ink,  the  writing  will  appear  of  a  light  blue  color,  and  will  continue  to  ap- 
pear so  until  the  retina  loses  the  impression  made  by  the  red  ink  upon  it.     In 


POPULAR  FALLACIES. 


95 


passing,  however,  from  the  black  to  the  red,  no  illusion  is  produced,  the  black  not 
acting  on  the  retina  so  as  to  excite  it. 

If  small  holes  be  made  in  a  red  curtain,  so  as  to  admit  the  rays  of  the  sun 
through  them,  the  light  which  will  be  thrown  upon  a  sheet  of  white  paper  will 
be  the  general  redness  produced  by  the  semi-transparency  of  the  curtain,  with 
the  white  spots  produced  by  the  light  passing  through  the  holes  ;  but  these 
white  spots  will  appear  to  the  eyes  blue. 

It  will  appear,  from  these  observations,  that  effects  are  produced  by  the  juxta- 
position of  colors  in  objects  of  art  independent  of  the  separate  properties  of  the 
colors  themselves.  Two  colors,  when  seen  in  juxtaposition,  do  each  of  them 
appear  to  the  eye  different  from  what  either  would  appear  to  be  if  seen  separate- 
ly from  the  other. 

The  senses  of  smelling,  tasting,  and  even  of  feeling  or  touch,  are  liable  to 
innumerable  causes  of  deception.  If  the  organ  at  the  time  it  receives  an  im- 
pression be  in  any  unusual  condition,  or  even  out  of  its  usual  position,  the  in- 
dication of  the  impression  will  be  fallacious. 

If  two  fingers  of  the  same  hand,  being  crossed,  be  placed  upon  a  table, 
and  a  marble  or  a  pea  is  rolled  between  them,  the  impression  will  be,  if  the 
eyes  are  closed,  that  two  marbles  or  two  peas  are  touched. 

If  the  nose  be  pinched,  and  cinnamon  be  tasted,  it  will  taste  like  a  common 
stick  of  deal.  This  is  not  a  solitary  instance.  Many  substances  lose  their 
flavor  when  the  nostrils  are  stopped.  Nurses,  therefore,  upon  right  and  scien- 
tific principles  stop  the  noses  of  children  when  they  give  them  doses  of  disa- 
greeable medicine. 

If  things  having  different  or  opposite  flavors  be  tasted  alternately,  in  such 
rapid  succession  as  not  to  allow  the  nerves  of  tasting  to  recover  their  state  of 
repose,  the  power  of  distinguishing  flavor  will  be  lost  for  the  moment,  and  the 
substances,  however  different,  will  be  undistinguishable  from  one  another. 
Thus,  if  the  eyes  be  blindfolded,  and  buttermilk  and  claret  be  alternately  tasted, 
the  person  tasting  them,  after  a  few  repetitions  of  the  process,  will  be  unable 
to  distinguish  one  from  the  other. 

Tastes,  like  colors,  in  order  to  produce  agreeable  effects,  should  succeed 
each  other  in  a  certain  order.  Eating,  considered  as  one  of  the  fine  arts  in  the 
most  refined  state  of  society,  is  regulated  by  principles,  and  nothing  can  shock 
the  habits  and  rules  of  epicureanism  more  than  the  violation  of  certain  rules 
in  the  succession  and  combination  of  dishes.  It  is  maintained  that  perfection 
in  the  art  of  cookery  and  the  observance  of  its  principles  at  table  is  the  surest 
mark  of  a  nation's  attainment  and  of  the  highest  state  of  civilization. 

Of  all  the  organs  of  sense,  that  whose  nervous  mechanism  appears  to  be  most 
easily  deadened  by  excessive  action  is  that  of  smelling.  The  most  delightful 
odors  can  only  be  enjoyed  occasionally,  and  for  short  intervals.  The  scent 
of  the  rose,  or  the  still  more  delicate  odor  of  the  magnolia,  can  be  but  fleeting 
pleasures,  and  are  destined  only  for  occasional  enjoyment.  He  who  lives  in 
the  garden  cannot  smell  the  rose,  and  the  woodcutter  in  the  southern  forests  is 
insensible  to  the  odor  of  the  magnolia. 

Persons  who  indulge  in  the  use  of  artificial  scents  soon  cease  to  be  conscious 
of  their  presence,  and  can  only  stimulate  their  jaded  organs  by  continually  chan- 
ging the  objects  of  their-  enjoyment. 

One  of  the  most  curious  and  most  incomprehensible  illusions  of  the  senses 
is  the  singularly  erroneous  estimate  which  we  make  of  the  number  of  objects 
of  any  kind  that  are  presented  to  us.     A  striking  example  of  this  is  presented 
by  tile  impression  made  upon  the  eye  by  a  view  of  the  firmament  on  a  clear  . 
starlight  night.     The  number  of  visible  stars  are  always  immensely  over-esti-  ' 
mated.     Although  it  be  true  that  the  stars  are,  strictly  speaking,  countless  in  ( 


96 


POPULAR  FALLACIES. 


number,  yet  the  number  distinctly  seen  by  the  naked  eye  at  any  one  time,  un- 
aided by  the  telescope,  is  not  great.  Any  one  can  satisfy  himself  of  this 
by  examining  any  good  map  of  the  stars  ;  yet,  when  we  look  at  the  firmament  < 
on  a  clear  night,  these  objects  appear  to  be  inconceivably  numerous.  This  ; 
illusion  is  dispelled  by  examining  the  heavens  through  the  most  ordinary  tele-  < 
scope,  or  even  by  looking  through  a  long  tube,  which  will  limit  the  view  at  any  < 
one  moment  to  a  small  portion  of  the  firmament.  On  the  entire  sphere  of  the  j 
heavens  there  are  not  above  twenty  stars  of  the  first  magnitude,  and  it  is  seldom  < 
that  as  many  as  six  or  eight  of  these  can  be  seen  at  once.  The  number  of  ] 
stars  of  the  second  magnitude  does  not  exceed  fifty,  and  of  these  twenty  are  ( 
seldom  seen  at  any  one  time.  The  stars  of  the  third  magnitude  may  amount  * 
to  about  two  hundred,  half  of  which  only  can  be  at  the  same  time  above  the  ( 
horizon.  The  smaller  stars  are  much  more  numerous,  but  they  are  discernable  ' 
with  difficulty,  and  do  not  produce  upon  the  mind  the  impression  of  multitude  i 
that  we  conceive.  ' 

I  have  explained,  on  another  occasion,  that  the  membrane  of  the  eye,  which 
is  affected  by  light,  retains  the  impression  it  has  received  for  about  the  tenth 
of  a  second  after  the  cause  which  produced  the  impression  has  been  removed. 
When  a  lighted  stick  is  whirled  in  a  circle,  the  circle  will  appear  to  be  one 
continuous  line  of  light,  because  the  eye  retains  the  impression  which  the  light 
produces  upon  it  at  any  point  in  the  circle  until  the  stick  returns  to  that  point. 
The  light  is,  therefore,  visible  at  the  same  time  at  every  point  of  the  circle. 

Ingenious  optical  toys  are  constructed,  the  effects  of  which  are  explicable  on 
these  principles.  The  same  object  is  painted  on  the  several  divisions  of  cir- 
cumference of  a  circle  in  a  succession  of  different  attitudes,  and  while  the  eye 
is  directed  to  the  highest  point  of  the  circle,  through  an  opening  made  for  that 
purpose,  the  circle  is  made  to  revolve,  and  the  object  passes  before  the  eye  in 
a  succession  of  diff^erent  attitudes.  If  the  velocity  with  which  the  circle  turns 
be  such  that  the  eye  shall  retain  the  impression  of  the  object  in  one  attitude 
until  its  picture  in  another  attitude  comes  into  view,  it  will  have  all  the  effect 
of  a  moving  object.  Waltzing  figures  and  other  similar  devices  are  painted  on 
circular  cards  and  mounted,  so  as  to  give  these  effects. 

If  the  eye  is  supplied  with  no  external  means  of  knowing  the  distance  of  a 
visible  object,  it  estimates  that  distance  by  its  apparent  magnitude,  and  if  there 
be  any  means  of  causing  the  magnitude  of  the  same  objects  to  undergo  a  grad- 
ual change,  the  impression  on  the  spectator  is  as  if  the  object  advanced  to  or 
receded  from  him.  It  is  upon  this  principle  that  the  exhibitions  of  phantasma- 
goria are  made.  The  image  of  an  object  is  formed  on  some  surface  prepared 
to  receive  it,  the  apartment  being  elsewhere  in  complete  darkness,  so  that  the 
observer  has  no  means  of  knowing  where  the  image  is  formed.  The  magic 
lantern  has  a  power,  by  advancing  it  gradually  toward  the  surface,  to  diminish 
the  size  of  the  image  indefinitely,  and  by  drawing  it  from  the  surface  to  aug- 
ment it.  The  spectators,  therefore,  see  the  images  gradually  increase  and  di- 
minish, and  imagine  it  gradually  to  approach  to  and  recede  from  them. 


■*..^S*^Vrt 


PROTECTION  FROM  LIGHTIIIG. 


Danger  proportionate  to  the  Magnitude,  not  to  the  frequency  of  the  Evil. — Ancient  Methods  of  avert- 
ing Lightning. — Persons  in  Bed  not  Secure,  as  some  think. — Augustus's  Sealskin  Cloak  as  a  Light- 
ning Protector. — Influence  of  Color  on  the  Electric  Fluid. — Tiberius's  Crown  of  Laurel  as  a  Light- 
ning Protector. — The  Danger  of  taking  Shelter  beneath  Trees. — Futility  of  taking  Shelter  in  Glass 
Cages. — Metal  about  the  Person  destroyed  by  Lightning. — Metal  Appendages  to  be  laid  aside. — 
Lightning  Explosions  occur  at  the  Points  where  it  leaves  or  enters  a  Metal. — Part  of  a  Room 
which  is  most  Safe. — Lightning  more  likely  to  discharge  among  a  Crowd  than  on  a  single  Individ- 
ual.— Influence  of  the  Vapor  of  Transpiration,  &c. — Certain  Individuals  are  comparative  Non- 
conductors.— Thunder-Clouds  have  been  traversed  with  Impunity. — Thunder-Storms  below  the 
Place  of  Observation. — Paratonnerres,  or  Lightning  Conductors. — Lightning  Conductors  pro- 
tective even  when  no  Flash  strikes  them. — Sparks  at  the  Interval  where  a  Conductor  is  dis- 
jointed.— Lightning  Conductors  drain  oS^  the  Electricity  of  Clouds. — Sparks  or  luminous  Aigrettes 
on  the  Point  of  Conductors. — More  frequent  Occurrence  at  Sea. — Influence  of  Elevation  of  a  Par- 
atonnerre. — Experimental  Illustration. — Electric  Kites. — Captive  Balloons  as  Paragreles  and  for 
Meteorological  Research. — Pointed  and  blunt  Conductors. — duantity  of  Lightning  drawn  down 
by  a  Conductor. — Mr.  Harris's  Conductors  for  Ships. — Assumed  Extent  of  the  protecting  Power 
of  a  Paratonnerre. — Not  based  on  experimental  Grounds. — Cases  against  its  general  Application. — 
Lightning  does  not  alway  strike  the  highest  Points. — Lightning  Conductors  with  many  Points. — 
A  Lightning  Conductor  must  have  sufiicient  Capacity. — A  Lightning  Conductor  must  be  in  good 
Connexion  with  the  moist  Sub-Soil. — Charcoal  Beds  to  receive  the  Base  of  the  Conductor. — "Vici- 
nal metallic  Conductors. — Conductors  of  metallic  Wire-Rope  ;  Insulation  not  needed. — Conduct- 
ors for  Powder  Magazines. — EiRcacy  of  Lightning  Conductors. — Lateral  or  divided  Discharge 
defined  ;  its  Cause. — More  readily  obtained  from  Conductors  than  from  Leyden  Discharges. — 
Line  or  Lines  of  least  Resistance. — Absolute  Necessity  of  connecting  the  Conductor  with  vici- 
nal Bodies. — Artificial  Means  of  producing  the  Electrical  Odor. — Chemical  Changes. — Fusion. — 
Fulgurites. — Mechanical  EiTects. — Effects  of  conducting  Bodies. 

VOIi.  II.— 7 


PROTECTION  FROM  LIGHTNING. 


PROTECTION  FROM  LIGHTIING. 


The  apprehension  of  danger  from  lightning,  and  the  solicitude  to  discover 
and  adopt  means  of  security  against  it,  are  proportionate  to  the  magnitude  of 
the  evils  it  produces  rather  than  the  frequency  of  their  occurrence.  The  chan- 
ces which  any  individual  of  the  population  of  a  large  city  incurs  of  being  struck 
with  lightning  during  a  storm  are  infinitely  less  than  those  which  he  encoun- 
ters in  his  daily  walks  of  being  destroyed  by  the  casual  fall  of  the  buildings 
near  which  he  passes,  or  by  the  encounter  of  carriages  crossing  his  path,  or 
from  the  burning  of  the  house  in  which  he  lodges,  or  from  a  thousand  other 
causes  of  danger  to  which  he  exposes  himself  without  apprehension.  Still, 
even  those  who  possess  the  greatest  animal  courage  are  struck  with  awe,  and 
affected  more  or  less  by  fear,  when  exposed  to  the  war  of  the  elements  in  a 
violent  storm ;  and  there  are  none  who,  in  such  cases,  will  not  willingly  avail 
themselves  of  any  means  of  protection  which  they  believe  to  be  availing.  Au- 
gustus entertained  such  a  dread  of  lightning  that  in  storms  he  took  refuge  in 
caves,  thinking  that  lightning  never  penetrates  to  any  considerable  depth  in  the 
ground. 

Strong  fear,  operating  on  ignorance,  has  prompted,  in  times  past  and  present, 
a  multitude  of  absurd  and  unavailing  expedients,  among  which,  nevertheless, 
chance  seems  to  have  flung  some  in  which  analogies  to  the  results  of  modern 
science  are  apparent.  When  a  cloud  menaced  thunder,  the  Thracians  shot 
their  arrows  at  it.  The  arrows  being  metal,  were  conductors,  and,  being- 
pointed,  had  the  virtue  of  attracting  lightning.  Pliny  states  that  the  Etruscans 
had  a  secret  method  by  which  they  could  draw  liiyhtning  from  the  clouds,  and 
guide  it  at  their  pleasure.  Numa  possessed  the  method,  and  Tullus  Hostil- 
ips,  committing  some  oversight  in  the  performance  of  the  ceremony,  was  him- 
self struck.  For  Numa  substitute  Franklin,  and  for  Tullus,  Richmann,  and 
the  Roman  legend  is  converted  into  a  true  historical  record  of  the  last  century. 

It  was  formerly  believed  that  persons  in  bed  were  never  stricken  by  light- 
ning ;  and  a  modem  meteorologist,  Mr.  Howard,  apparently  favors  such  an 


100 


PROTECTION  FROM  LIGHTNING. 


idea,  by  relating  two  cases  in  1828,  in  which  beds  were  completely  destroyed 
by  lightning,  while  the  persons  who  lay  in  them  were  uninjured.  Against 
this,  however,  many  contrary  instances  may  be  cited.  On  the  29th  of  Septem- 
ber, 1779,  Mr.  Hearthhy  was  killed  in  his  bed,  by  lightning,  at  Harrnwgate, 
while  his  wife,  who  lay  beside  him,  escaped.  On  the  27th  of  September, 
18J9,  a  servant  was  killed  in  her  bed  at  Confolens,  in  France.  In  1837,  a 
house  was  struck  with  lightning  at  Kensington,  near  London,  where  a  man  and 
his  wife  were  killed  in  their  bed. 

The  Romans  believed  that  seaVs  skin  was  a  preservative  against  lightning ; 
and  tents  were  made  of  this  material  for  timid  persons  to  shelter  under  in 
storms.  Augustus  was  always  provided  with  a  seal's  skin  cloak.  However 
ineffectual  may  be  such  an  expedient,  experience  abundantly  proves  that  the 
material  of  the  dress  is  not  without  considerable  influence  on  the  course  which 
lightning  follows,  and  may,  therefore  augment  or  diminish  the  peril  of  the  wear- 
ers. When  lightning  struck  the  church  at  Chaleau-neuf-les-Moutiers ,  during 
the  celebration  of  mass,  of  the  three  priests  who  officiated  at  the  altar,  two  were 
struck  dead  and  the  third  was  uninjured.  The  vestments  of  the  last  were  of 
silk. 

There  are  some  well-attested  facts  which  indicate  a  relation  between  color 
and  the  movements  of  the  electric  fluid.  Three  cases  are  cited  in  which  hor- 
ses and  oxen  having  white  spots  were  struck  by  lightning,  and  had  all  the 
white  hair  burned  off,  while  the  remainder  of  the  hide  remained  unaltered. 

It  has  been  supposed  that  certain  species  of  trees  are  proof  against  lightning, 
and  never  struck  by  it.  Tiberius  was  accustomed  to  wear  a  crown  of  laurel, 
from  the  idea  that  lightning  never  struck  it.  Observations  made  in  districts 
where  extensive  forests  present  all  varieties  of  trees  to  the  chances  of  the 
storm,  afford  no  grounds  for  any  certain  conclusions  on  this  subject. 

When  assailed  by  a  storm  in  an  open  plain,  the  danger  is  greatly  augmented 
by  seeking  the  shelter  of  a  tree.  Experience  and  theory  combine  to  prove 
this.  The  position  of  greatest  safety  is  such  a  distance  from  the  tree  that  it 
shall  act  as  a  conductor,  diverting  the  lightning  from  the  place  assumed  for 
safety.     A  distance  of  half  a  dozen  yards  may  serve  for  this  purpose. 

Glass,  being  a  non-conductor  of  electricity,  is  generally  supposed  to  have  a 
protective  virtue.  Thus  it  has  been  presumed  that  a  person  enclosed  in  a  cage 
of  glass  exposed  to  a  thunder-storm  would  be  in  absolute  safety.  This  is 
proved  to  be  a  fallacy  by  many  examples  of  lightning  striking  and  penetrating 
the  panes  of  windows  and  the  frames  of  conservatories. 

Nothing  is  more  clearly  established  than  that  pieces  of  metal  of  any  kind,  car- 
ried about  the  person,  augment  the  danger  of  being  struck  by  lightning ;  and  this 
increase  of  peril  is  greater  in  proportion  to  the  magnitude  of  the  metallic  appen- 
dages. That  this  material  principle,  illustrating,  as  it  does,  one  of  the  elemen- 
tary laws  of  electricity,  may  be  appreciated  as  fully  as  it  ought  to  be,  we  shall 
here  cite  some  of  the  numerous  recorded  examples  of  it. 

On  the  21st  of  July,  1819,  lightning  struck  the  prison  of  Biberac,  in  Swabia, 
and,  passing  into  the  grand  hall,  struck  an  individual  prisoner  who  was  one  in 
a  group  of  twenty  ;  the  nineteen  others  were  untouched.  This  individual  was 
a  brigand  chief,  who,  being  under  sentence,  was  chained  round  the  waist. 

When  Saussure  and  his  party  were  at  Breven,  in  1767,  the  metal  band  and 
gold  button  on  the  hat  of  M.  Jallabat  emitted  sparks. 

CoNSTANTiNi  relates,  that,  in  1749,  a  lady,  wearing  on  her  arm  a  gold  brace- 
let, raised  her  hand  to  shut  the  window  during  a  thunder-storm ;  the  bracelet 
suddenly  disappeared ;  not  the  slightest  trace  of  it  remained.  The  lady  was 
slightly  wounded. 

Brydone  relates  that  a  lady  of  his  acquaintance,  Mrs.  Douglas,  sitting  at  an 


PEOTECTION  FROM  LIGHTNING. 


101 


open  window,  during  a  storm,  had  her  bonnet  completely  destroyed,  but  suffered  ' 
no  injury  in  her  person.  He  accounts  for  this  by  the  wire  of  the  form  of  the  i 
bonnet  attracting  the  Hghtning. 

These,  and  many  other  instances  which  might  be  mentioned,  sufficiently 
prove  that  safety  is  best  consulted  in  time  of  storm,  by  laying  aside  all  metal- 
lic appendages  of  the  person,  snch  as  chains,  watches,  ear-rings,  hair  orna- 
ments. &c.  The  source  of  the  greatest  danger  is  in  the  bars  or  plates  of  steel 
which  are  used  in  the  corsets  of  females,  and  which  ought  to  be  abandoned  by 
all  ladies  who  do  not  desire  to  invite  the  approach  of  lightning. 

It  has  been  already  shown  that  when  lightning  passes  along  a  line  of  con- 
ducting matter,  the  only  points  where  explosion  takes  place  and  damage  en- 
sues, are  at  the  parts  where  lightning  enters  and  leaves  the  conductor ;  and  as 
a  necessary  consequence  of  this,  all  interruption  of  continuity  in  any  part  of 
a  conductor  or  series  of  conductors  is  attended  with  explosion  and  correspond- 
ing damage.  Since,  then,  the  bodies  of  men  and  animals  afford  a  free  passage 
to  the  electric  fluid,  it  may  be  expected  by  analogy  that  when  lightning  is  trans- 
mitted through  a  chain  of  animals,  either  in  mutual  contact  or  connected  by 
conductors,  the  chief  if  not  the  only  injury  Avould  be  sustained  by  the  first  and 
last  individuals  of  the  series.  This  principle  is  accordingly  supported  by  the 
results  of  experience.     The  following  instances  will  illustrate  it : — 

On  the  2d  of  xA.ugust,  1785,  a  stable  at  Rambouillet  was  struck  by  lightning. 
A  file  of  thirty-two  horses  received  the  fluid :  of  these,  the  first  was  laid  stiff 
dead,  and  the  last  was  severely  wounded.  The  intermediate  thirty  were  only 
thrown  down. 

On  the  22d  o(  August,  1808,  lightning  struck  a  schoolroom  in  Knonaii,  in 
Switzerland.  Five  children  read  together  on  the  same  bench:  the  first  and 
last  were  struck  dead,  the  other  three  only  sustained  a  shock. 

At  Flavigny  [Cote-d'Or)  lightning  struck  a  chain  of  five  horses,  killing  the 
first  two  and  the  last  two,  the  middle  horse  suffering  nothing.  At  a  village  in 
Fra7iche-Co7nte,  lightning  struck  a  chain  of  five  horses,  killing  the  first  and  last 
only.  At  Pravilie,  near  Char/res,  a  miller  walked  between  a  horse  and  a 
mule  loaded  with  grain;  lightning  struck  them,  killing  the  horse  and  mule. 
The  man  was  unhurt,  except  that  his  hat  was  burnt  and  his  hair  singed. 

The  danger  from  lightning  during  storms  may  be  lessened  by  observing 
some  precautions  suggested  by  the  known  properties  of  the  electric  fluids. 
Chimneys  often  afford  an  entrance  to  lightning,  the  soot  which  lines  them  be- 
ing a  conductor.  Keep,  therefore,  at  a  distance  from  them.  Avoid  the  neigh- 
borhood of  all  pieces  of  metal,  gilt  objects,  such  as  the  frames  of  glasses,  pic- 
tures, and  chandeliers.  Mirrors,  being  silvered  on  the  back,  augment  the 
danger.  Avoid  the  proximity  of  bell-wires.  The  middle  of  a  large  room  in 
which  no  chandelier  is  suspended  is  the  safest  position,  and  is  rendered  still 
more  so  by  standing  on  a  plate  of  glass,  or  a  cake  of  resin  or  pitch,  or  sitting 
on  a  chair  suspended  by  silken  cords. 

The  danger  of  being  struck  with  lightning  is  augmented  by  being  placed  in 
a  crowd  of  persons.  The  living  body  being  a  conductor  of  electricity,  a  con- 
nected mass  of  such  bodies  is  more  likely  to  be  stricken,  for  the  same  reason 
that  a  large  mass  of  metal  is  more  liable  than  a  small  one. 

Besides  this,  the  vapor  which  arises  from  the  transpiration  of  a  crowd  of  per- 
sons, rising  through  the  air,  plays  the  part  of  a  conductor,  and  attracts  the  light- 
ning in  the  same  manner  as  a  metallic  rod,  though  in  a  less  degree.  For  these 
reasons,  those  who  are  very  solicitous  for  their  personal  security,  should  not  re- 
main in  churches,  theatres,  or  other  places  of  public  assembly,  during  a  storm. 
The  same  causes  expose  flocks  of  sheep  and  herds  of  cattle  or  horses  collected 
together  in  the  same  stable,  to  increased  danger.     Barns  and  granaries  are  lia- 


PROTECTION  FROM  LIGHTNING. 


ble  to  exhale  vapor  in  such  quantities  as  to  produce  a  column  of  conducting 
matter  above  them,  and  are,  for  this  reason,  often  struck  by  lightning,  when 
not  provided  wllh  the  means  of  protection  afforded  by  Paratonnerres. 

It  sometimes  happens  that  lightning  falling  among  a  crowd  selects  an  indi- 
vidual through  whose  body  it  passes  to  the  ground,  neglecting  the  rest,  and  this 
without  any  discoverable  cause. 

A  case  has  been  already  mentioned  in  which  this  occurred  from  the  influence 
of  a  mass  of  metal  concealed  behind  the  wall  against  which  the  person  who 
suffered  stood.  But  cases  are  not  wanting  in  which  we  are  compelled  to  admit 
that  different  individuals  are  endowed  with  the  conducting  power  in  different 
degrees,  and,  therefore,  that  the  lightning  strikes  by  preference  the  best  con- 
ductor. The  results  of  experiments  with  artificial  electricity  corroborate  this, 
for  in  transmitting  the  electric  discharge  through  a  chain  of  persons,  it  has 
sometimes  happened  that  one  individual  in  the  chain  stops  the  fluid.  From 
some  unknown  peculiarity  of  his  organization,  his  body  is  a  non-conductor.  If, 
then,  it  be  ascertained  that  in  some,  though  very  rare  instances,  individuals 
are  found  who  are  non-conductors,  analogy  leads  to  the  inference  that  different 
individuals  have  the  conducting  quality  in  different  degrees. 

The  fear  engendered  by  the  proximity  of  the  cloud  in  which  lightning  is 
elaborated,  is  founded,  not  on  any  distinct  and  explicable  principles,  but  on  a 
vague  impression  that  the  chances  of  damage  are  augmented  as  we  approach 
the  cause  of  danger,  whatever  that  cause  may  be.  If,  then,  the  risk  of  injury 
be  admitted  to  increase  as  the  distance  from  the  thunder-cloud  is  diminished,  it 
would  follow,  by  necessary  inference,  that  destruction  would  be  inevitable  to 
those  whose  temerity  or  misfortune  might  place  them  actually  within  the  dimen- 
sions of  the  cloud.  Experience,  however,  does  not  justify  this.  On  the  con- 
trary, thunder-clouds  have  been  repeatedly  traversed  with  impunity.  In  August, 
1770,  the  abbe  Richard,  passed  through  a  thunder-cloud  on  the  small  mountain 
called  Boyer,  between  Chalons  and  Tournus.  Before  he  entered  the  cloud  the 
thunder  rolled  as  it  is  wont  to  do.  When  he  was  enveloped  in  it,  he  heard  only 
single  claps  with  intervals  of  silence,  without  roll  or  reverberation.  After  he 
passed  above  the  cloud,  the  thunder  rolled  below  him  as  before,  and  the  lightning 
flashed. 

The  sister  of  M.  Arago  witnessed  similar  phenomena  between  the  village  of 
Estagel  and  Limoiix ;  and  the  officers  of  engineers  engaged  in  the  trigonometri- 
cal survey  repeatedly  experienced  the  same  occurrences  on  the  Pyrenees. 

TYie  paratonnerres,  appended  to  buildings  and  ships,  consist  of  a  pointed  metal- 
lic rod,  attached  to,  and  projecting  upward  from  the  highest  point  of  the  structure 
placed  under  their  protection.  The  lower  end  of  this  rod  is  connected  with  a 
series  of  other  metallic  rods,  or  with  a  metallic  chain,  which  is  continued  to 
the  ground.  If  the  paratonnerre  be  applied  to  a  building,  the  series  of  rods 
being  attached  to  the  walls  and  carried  to  the  ground,  must  be  continued  to  such 
a  depth,  and  brought  to  such  a  position,  that  its  inferior  extremity  shall  either 
be  immersed  in  water,  or  in  soil  which  is  in  a  permanent  state  of  moisture. 
The  water,  or  moist  soil,  possessing  the  conducting  power,  receives  the  elec- 
tricity from  the  extremity  of  the  rod  without  explosion  ;  but  if  the  rod  termi- 
nated in  dry  earth  the  fluid  would  escape  from  the  extremity,  or  worse  still, 
from  some  other  part  of  the  series  of  rods,  with  an  explosion,  and  would  dam- 
age whatever  bodies  might  be  adjacent  to  it.  If  it  be  applied  to  a  ship,  the 
pointed  rod  is  attached  to  the  point  of  the  main-top-mast,  and  the  lower  end  of 
the  rod  is  connected  with  a  chain  which  is  carried  down  the  mast  and  rigging 
over  the  side  of  the  vessel,  and  finally  plunged  in  the  sea.  The  highest  point 
of  the  rod  being  liable  to  be  heated  by  lightning,  and  to  be  oxydated,  is  fornied 
of  platinum,  or  gilt,  so  as  to  restrict  oxydation. 


PROTECTION  FROM  LIGHTNING. 


103 


That  paratonnerres  exert  their  protective  power  only  when  lightning  strikes  < 
the  structure  over  which  they  preside,  is  an  error  easily  corrected,  by  immedi-  | 
ate  experiment,  independently  of  the  refutation  it  might  receive  on  theoretical  ( 
grounds.  Let  the  continuity  of  one  of  these  apparatuses  be  broken,  by  sepa-  \ 
rating  any  two  bars  of  the  series,  so  that  their  ends,  instead  of  being  in  imme-  ' 
diate  contact,  shall  be  distant  by  the  eighth  or  tenth  part  of  an  inch  from  each  ] 
other.  When  stormy  clouds  pass  over  the  apparatus,  a  continual  stream  of  elec-  < 
trical  light  will  be  visible  in  the  interval  between  the  separated  points  of  the  ] 
bars.  If  their  distance  be  increased  to  an  inch,  sparks  will  be  observed  to 
pass  between  them,  in  rapid  and  continual  succession,  accompanied  by  deto- 
nations as  loud  as  the  report  of  a  pistol. 

Captain  Wynne,  who  commanded  a  British  frigate,  lately  observed,  during  a 
storm,  at  a  point  where,  by  accident,  an  interruption  of  the  metallic  continuity 
of  his  paratonnerre  occurred,  an  almost  unintermitting  succession  of  sparks, 
which  continued  for  two  hours  and  a  half,  the  whole  interval  during  which  the 
thunder-clouds  were  over  the  vessel. 

It  is  apparent,  then,  that  paratonnerres  are  not  merely  instrumental  in  saving 
a  structure  when  lightning  actually  falls  upon  it,  but  they  also  possess  a  pre- 
ventive power,  and  gradually  and  silently  disarm  the  clouds  by  draining  the 
electric  fluid  from  them ;  and  this  process  commences  the  moment  the  clouds 
approach  a  position  vertically  over  the  paratonnerre. 

The  explanation  of  these  phenomena  is  easy,  when  the  principles  which 
govern  the  movements  of  the  electric  fluids  are  understood.  From  the  mo- 
ment that  a  stormy  cloud  passes  over  a  paratonnerre,  and  comes  within  the 
range  of  its  influence,  the  electricity  of  the  cloud  decomposes  the  natural  elec- 
tricities of  the  rod,  attracting  that  of  the  contrary  name,  which  is  accordingly  ac- 
cumulated at  the  point,  and  repelling  that  of  the  same  name,  which  is  driven 
into  the  crust  of  the  earth,  or  into  the  water  with  which  the  lower  extremity  of 
the  paratonnerre  is  in  communication.  The  electricity  of  the  contrary  name, 
collected  at  the  point,  soon  acquires  so  great  a  tension  that  it  overcomes  the 
restraining  pressure  of  the  air,  and  escapes  in  a  jet,  which  may  often  be  seen 
in  the  dark,  in  the  form  of  a  luminous  aigrette,  issuing  from  the  metallic  point. 
The  fluid  which  thus  escapes,  enters  into  combination  with  the  fluid  of  a  con- 
trary name,  with  which  the  cloud  is  charged,  and  neutralizes  it. 

On  land,  and  especially  in  cities,  numerous  objects  are  presented  to  the  elec- 
tricity of  the  air,  which  have  this  tendency  to  neutralize  it,  and  marked  effects, 
such  as  that  now  referred  to,  are  of  more  rare  occurrence  ;  but  at  sea  such  ap- 
pearances are  common,  as  is  proved  by  the  familiarity  of  all  seamen  with  the 
fire  of  St.  Elmo,  Castor  and  Pollux,  and  Helen,  already  mentioned.  Experi- 
ence proves  that,  ceteris  paribus,  the  more  elevated  a  paratonnerre  is,  the  more 
efficacious  it  will  be. 

This  is  easily  verified  by  immediate  experiment.  The  influence  of  a  para- 
tonnerre, or  what  is  the  same,  the  rate  at  which  it  neutralizes  the  electricity  of 
,  the  air,  is  estimated  by  the  number  of  sparks  which  pass  in  a  given  time  through 
I  a  space  of  a  given  length — suppose,  for  example,  an  inch— by  which  its  me- 
,  tallic  continuity  is  broken.  It  is  found,  that,  according  as  the  elevation  of  the 
[  point  of  the  rod  is  increased,  the  number  of  sparks  transmitted  undergoes  a  corre- 
,  spending  increase.  The  height  of  the  point  being  preserved,  the  number  of 
[  sparks  transniitted  in  a  given  time  is  diminished  by  bringing  other  pointed  con- 
)  ductors  near  it,  and  still  more  so  if  these  conductors  are  more  elevated. 
[  The  increased  eflicacy  obtained  by  augmenting  the  elevation  of  the  metallic 
I  point  of  a  paratonnerre,  is  strikingly  illustrated  by  the  experiments  which  the 
[  contemporaries  and  successors  of  Franklin  made  with  kites.  Romas,  having 
I  elevated  kites  by  means  of  cord  lapped  wiih  metallic  wire,  like  the  base-strings 


104 


PROTECTION  FROM  LIGHTNING. 


of  a  harp  or  violin,  drew  from  the  lower  extremity  of  the  cord  flashes  of  light- 
ning from  three  to  four  yards  long,  and  an  inch  in  thickness,  accompanied  by  a 
report  as  loud  as  that  of  a  gun.  It  was  remarked  on  several  occasions  that 
thunder  and  lightning  ceased  when  the  fire  was  thus  drawn  from  the  cord.  By 
the  same  expedient  thunder-clouds  were  drained  of  their  fire,  and  converted 
into  common  clouds,  by  Dr.  Lining,  of  Charleston,  and  M.  Charles. 

M.  Arago  proposes  this  expedient  for  averting  the  calamitous  eff'ects  of  hail- 
stones which  are  so  great  a  scourge  to  the  agriculturist  in  several  parts  of 
France.  As  the  formation  of  hail  is  undoubtedly  an  effect  of  the  sudden  dis- 
turbance of  the  electric  equilibrium  of  the  clouds,  if  the  electric  fluid  could  be 
quietly  and  gradually  drawn  away,  hail  would  be  altogether  prevented.  Cap- 
tive balloons  might  be  substituted  with  advantage  for  kites,  since  they  could 
be  elevated  in  a  calm,  and  maintained  at  any  required  height.  By  such  means 
a  multitude  of  experimental  researches  in  electro-meteorology  could  be  prosecu- 
ted. The  atmosphere  could  be  sounded  and  the  clouds  themselves  searched, 
and  their  electrical  contents  submitted  to  careful  and  deliberate  examination. 

The  contest  respecting  pointed  and  blunt  conductors,  which  was  maintained 
about  the  middle  of  the  last  century,  has  been  already  noticed.  Although  the 
electrical  laws,  which  have  since  then  been  so  fully  and  clearly  established, 
can  leave  no  doubt  as  to  that  question,  an  experiment  decisive  of  it  made  by 
Beccaria  may  be  mentioned  here.  This  philosopher  placed  on  the  roof  of 
San  Giovanni-di-Dio  at  Turin,  a  bar  of  iron,  at  the  lower  part  of  which  was 
such  an  interruption  of  continuity  as  to  produce  sparks  when  electricity  passed 
along  it.  The  metallic  point  at  the  top  was  moveable  on  a  joint,  and  con- 
nected with  a  silken  cord,  by  drawing  which  the  observer  could  at  pleasure 
convert  it  into  a  blunt  conductor,  or  restore  to  it  the  pointed  form.  In  a 
storm,  so  long  as  the  point  was  presented  upward,  a  stream  of  sparks  was 
seen  at  the  place  where  the  breach  of  continuity  was  provided,  but  the  moment 
it  was  converted  into  a  blunt  conductor,  the  sparks  either  disappeared  alto- 
gether (which  generally  happened),  or  passed  in  much  less  rapid  succession. 

An  ingenious  calculation  of  the  quantity  of  lightning  drawn  from  the  clouds 
by  paratonnerres,  has  been  made  by  M.  Arago.  He  states  that  in  an  ordinary 
storm  a  hundred  sparks  would  be  transmitted  through  a  small  break  of  con- 
tinuity in  the  conductor  of  which  the  combined  effect  would  be  sufficient  to 
kill  a  man,  and  these  would  pass  in  ten  seconds.  As  much  lightning  would 
therefore  pass  per  minute  as  would  destroy  six  men,  as  much  per  hour  as 
would  kill  three  hundred  and  sixty  men.  He  calculates  in  this  way  that  the 
paratonnerres  erected  by  Beccaria  on  the  palace  of  Valentino,  combined  with 
the  effects  of  the  pointed  parts  of  the  roof,  must  take  as  much  lightning  per 
hour  from  the  clouds  as  would  be  sufficient  to  destroy  three  thousand  men. 

The  quantity  of  electricity  which  pointed  conductors  neutralize,  may  be  im- 
agined from  the  following  circumstance  :  The  British  frigate  Dryad,  provided 
with  a  paratonnerre  (constructed  according  to  the  method  proposed  by  Mr. 
Snow  Harris,  by  fixing  to  the  mast  itself  narrow  plates  of  thin  copper),  was 
several  times  exposed  to  violent  tornadoes  off"  the  coast  of  Africa.  The  elec- 
tric fluid  was  seen  on  every  part  of  these  copper  plates  in  such  quantity  as  to 
produce  around  them  a  sort  of  luminous  atmosphere,  accompanied,  by  a  noise 
like  that  of  water  boiling  violently. 

In  the  practical  adaptation  of  paratonnerres,  the  determination  of  the  range 
of  their  protective  influence  is  a  problem  of  great  importance.  The  physical 
section  of  the  Academy  of  Sciences  at  Paris,  being  consulted  by  the  minister 
of  war  on  this  point  in  1823,  adopted  the  estimate  of  M.  Charles,  and  assumed 
that  a  circle  of  double  the  height  of  the  rod  would  be  protected. 

If  this  estimate  be  interpreted  with  geometrical  rigor,  it  would  appear  that 


PROTECTION  FROM  LIGHTNING. 


105 


the  space  over  which  a  pointed  metallic  rod  extends  its  protection,  is  a  cone,  of 
lohich  the  vertex  is  the  point  of  the  rod,  of  which  the  rod  is  the  axis,  and  of  which 
the  section  made  by  any  horizontal  plane  is  a  circle,  whose  diameter  is  four  times 
the  distance  of  such  plane  from  the  point  of  the  rod. 

This  estimate,  which  is  evidently  empyrical,  and  of  which  the  experimental 
grounds  are  not  staled,  requires  much  elucidation  before  it  can  receive  un- 
qualified assent.  Does  the  conductor  extend  no  protection  to  any  surrounding 
points  at  the  level  of  its  own  points  1  To  what  depth  below  the  point  does  the 
surface  of  the  cone  bounding  the  protected  space  extend  ?  or  what  is  the  posi- 
tion of  the  base  which  limits  the  protected  space  taken  in  the  vertical  direction 
downward  ?  Does  the  same  form  of  cone  limit  the  protected  space  for  all 
kinds  of  structures  ?  Is  the  angle  of  the  cone  affected  by  the  presence  of 
large  masses  of  metal,  such  as  the  guns  in  a  battery,  or  the  machinery  used  in 
certain  large  factories,  or  the  armament  of  a  ship-of-war,  or  the  engines  of  a 
large  steamship  ? 

Theory  affords  no  grounds  for  the  law  laid  down  by  M.  Charles,  and  obser- 
vation is  not  wanting  to  show  its  fallacy. 

The  foremast  of  the  ship  Endymion  was  struck  by  lightning  at  Calcutta,  in 
March,  1842.  The  mainmast,  not  fifty  feet  distant,  had  a  chain-conductor, 
which,  according  to  the  above  law,  would  protect  a  circle  of  one  hundred  and 
fifty  feet  diameter. 

The  bow  of  the  ship  Etna  was  struck  at  Corfu,  January,  1830,  although  the 
mainmast  had  a  chain-conductor.  Other  cases  of  similar  character  have  oc- 
curred to  buildings  on  shore,  one  of  which  has  very  recently  been  communi- 
cated to  the  French  Academy.  M.  Arago,  and  many  with  him,  were  un- 
willing to  admit  so  vague  a  law,  and  experience  confirms  their  decision.  To 
protect  an  extensive  building,  several  paratonnerres  would  be  necessary,  and 
the  less  the  height  of  each,  the  greater  should  be  their  number,  which,  as  well 
as  their  position,  must  be  determined  by  the  condition  that  no  part  must  be 
more  distant  from  the  foot  of  the  rod  than  twice  its  height. 

Although  lightning  falls  generally  by  preference  on  the  highest  points,  of 
buildings,  it  does  not  always  do  so.  Many  cases  are  recorded  in  which,  with- 
out damaging  the  summit,  it  has  struck  at  the  middle  of  the  height.  In  some 
cases  it  has  been  seen  distinctly  to  move  in  the  horizontal  direction,  and  strike 
the  side  of  a  steeple.  Cases  are  also  cited  in  which  it  has  entered  by  the 
ground-floor,  where  it  has  struck  persons  and  caused  their  deaths,  doing  slight 
damage  to  the  first  floor,  and  none  to  the  higher  parts  of  the  house.  Such 
facts  suggest  the  utility  of  paratonnerres  with  points  presented  laterally  and 
obliquely. 

In  some  countries  the  superior  extremities  of  paratonnerres  are  formed  into 
a  group  of  points,  radiating  in  various  directions  like  a  star.     This  method  has 
been  suggested  by  the  supposed  advantages   of  horizontal  and  oblique  points.  , 
Experience  has  not  yet  supplied  data  on  which  any  certain  judgment  can  be  ' 
formed  as  to  the  efficiency  of  this  expedient.  i 

The  rod  of  a  paratonnerre,  by  which  it  is  intended  to  conduct  the  electric  ' 
influence  to  or  from  the  earth,  should  be  of  such  thickness  that  it  may  not  be  ( 
lused  by  the  most  powerful  current  of  electricity  which  is  likely  to  pass  through  J 
it.  Experience  indicates  that  this  purpose  will  be  sufljciently  attained  if  it  be  < 
a  square  of  three  quarters  of  an  inch  in  the  side,  or  a  circle  of  the  same  di-  | 
ameter.  Toward  the  base,  an  increased  thickness  is  sometimes  given  to  it,  ( 
with  a  view  to  its  stability.  Paratonnerres  are  sometimes  painted  to  protect  ) 
them  from  rust,  and  lampblack  is  selected  as  the  material  of  the  paint,  in  con-  < 
sequence  of  its  conducting  power.  ) 

It  has  been  already  slated,  ihat  the  inferior  extremity  of  the  paratonnerres  < 


106 


PROTECTION  FROM  LIGHTNING. 


ought  to  be  immersed  in  water  or  in  wet  soil.  If  is  necessary  to  add,  that  if  ! 
it  be  in  water,  an  artificial  cistern  will  not  serve  the  purpose,  as  it  is  in  general 
stanch,  and  enclosed  on  every  side  by  non-conductors  of  electricity.  Exam- 
ples of  the  inefficiency  of  such  a  termination  to  the  conductor  are  not  wanting. 
The  cathedral  of  Milan  was  struck  by  lightning  on  the  9th  of  June,  1819,  and 
the  lighthouse  at  Genoa,  on  the  4th  January,  1827,  and  in  both  cases  damage 
was  sustained,  notwithstanding  the  paratonnerres.  On  examination,  it  proved 
that  the  inferior  extremities  of  these  apparatus  were  immersed  in  artificial 
cisterns. 

To  increase  the  surface  of  contact  of  the  conductor  with  the  ground  it  has 
been  proposed  to  make  it  diverge  into  several  points  at  its  lower  end,  or  to  flat- 
ten it  into  a  thin  broad  plate.  It  has  also  been  proposed  to  immerse  it  in  a  bed 
of  charcoal,  previously  raised  to  a  red-heat,  this  being  a  good  conductor  of 
electricity. 

When  several  paratonnerres  are  erected  on  the  same  building,  each  should 
communicate  with  the  ground  by  the  nearest  and  most  direct  route,  the  fluid 
by  such  means  passing  more  freely  through  them.  Their  efficiency  will  be 
still  more  augmented  if  they  communicate  with  each  other,  and  with  all  the 
metallic  parts  of  the  roof. 

Flexible  metallic  wires  combined  together  so  as  to  form  a  metallic  rope,  such 
as  are  sometimes  used  for  suspension  bridges,  have  been  proposed  as  substi- 
tutes for  rigid  bars  in  paratonnerres  as  being  more  capable  of  adapting  them- 
selves to  the  inequalities  of  buildings,  and  less  liable  to  lose  their  metallic 
continuity  by  the  effects  of  rust. 

When  iron  beams  or  cramps  are  used  in  the  construction  of  a  building,  they 
are  sometimes  carefully  separated  from  the  paratonnerres  by  non-conductors, 
such  as  resin  or  pitch.  If  the  paratonnerres  be  properly  constructed,  this  pre- 
caution is  unnecessary.  The  lightning  will  go  to  the  earth  in  preference  to 
any  lesser  mass  of  conducting  matter. 

In  the  adaptation  of  paratonnerres  to  powder-magazines,  danger  is  supposed  to 
arise  from  the  electric  sparks,  which  issue  at  parts  of  the  conductor  where  mi- 
nute and  imperceptible  breaches  of  continuity  may  take  place.  The  sparks, 
catching  the  powder  which  may  be  accidentally  scattered  on  the  projecting 
parts  of  the  building,  or  lodged  in  crevices  by  the  wind,  may  produce  fatal  ef- 
fects., t'or  this  reason  it  has  been  proposed  that  the  paratonnerres  for  such 
structures  should  not  be  erected  on  the  building,  but  that  they  should  be  planted 
in  the  ground  near  it.  In  that  case,  the  practical  principle  already  explained, 
by  which  the  range  of  the  protective  influence  of  the  conductor  is  limited,  must 
be  attended  to,  and  a  sufficient  number  of  paratonnerres  be  placed  round  the 
building  to  defend  every  part  of  it. 

With  the  view  to  prove  the  efficacy  of  paratonnerres,  independently  of  all 
reasoning  based  on  theory,  M.  Arago  has  collected  a  number  of  facts,  which 
are  too  interesting,  and  have  too  strong  a  bearing,  to  be  passed  without  some 
notice  here.     We  shall,  therefore,  briefly  state  the  most  important  of  them. 

The  temple  at  Jerusalem  stood  from  the  time  of  Solomon  till  the  year  70  of 
the  Christian  era,  a  period  of  above  1000  years.     It  was  completely  exposed 
to  the  violent  storms  incidental  to  Palestine.     It  was  never  struck  by  lightning. 
Neither  the  Bible  nor  Josephus  mentions  any  such  fact,  which,  if  it  had  occur- 
red, must  have  strongly  excited  attention,  and  certainly  been  recorded.     Be- 
\  sides,  it  was  covered  with  wood  both  within  and  without,  and  must  have  been 
I  set  fire  to  if  it  had  been  struck.     Michaelis  rightly  infers  that,  in  the  course  of 
\  ten  centuries,  in  the  midst  of  continual  thunder-storms,  and  ages  before  the  in- 
I   vention  of  paratonnerres,  this  building  was  never  struck  by  lightning.    The  cause 
[,  is  easily  explained.     By  a  circumstance  apparently  fortuitous,  the  temple  was 


PROTECTION  FROM  LIGHTNING. 


107 


provided  with  paratonnerres  similar  in  principle  to  those  of  Franklin !  The 
roof  of  the  building  was  formed  of  cedar,  covered  with  thick  gilding,  and  from 
end  to  end  was  adorned  by  a  row  of  long  lances  of  iron  or  steel,  pointed  and 
gilt.  According  to  Josephus,  the  architect  intended  these  numerous  points 
to  prevent  birds  from  defiling  the  roof.  The  several  fronts  of  the  building 
were  constructed  throughout  their  whole  extent  of  wood  thickly  gilt.  Finally, 
under  the  porch  were  cisterns,  into  which  the  waters  of  the  roof  were  dis- 
charged through  metallic  pipes  provided  for  that  purpose.  It  appears,  there- 
fore, that  the  roof  was  protected  by  a  vast  number  of  pointed  metallic  rods 
communicating  with  a  superabundance  of  metallic  conductors,  which  were  con- 
tinued to  cisterns  of  water  below,  so  that  the  most  carefully-constructed  para- 
tonnerres of  the  present  day  could  not  confer  greater  security. 

The  church  of  the  chateau  of  Count  Orsini,  in  Carinthia,  standing  on  an  em- 
inence, was  so  often  struck  by  lightning,  and  so  many  fatalities  occurred  in 
consequence,  that,  at  length,  the  celebration  of  divine  service  was  discontinued 
there  in  summer.  In  the  course  of  the  year  1730  the  steeple  was  entirely  de- 
stroyed by  lightning.  After  it  was  reconstructed,  it  continued  to  be  struck  four 
or  five  times  a  year.  In  1778  it  was  entirely  demolished,  and  being  immedi- 
ately rebuilt,  it  was  now  supplied  with  a  paratonnerre.  From  that  time  the 
building  was  free  from  damage  by  lightning.  In  five  years  it  was  struck  but 
once,  and  then  the  fluid  was  conducted  to  the  earth  by  the  paratonnerre,  with- 
out injury  to  the  church. 

In  1750  and  1763  the  Dutch  church  at  New- York  was  struck  by  lightning, 
and  sustained  great  injury.  It  was  after  that  provided  with  a  paratonnerre, 
and,  being  again  struck  in  1765,  sustained  no  injury. 

The  church  of  St.  Michael,  at  Charleston,  used  to  be  struck  and  damaged 
once  at  least  in  two  or  three  years.  It  was  provided  with  a  paratonnerre,  after 
which  it  sustained  no  damage. 

Before  the  time  of  Beccaria,  the  palace  of  Valentino,  at  Turin,  was  con- 
stantly struck  by  lightning  and  damaged.  Beccaria  erected  paratonnerres  upon 
it,  and  the  damage  ceased. 

The  tower  of  St.  Mark,  at  Venice,  was,  until  the  year  1776,  constantly  struck 
by  lightning,  and  sustained  occasionally  great  damage.  In  that  year  a  para- 
tonnerre was  placed  upon  it,  and  no  damage  has  occurred  since. 

Mr.  Snow  Harris  states  that,  of  six  steeples  in  Devonshire,  all  have  been 
within  a  short  period  struck  by  lightning.  One  only  sustained  no  damage,  and 
that  one  alone  was  provided  with  a  paratonnerre. 

The  present  lecture  would  be  incomplete,  were  we  to  close  it  without  advert- 
ing to  the  phenomena  termed  "  the  lateral  discharge ;"  it  bears  intimately  on 
the  practical  part  of  the  subject,  and  will  enable  us  at  the  same  time  to  present 
certain  illustrations  of  the  action  of  electricity  which  have  not  been  included 
elsewhere.  When  a  portion  of  the  discharge  from  a  prime  conductor,  for  in- 
stance, or  a  Leyden  jar,  leaves  the  course  marked  out  for  it  to  pursue  a  side- 
path,  the  spark  consequent  on  such  deviation  is  termed  the  lateral  spark ;  it  is, 
in  fact,  a  spark  produced  by  the  division  of  the  discharge.  It  may  be  shown 
in  the  following  manner :  Let  a  powerful  electrical  machine  be  in  action,  and 
sparks  be  thrown  on  a  wire  held  by  an  insulated  rod,  but  having  its  extremity 
connected  with  the  earth  ;  on  applying  the  knuckle,  or  a  brass  ball,  to  any  part 
of  this  wire,  sparks  may  be  obtained  ;  not  that  the  wire  is  incapable  of  carryino- 
away  the  whole  charge  safely,  but  because  of  the  repulsive  action  of  the  elec° 
tricity,  by  which  it  has  a  tendency  to  spread  over  the  surface  of  conductors,  ' 
and  take  the  widest  path  it  can.  The  tendency  is  even  developed  when  the 
side-path  only  lasts  for  a  part  of  the  course  to  the  earth,  and  the  electricity  has  ' 
to  return  again  to  its  original  wire,  for,  if  the  insulated  discharging-rod  have  ', 


108 


PROTECTION  FROM  LIGHTNING. 


one  ball  placed  very  close  to  one  part  of  the  wire,  and  the  other  ball  very  close 
to  another  part,  a  spark  will  appear  at  each  ball.  In  this  case,  it  is  evident 
that  the  metal  of  the  discharging-rod  was  of  no  ultimate  service  in  furnishing 
a  side-path  as  a  thoroughfare  to  the  charge,  but  merely  relieved  the  portion  of 
the  wire  intervening  between  the  balls.  The  same  effects  occur  during  the 
discharge  of  a  Leyden  battery,  especially  when  it  is  insulated.  But  not  only 
is  it  possible  to  obtain  a  spark  from  the  wire  itself,  but  even  from  any  metallic 
system  with  which  the  wire  is  connected.  We  have  ourselves  obtained  it  from 
gas-burners  in  all  parts  of  a  very  large  building,  when  the  wire  was  connected 
with  the  gas-pipes  in  one  part. 

This  spark  is  much  more  readily  obtained  from  the  prime  conductor  than 
from  the  Leyden  discharger,  obviously  on  account  of  the  low  intensity  of  the 
latter,  for  it  is  an  effect  of  intensity  alone  which  enables  electricity  to  pass  at 
all  through  the  air.  Voltaic  electricity,  of  which  we  shall  hereafter  speak,  is 
abundant  in  quantity,  but  of  such  low  tension  as  not  to  pass  at  all  before  contact, 
unless  from  a  very  extensive  series  of  the  pile. 

Now  the  law  which  regulates  all  discharges  is,  that  they  pursue  the  line  or 
lines  of  least  resistance.  When,  therefore,  the  sum  of  two  paths,  including  the 
interval  or  intervals  of  air,  involves  less  resistance  than  does  the  one  original 
path,  the  division  occurs  ;  when  it  involves  greater  resistance,  it  does  not  oc- 
cur ;  and  this  readily  explains  the  greater  facility  for  lateral  discharge  display- 
ed by  the  electricity  from  the  conductor,  as  contrasted  with  the  Leyden  flash. 
At  the  very  outset  the  former  will  overcome  the  resistance  of  many  inches  of 
air,  while  the  latter  is  insulated  by  less  than  one  inch,  and  hence  the  former 
has,  throughout  its  brief  existence,  a  power  greatly  exalted  over  that  of  the 
other.  And  this  path,  or  paths,  is  not  a  mere  matter  of  choice,  determined  on 
by  the  charge  in  its  progress  onward  ;  it  is  a  course  entirely  marked  out  by  the 
action  of  induction,  antecedent  to  the  original  discharge.  Indeed,  it  is  the  mere 
fact  of  the  inductive  action  being  able  to  find  a  path  offering  a  resistance  which 
the  charge  can  overcome  that  first  causes  the  discharge  to  take  place.  There 
are  other  instructive  facts  connected  with  the  lateral  discharge,  for  which  we 
have  not  space  here,  and  to  which  the  reader  must  refer.* 

*  Vide  Naut.  Mag.,  Jan.,  1840 ;  Report  of  Committee  of  House  of  Commons  on  Lightning ;  Ann, 
Elect,  1840     Vro'jeed  Elect.  Soc,  1842;  Harris,  on  Thunder-Storms,  1843. 


MAGNETISM. 


Magnetic  Attraction  and  Polarity. — Magnetic  Meridian,  Variation. — Dip  of  the  Magnetic  Needle. — 
Magnetic  Attraction  known  to  the  Ancients. — Invention  of  the  Mariner's  Compass  of  uncertain 
Date. — Discovery  of  the  Variation. — Tables  of  Variation  constructed. — Robert  Norman  discovers 
the  Dip. — Invention  of  the  Dipping  Needle. — The  Variation  of  the  Variation  discovered. — Influ- 
ence of  Magnets  on  soft  Iron  ob.served. — Polarity  of  Magnets  ob.served. — Construction  of  artificial 
Magnets. — Magnetism  imparted  to  Iron  by  the  Earth. — Laws  of  Magnetic  Attraction  discovered 
by  Coulomb. — Methods  of  making  artificial  Magnets — consequent  Points. — Knight's  improved 
Method. — Duhamel's  Improvement. — Coulomb's  Researches  on  artificial  Magnets. — Influence  of 
Heat  on  Magnetism. — Local  and  periodical  Changes  of  the  Variation. — Diurnal  Variation. — Cas- 
'sini's  Observations  at  Paris — Advancement  of  Magnetic  Geography. — Magnetic  Equator. — Mag- 
netic Poles. 


i 


MAGNETISM. 


The  substances  endowed  with  magnetism  exhibit  that  property  by  three 
distinct  effects  :  — 

1.  They  attract  iron  and  all  ferruginous  matter. 

2.  Two  bodies  endowed  with  the  property  of  magnetism  will  attract  each 
other  at  one  part  of  their  dimensions,  and  repel  each  other  at  another  part. 
These  contrary  effects,  belonging  to  opposite  sides  or  ends,  are  called  mag- 
netic polarity. 

3.  When  a  magnet  is  placed  on  a  vertical  axis  through  its  centre  of  gravity, 
on  which  it  is  free  to  revolve,  the  axis  being  between  its  poles,  it  will  oscillate 
on  each  side  of  a  certain  determinate  position,  in  which  it  will  at  length  come 
to  rest.  When  in  this  position,  a  vertical  plane  passing  through  the  axis  and 
the  poles  will  be  nearly,  but  not  exactly,  coincident  with  the  plane  of  the  me- 
ridian of  the  place  in  which  the  magnet  is  situate.  For  all  magnets  similarly 
supported,  in  the  same  situation,  these  planes  will  be  parallel.  This  plane  is 
called  the  magnetic  meridian.  The  angle  which  the  magnetic  meridian  makes 
with  the  terrestrial  meridian  is  called  the  variation  of  the  magnet. 

4.  If  a  magnet  be  placed  on  a  horizontal  axis  passing  through  its  centre  of 
gravity  at  right  angles  to  the  magnetic  meridian  and  between  its  poles,  it  will 
oscillate  on  each  side  of  a  certain  determinate  position,  in  which  it  will  at 
length  come  to  rest.  When  in  this  position,  a  plane  passing  through  the  axis 
and  the  poles  of  the  magnet  will  not  be  horizontal,  but  will  make  a  certain 
angle  with  a  horizontal  plane  through  the  axis.  This  angle  is  called  the  dip 
of  the  magnet. 

The  power  of  the  magnet,  when  placed  on  a  vertical  axis,  to  fix  itself  in  the 
magnetic  meridian  of  any  place  to  which  it  may  be  transported,  is  called  its 
directive  power,  and  is  the  principle  on  which  its  application  to  navigation  de- 
pends. 

The  attractive  power  of  the  magnet  for  iron  was  the  property  which  was 
first  observed.     This  property  was  known  to  the  ancients,  who  gave  to  the 


112 


MAGNETISM. 


1 


natural  magnet  (an  oxide  of  iron)  the  name  Magnes  [liayvns)  •  derived,  as  is 
supposed,  from  Magnesia,  a  district  of  Lydia,  in  which  the  natural  magnet  was 
found  in  greatest  abundance.  It  was  also  called  Lapis  Herachnis,  from  He- 
raclea,  a  city  of  Lydia.  From  some  passages  in  ancient  authors,  it  would 
seem  that  the  force  of  magnetic  attraction  in  very  high  degrees  of  intensity  was 
then  generally  known.  Pliny  relates  that  Dinochares  proposed  to  Ptolemy 
Philadelphus  to  erect  a  temple  at  Alexandria,  the  dome  of  which  should  be 
built  of  loadstone,  so  as  to  sustain  in  the  air  an  iron  statue  of  Arsinoe.  Saint 
Augustine  also  alludes  to  a  statue  thus  suspended  in  the  air  in  the  middle  of 
the  temple  of  Serapis,  at  Alexandria. 

The  polarity  and  directive  powers  of  the  magnet  were  discoveries  of  a  much  ) 
more  recent  date.  The  application  of  the  magnetic  needle  to  navigation  must  ^ 
have  immediately  succeeded  the  first  knowledge  of  its  directive  power,  but  the 
discoverer  is  unknown  ;  and  even  the  century  vv^hich  was  honored  by  the  in- 
vention of  this  most  beautiful  application  of  science  to  the  uses  of  man  is  un- 
certain. It  is  stated,  in  the  account  of  the  Chinese  empire  by  Du  Halde,  that 
the  directive  power  of  the  magnet  was  used  in  that  part  of  the  globe,  for  the 
purpose  of  land-journeys,  more  than  a  thousand  years  before  the  birth  of  Christ. 
If  such  were  the  case,  it  is  difficult  to  imagine  that  its  use  for  sea-voyages 
should  have  failed  to  spread  itself  westward  until  two  thousand  years  later. 
But,  besides  this,  there  are  other  reasons  why  little  credit  is  to  be  given  to  the 
accounts  which  ascribe  this  invention  to  the  Chinese.* 

The  earliest  work  in  which  the  use  of  the  mariner's  compass  is  distinctly 
mentioned  is  a  manuscript  poem  of  the  twelfth  century,  preserved  in  the  Royal 
Library  at  Paris,  the  authorship  of  which  is  attributed  to  Guiot  de  Provins. 
Guiot  was  at  the  court  of  the  emperor  Frederick  Barbarossa,  held  at  Mentz  in 
the  year  1181. 

Hansteen,  in  his  work  on  the  "  Magnetism  of  the  Earth,"  quotes  an  Icelandic 
historian,  to  show  that  the  directive  power  of  the  loadstone  was  known  a  cen- 
tury antecedent  to  the  date  of  this  poem.  That  annalist,  relating  a  voyage 
made  in  those  seas,  says  incidentally,  that  "  in  those  times,  seamen  had  no 
loadstone  in  the  northern  countries."  It  appears  that  this  writer.  Arc  Frode, 
was  born  about  the  year  1068,  and  therefore  probably  published  his  account 
early  in  the  twelfth  century. 

Cardinal  Jacques  de  Vitri,  who  lived  about  the  year  1200,  speaks  of  the 
magnetic  needle,  in  his  "  History  of  Jerusalem,"  as  indispensable  to  those  who 
make  sea-voyages.  It  has  also  been  said  that  it  was  first  brought  to  Europe, 
from  the  East,  by  Marco  Polo.  It  is,  however,  certain  that  Vasco  de  Gama, 
the  Portuguese  navigator,  used  the  compass  in  his  voyage  to  India  in  1497. 

Before  it  became  the  subject  of  accurate  investigation,  it  was  supposed  that 
the  direction  of  the  compass  was  identical  with  that  of  the  terrestrial  meridian, 
and  that  it  pointed  due  north  and  south.  The  discovery  of  its  variation,  and 
that  the  amount  and  direction  of  the  variation  are  difTerent  in  different  places, 
is  generally  ascribed  to  Columbus  in  1492.  There  appears,  however,  in  a 
volume  of  MS.  tracts  in  the  University  of  Leyden,  a  letter  dated  1269,  by  Peter 
Alsiger,  in  which  the  principal  properties  of  the  magnet  are  mentioned  ;  and, 
among  others,  the  variation.  The  honor  of  this  discovery  has  also  been  ascribed 
to  Grignon,  a  pilot  of  Dieppe,  Sebastian  Cabot,  Gonzales,  and  others. 

Accurate  observations  of  the  variation  of  the  needle  began  to  be  made  at 
Paris  about  the  year  1550.  At  this  time  the  variation  was  toward  the  east. 
It  diminished  in  quantity,  and  became  nothing  in  1663  ;  after  which  it  passed 
to  the  west,  increasing  gradually  till  it  attained  a  certain  limit,  after  which  it 
diminished. 

^  *  See  Kircher,  "  De  Magnete." 


MAGNETISM. 


113 


The  Dutch  navigators,  in  1599,  also  constructed  accurate  tables  of  varia-  i 
tion.  ' 

In  the  year  1576,  Robert  Norman,  a  mathematical  instrument  maker  in 
London,  discovered  the  dip.  He  found  that  the  card  of  the  compass  near  the 
north  point  was  always  depressed  or  inclined  downward,  so  that  he  was 
obliged  to  put  a  counterpoise  on  the  southern  pole  of  the  needle,  to  keep  it 
level. 

Mentionino-  this  circumstance  to  some  scientific  friends,  he  was  advised  to 
construct  a  needle  on  a  horizontal  axis,  and  to  observe  the  position  to  which 
this  downward  inclination  would  bring  the  northern  pole.  He  accordingly 
constructed  the  first  dipping  needle,  and  found  the  dip  to  be  about  seventy-one 
and  a  half  degrees. 

The  variation  of  the  needle  was  accurately  observed  at  London  by  Burrough, 
the  friend  of  Norman,  who  found  that  in  the  year  1581  it  was  eleven  degrees 
and  fifteen  minutes  east.     In  the  treatises  extant  by  Norman  and  Burrough,  no 
reference  is  made  to  any  change,  periodical  or  otherwise,  either  in  the  varia- 
tion or  the  dip. 
\       In  the  following  century,  the  change  to  which  the  variation  is  subject  was 
'  observed  by  Mair,  Gunter,  Gellibrand,  and  Bond.     In  the  year  1599,  Edward 
\  Wright  wrote  a  work  on  the  compass,  which  was  published  by  Prince  Mau- 
rice, lord  high  admiral  of  the  United  Provinces,  in  which  the  advantage  of 
keeping  registers  of  the  variations  observed  on  all  voyages  is  urged.     Thus 
the  variation  of  the  variation,  not  only  as  to  time,  but  as  to  place,  had  at  this 
period  begun  to  receive  the  attention  of  those  engaged  in  navigation. 

When  the  influence  of  magnets  on  ferruginous  matter  came  to  be  examined, 
it  was  soon  apparent  that  they  not  only  enjoyed  the  property  of  attraction,  but 
that  soft  iron,  so  long  as  it  remained  within  the  sphere  of  their  influence,  actu- 
ally acquired  their  own  nature,  and  became  magnetic  also.  When  withdrawn 
from  the  influence  of  the  magnet,  the  iron  was  found  to  return  to  its  natural 
state.  If,  however,  the  iron,  while  influenced  by  the  magnet,  were  twisted, 
filed,  hammered,  or  submitted  to  other  violence  affecting  its  structure,  it  was 
then  found  to  preserve  the  magnetism  it  had  acquired,  even  when  withdrawn 
from  the  magnet. 

When  iron  filings  were  scattered  over  a  sheet  of  paper  under  which  a  mag- 
'  netic  bar  was  placed,  it  was  found  that  the  metallic  powder  arranged  itself  in 
I  a  particular  manner,  indicating  different  intensities  of  attraction  in  different 
'  parts  of  the  bar.  At  a  point  near  the  centre  the  attraction  seemed  to  cease, 
I  and  to  be  augmented  in  each  direction  toward  the  extremities.  The  polarity 
'  of  the  magnet  was  consequently  apparent.  The  points  where  the  attraction 
,  seemed  to  be  most  intense  were  called  the  poles. 

'  When  a  magnetic  bar  was  broken  in  the  middle,  or  at  the  neutral  point,  each 
(  part  was  found  to   acquire  separate  polarity,  and,  like  the  original  magnet,  to 

>  have  two  poles  with  neutral  points  intermediate.  When  magnetism  was  im- 
f  parted  by  a  magnet  to  a  bar  of  iron,  the  former  lost  none  of  its  own  magnetic 
)  force.  Hence  it  was  inferred  that,  in  giving  magnetism,  the  magnet  lost  none 
(  of  the  magnetic  fluid. 

)  When  a  magnet  was  brought  in  contact  with  a  piece  of  steel,  the  efl^ect  was 
(  first  discovered  to  be  feebly  but  gradually  increased,  until  the  steel  itself  be- 
)  came  a  permanent  magnet,  but  that  this  might  be  effected  suddenly  by  friction. 
(  Bars  of  steel,  thus  magnetized,  were  called  artificial  magnets. 
)  Gilbert,  in  his  work  already  referred  to  published  in  the  sixteenth  century, 
)  mentions  that  the  fact  of  magnetism  being  imparted  to  a  bar  of  iron  by  the  earth 

>  itself,  was  first  discovered  by  examining  the  rod  of  the  weathercock  of  the 
)  church  of  the  Auguslines  at  Mantua. 

)  VOL,.  II.— 8 


114 


MAGNETISM. 


The  possibility  of  conferring  magnetism  on  substances  which  are  not  ferru- 
ginous, was  shown  in  1733  by  Brandt,  who  imparted  magnetism  to  the  metal 
cobalt.  Cronstedt,  in  1750,  showed  that  nickel  is  also  susceptible  of  this  prop- 
erty. 

After  philosophers  had  become  familiar  with  the  attractions  and  repulsions, 
the  polarity  and  directive  power  of  magnets,  their  attention  was  directed  to 
the  establishment  of  a  numerical  measure  of  the  actual  amount  of  attractive  or 
repulsive  force  which  they  exerted  under  given  circumstances.  For  a  long 
period,  no  estimate  of  this  was  formed  more  accurate  than  the  weight  which, 
by  attraction,  the  magnet  was  capable  of  supporting  attached  to  a  piece  of  soft 
iron  adhering  to  it.  In  1780,  Coulomb  applied  to  magnetism  those  beautiful 
and  accurate  instruments  of  investigation  which  were  so  successfully  employed 
in  electricity  and  other  departments  of  experimental  physics,  and  determined 
by  their  means  the  intensities  and  laws  of  magnetic  forces.  Two  methods  of 
measuring  the  force  exerted  were  practised  by  him,  similar  to  those  by  which 
electric  attractions  and  repulsions  had  been  measured.  These  were,  the  bal- 
ance of  torsion,  by  which  the  amount  of  the  force  was  estimated  by  the  action 
of  a  twisted  wire,  or  fibre  of  silk  ;  and  the  observation  of  the  number  of  oscil- 
lations which  the  attracted  orrepelled  body  made  in  a  given  time,  on  each  side 
of  the  line  of  attraction  or  repulsion.  By  these  means  it  was  demonstrated 
that  the  force  of  a  magnet  was,  cmteris  paribus,  in  the  direct  ratio  of  the  abso- 
lute intensity  of  the  magnetism,  and  inversely  as  the  square  of  the  distance  of 
the  attracted  or  repelled  body  from  it :  a  law  identical  in  all  respects  with  that 
by  which  electrical  attractions  and  repulsions  are  governed.  He  also  esti- 
mated, as  he  had  done  with  electrified  conductors,  the  distribution  of  magnet- 
ism on  the  surface  of  magnetized  bars ;  and  found  that  in  bars  of  equal  trans- 
verse section,  of  which  the  length  was  considerable  compared  with  the  mag- 
nitude of  the  section,  the  poles  or  points  of  maximum  intensity  were  always 
at  a  distance  of  about  an  inch  and  a  half  from  the  extremities  ;  and  that,  in  very 
short  bars,  the  poles  are  at  one  third  of  their  length  from  the  extremities,  and 
that  this  latter  position  is  the  limit  to  which  the  poles  approach  as  the  bars  are 
diminished  in  length. 

In  making  artificial  magnets,  either  by  means  of  natural  magnets  or  by  other 
artificial  magnets  already  made,  the  process  first  adopted  was  to  rub  the  bar  to 
be  magnetized,  from  end  to  end,  with  one  of  the  poles  of  the  magnet  by  which 
it  was  to  be  magnetized.  This  method  succeeded  sufficiently  well  in  magnet- 
izing short  needles  ;  but,  when  applied  to  bars  of  any  considerable  length,  it 
was  attended  with  the  liability  of  producing  consequent  points — that  is,  in  fact, 
making  the  bar  into  a  succession  of  magnets  instead  of  a  single  niagnet.  Thus 
a  certain  portion  of  the  entire  length,  measured  from  the  extremity,  would  pos- 
sess two  poles  and  an  intermediate  neutral  point ;  then  another  succeeding 
portion  of  the  length  would  possess  other  two  poles  with  another  intermediate 
neutral  point,  and  so  on. 

In  1745,  Dr.  Gowan  Knight,  of  London,  practised  an  improved  method. 
He  placed  two  strong  bar  magnets  end  to  end  in  the  same  line,  the  north  pole 
of  the  one  being  in  contact  with  the  south  pole  of  the  other.  Over  them  he 
laid  the  bar  to  be  magnetized,  its  centre  coinciding  with  the  united  ends  of  the 
two  magnets,  and  its  length  laid  along  them.  In  this  position  the  two  magnets 
were  drawn  asunder,  their  poles  passing  under  each  half  of  the  length  of  the 
bar  to  be  magnetized.  By  this  method  the  bar  acquired  much  stronger  mag- 
netism than  by  that  which  had  previously  been  practised. 

Dii  Hamel  further  improved'this  process.  The  bar  to  be  magnetized  being 
placed  between  the  pieces  of  soft  iron,  he  took  two  bar  magnets,  and  placing 
the  north  end  of  one  and  the  south  end  of  the  other  upon  the  centre  of  the  bar. 


MAGNETISM. 


and  inclining  them  at  an  angle  of  about  thirty  degrees  to  it,  he  drew  them  upon 
it  from  the  centre  to  the  extremities,  and  repeated  this  process  until  the  bar 
was  strongly  magnetized.  This  method  was  modified  by  Mitchell,  who  placed 
a  series  of  bars  to  be  magnetized  in  the  same  straight  line,  with  their  extremi- 
ties successively  in  contact.  He  then  placed  two  bundles  of  strong  magnets 
perpendicular  to  them,  with  their  ends  resting  upon  them,  the  northern  end  of 
one  bundle  and  the  southern  end  of  the  other  being  downward.  These  two 
bundles  of  magnets,  being  attached  to  each  other,  were  moved  over  the  series 
of  bars  to  be  magnetized. 

In  1789,  Coulomb  directed  his  investigations  to  the  processes  of  producing 
artificial  magnets.  He  showed  that  the  susceptibility  of  bars  of  steel  for  mag- 
netism depended  conjointly  on  the  temper  of  the  steel  and  the  force  of  the 
magnets,  and  that  there  was  a  certain  limit  to  the  magnetic  force  which  a  bar 
could  receive.  When  a  bar  attained  this  limit,  it  was  said  to  be  magnetized 
to  saturation. 

The  magnetic  needles  of  ships'  compasses  being  liable  to  great  vicissitudes 
of  temperature,  it  was  a  question  of  considerable  importance  to  navigation 
whether  heat  afl^ected  the  magnetic  virtue.  Gilbert  was  the  first  who  observed 
that  a  magnet  lost  all  its  power  when  raised  to  a  white  heat,  and  on  being 
cooled  did  not  recover  its  magnetism.  It  was  not,  however,  till  a  much  later 
period,  that  the  influence  of  heat  on  magnetism  was  submitted  to  accurate  in- 
quiry. 

It  was  natural  that  the  directive  power  of  the  magnet,  and  its  application  to 
na\dgation,  should  engross  a  large  share  of  attention  ;  and  that  the  govern- 
ments of  maritime  countries,  more  especially,  should  cause  to  be  carefully  and 
accurately  observed  all  those  phenomena  by  which  that  property  was  affected. 
The  variation  of  the  needle,  and  the  changes  periodical  and  local  to  which  it 
is  subject,  were  questions  of  the  highest  importance  to  national  and  commer- 
cial interests  in  every  part  of  the  world.  So  early  as  1722,  Graham  had  ob- 
served that  in  a  given  place  the  needle  was  subject  to  a  diurnal  variation, 
which  was  afterward  ascertained  with,  great  precision  in  different  parts  of 
Europe.  It  was  observed  by  Wargentin,  secretary  to  the  Swedish  Academy, 
in  1750,  and  by  Canton  in  London  in  1756  ;  and  subsequently  by  Van  Swie- 
ten,  with  nearly  the  same  results.  From  all  these  observations  it  appeared 
that  the  north  pole  of  the  needle  begins  to  turn  westward  at  seven  or  eight 
o'clock  in  the  morning,  and  continues  to  deviate  in  that  direction  till  about  two 
o'clock,  when  it  becomes  stationary,  and  soon  begins  to  return  eastward,  ar- 
riving at  the  position  it  had  in  the  morning  at  the  same  hour  in  the  evening. 
Canton's  observations  showed  that  the  amount  of  this  deviation  varied  from 
seven  to  thirteen  or  fourteen  minutes,  being  greatest  at  midsummer  and  least  at 
midwinter,  and  increasing  and  decreasing  gradually  between  these  seasons. 

More  recently  the  same  phenomenon  has  been  observed  by  Colonel  Beaufoy, 
Professor  Hansteen,  and  others. 

Cassini,  who  observed  the  diurnal  variation  of  the  needle  at  Paris,  found 
that  neither  the  solar  heat  nor  light  influenced  it ;  for  it  was  the  same  in  the 
deep  caves  constructed  under  the  Observatory  in  Paris,  where  a  sensibly  con- 
stant temperature  is  preserved,  and  from  which  light  is  excluded,  as  at  the  sur- 
face. In  northern  regions  these  diurnal  changes  are  greater  and  more  irregu- 
lar ;  while,  toward  the  line,  their  amplitudes  are  gradually  diminished  until  at 
length  they  disappear. 

The  investigation  of  the  changes  produced  in  the  direction  of  the  needle, 
and  in  the  intensity  of  the  earth's  attraction  upon  it,  by  change  of  place  upon 
the  surface,  being  a  matter  vitally  important  to  commerce  and  navigation,  has 
engaged  the  attention  of  all  maritime  and  commercial  countries,  from  an  early 


116 


MAGNETISM. 


period  in  the  history  of  the  mariner's  compass.  In  fact,  what  may  be  not  im- 
properly called  magnetic  geography  has  been,  and  still  is,  a  subject  of  profound 
interest,  as  well  to  the  merchant  as  to  the  philosopher. 

It  has  been  already  stated  that  the  discoverer  of  the  dip  found  that  at  London 
a  magnetic  needle,  free  to  move  on  an  axis  perpendicular  to  the  magnetic  me- 
ridian, presented  its  north  pole  downward,  forming  an  angle  of  above  seventy- 
one  degrees.  If  the  instrument  be  carried  northward,  it  is  found  that  the  dip 
gradually  increases  ;  and,  on  reaching  a  certain  region  near  the  pole,  the  nee- 
dle would  become  vertical,  the  dip  being  then  ninety  degrees,  and  its  north 
pole  pointing  downward.  At  such  a  place,  the  common  compass  needle,  moA^- 
ing  on  a  vertical  support,  would  lose  its  directive  power,  and  rest  indifTerently 
in  any  position.  A  place  where  these  effects  would  be  produced  is  called  a 
northern  magnetic  pole. 

If,  on  the  other  hand,  the  dipping  needle  were  carried  toward  the  equator, 
the  magnitude  of  the  dip  would  be  gradually  diminished,  until,  on  arriving  at  a 
certain  region  near  the  equator,  the  needle  would  become  horizontal,  and  the 
dip  would  become  nothing  ;  and  if  the  dipping  needle  could  be  carried  round 
the  globe,  always  following  such  a  course  as  would  allow  it  to  retain  its  hori- 
zontal position,  its  course  traced  on  the  globe  would  be  the  magnetic  equator. 

The  magnetic  equator  does  not  coincide  with  the  equator  of  the  globe,  nor 
is  it  a  great  circle  of  the  earth.  It  never  departs  from  the  equator,  however, 
more  than  twelve  or  thirteen  degrees. 

If,  after  passing  the  magnetic  equator,  the  dipping  needle  be  carried  south- 
ward, its  south  pole  will  dip  or  be  directed  downward  ;  and  this  dip  will  in- 
crease in  magnitude  as  the  needle  approaches  the  south  pole.  A  place  near 
that  pole,  where  the  needle  becomes  vertical,  is  a  southern  magnetic  pole. 

The  first  national  project  to  determine  the  elements  of  magnetic  geography 
was  undertaken  by  the  British  government  about  the  year  1700,  when  the  cele- 
brated Halley  was  commissioned  to  make  a  voyage  with  the  view  to  collect 
the  necessary  observations.  The  results  obtained  by  him  were,  however,  de- 
prived of  the  chief  part  of  the  advantages  which  ought  to  have  attended  them, 
because  of  the  absence  of  uniformity  in  his  instruments,  and  the  neglect  of 
making  proper  comparisons  of  them  with  others. 

Since  that  period,  observations  have  been  made  and  recorded  in  all  exten- 
sive voyages,  and  the  data  for  the  determination  of  the  elements  of  this  part 
of  physical  geography  have  been  greatly  augmented. 


ELECTRO-MAGNETISM. 


Electro-Mag-netism  very  recently  discovered. — Oersted's  Experiments  at  Copenhagen. — The  Law 
according  to  which  the  Needle  is  deflected. — The  Law  of  Attraction  and  Repulsion  of  Electric 
Currents. — Supposes  Electric  Currents  circulating  round  the  Globe. — Arago  show^s  that  the  con- 
ducting Wire  has  Magnetic  Properties. — Needles  magnetized  by  the  Electric  Current. — The 
Variation  of  the  Attraction  of  the  Cun-ent  at  ditferent  Distances  determined. — Laplace  reduces 
this  Result  to  an  analytical  Formula. — The  whole  Body  of  Electro-magnetic  Phenomena  reduced 
to  analytical  Calculation. — Faraday's  Researches. — Rotation  imparted  to  Mercury  by  means  of  the 
Magnet  and  Electric  Current. — The  Multiplier  or  Galvanometer. — Its  Construction  and  Applica- 
tion.— The  Earth  affects  Electric  Currents  in  the  same  Manner  as  it  affects  Magnets. — Ampere's 
Theory  of  Ten-estrial  Magnetism. — Researches  of  M.  de  la  Rive. — Magnetizing  Power  of  the 
Current  at  different  Distances,  and  the  Law  of  its  Variation. — The  Effect  produced  by  transmit- 
ting it  through  Metals. — The  undulatory  Theory  of  Electricity  similar  to  that  of  Light. —  Thermo- 
Electricity. — ThermoElectric  Effects  observed  by  Professor  Seebeck. — His  Experiment  with  An- 
timony and  Copper. — Researches  of  Yelin,  Marsh,  and  Cumming. — Oersted  and  Fourier  construct 
a  Thermo-Electric  Pile. — Becquerel  decomposes  Water  with  such  an  Instrument. — Thermo-elec- 
tric Sea' 6  "f  M'^tals. 


ELECTRO-MAGNETISM. 


119 


ELECTRO-MAGNETISM. 


Those  capital  experiments  by  which  the  science  of  magnetism  has  been  re- 
duced to  the  rank  of  a  branch  of  electricity,  by  showing  that  all  magnetical 
phenomena  are  merely  effects  of  electrical  currents  modified  by  physical  influ- 
ences peculiar  to  certain  substances,  are  of  so  very  recent  a  date  that  they  can 
scarcely  be  considered  as  yet  falling  within  the  scope  of  scientific  history. 
Nevertheless,  the  important  relations  they  bear  to  other  parts  of  physics,  the 
high  generality  of  the  phenomena  themselves,  and  especially  their  susceptibil- 
ity of  being  reduced  to  mathematical  analysis,  require  that  they  should  not  be 
passed  without  some  notice,  even  in  a  sketch  so  brief  and  rapid  as  the  present. 
Since,  however,  it  is  proposed  in  these  volumes  to  enter  very  fully  into  the 
details  of  the  chief  experiments  which  form  the  foundation  of  this  part  of  elec- 
trical science,  it  will  be  sufficient  here  to  notice  concisely  the  chief  results,  in 
the  order  of  their  discovery,  of  those  experimental  investigations  which  may 
be  regarded  as  forming  the  basis  of  the  division  of  the  science  now  denomina- 
ted electro-magnetism. 

At  a  very  early  period  in  the  progress  of  electrical  inquiries,  indications 
were  observed  of  a  relation  existing  between  electricity  and  magnetism. 
Ships'  compasses  had  their  directive  powers  impaired  by  lightning,  and  sewing- 
needles  were  rendered  magnetic  by  electric  discharges  passed  through  them. 
The  influence  of  electricity  over  the  magnetic  properties  of  iron  had  been  suf- 
ficiently noticed  to  suggest  to  the  clear  and  far-sighted  mind  of  Beccaria  a 
notion,  which  can  scarcely  be  called  a  vague  one,  of  that  theory  of  terrestrial 
magnetism  which  may  now  be  regarded  as  established  on  the  basis  of  electro- 
magnetical  phenomena. 

No  facts  sufficiently  clear  and  decisive  to  afford  general  conclusions  were 
produced  until  the  year  1820,  which  was  signalized  by  the  greatest  discovery 
in  physical  science  since  the  memorable  invention  of  the  pile. 

Professor  Oersted,  of  Copenhagen,  had  promulgated  certain  theoretical  views 
on  the  subject  of  the  relations  of  electricity  an,d  magnetism  in  the  year  1807, 


120 


ELECTRO-MAGNETISM. 


which  obtained  little  attention,  being  unaccompanied  by  any  new  facts,  and  the 
community  of  science  being  then  engrossed  by  the  various  and  interesting  ex- 
perimental applications  of  the  pile,  and  the  magnificent  series  of  discoveries 
which  Davy  was  beginning  to  unfold.  In  1 820,  however.  Oersted  supplied 
all  that  was  wanting  in  1807  to  fix  the  attention  of  scientific  inquirers — a  cap- 
ital experiment.  In  that  year  he  announced  the  fact,  that  a  magnetized  needle 
placed  near  a  metallic  wire  connecting  the  poles  of  a  pile  was  compelled  to 
change  its  direction  ;  that  the  new  direction  which  it  assumed  was  determined 
by  its  position  in  relation  to  the  wire,  and  to  the  direction  of  the  current  trans- 
mitted along  the  wire  ;  that  when  the  current  was  sufficiently  strong,  and  the 
needle  sufficiently  sensitive,  the  latter  always  assumed  a  position  at  right  an- 
gles to  the  wire  ;  and  that  whenever  the  direction  of  the  current  along  the  wire 
is  reversed,  the  needle,  making  half  a  revolution,  reverses  the  direction  of  its 
poles,  keeping  still  perpendicular  to  the  wire.  This  discovery  being  made 
known,  caused  unqualified  astonishment  throughout  Europe  ;  the  more  espe- 
cially, as  all  the  attempts  made  before  to  trace  the  relation  between  the  elec- 
tric current  and  the  magnet  had  been  unavailing.  The  enthusiasm  which  had 
been  lighted  up  by  the  great  discovery  of  Volta  twenty  years  before,  and  which 
time  had  moderated,  was  relumined,  and  the  experimental  resources  of  every 
cabinet  and  laboratory  were  brought  to  bear  on  the  pursuit  of  the  consequences 
of  this  new  relation  between  sciences  so  long  suspected  of  closer  ties.  The 
inquiry  was  taken  up  by  Ampere,  Arago,  Biot,  Savart,  and  Savary,  in  France  ; 
by  Davy,  Gumming,  and  Faraday,  in  England  ;  and  by  De  la  Rive,  Berzelius, 
Seebeck,  Schweiger,  Nobili,  and  others,  in  various  parts  of  Europe. 

Among  these,  in  the  inquiry  now  before  us,  Ampere  has  assumed  the  first 
and  highest  place.  No  sooner  was  the  fact  discovered  by  Oersted  made  known, 
than  that  philosopher  commenced  the  beautiful  series  of  researches  which  has 
since  surrounded  his  name  with  so  much  lustre,  and  brought  electro-dynamics 
within  the  pale  of  mathematical  physics.  On  the  18th  of  September,  1820, 
within  less  than  three  months  of  the  publication  of  Oersted's  experiments  in 
France,  Ampere  communicated  his  first  memoir  on  electro-magnetism  to  the 
Academy  of  Sciences. 

In  this  paper  was  explained  the  law  which  determined  the  position  of  the 
magnetic  needle  in  relation  to  the  electric  current.  In  order  to  illustrate  this, 
he  proposes  that  a  man  should  imagine  the  current  to  be  transmitted  through 
his  body,  the  positive  wire  being  applied  to  his  feet  and  the  negative  wire  to 
his  head,  so  that  the  current  of  positive  fluid  shall  pass  upward  from  the  feet 
to  the  head,  and  that  of  the  negative  fluid  downward  from  the  head  to  the  feet. 
This  being  premised,  a  magnetic  needle  freely  supported  on  its  centre  of  grav- 
ity, so  as  to  be  capable  of  assuming  any  direction,  and  placed  before  him,  will 
throw  itself  at  right  angles  to  him  :  the  north  pole  of  the  needle  pointing  toward 
his  left,  and  the  south  pole  toward  his  right. 

If  the  person  through  whose  body  the  current  thus  passes  turn  round,  so  as 
to  present  his  face  in  diflferent  directions,  a  magnetic  needle,  still  placed  before 
him,  will  have  its  direction  determined  by  the  same  condition  :  the  north  pole 
pointing  always  to  the  left,  and  the  south  to  the  right. 

In  the  same  memoir  were  described  several  instruments  intended  to  be  con- 
structed-; especially  spiral,  or  helical  wires,  through  which  it  was  proposed  to 
transmit  the  electric  currents,  and  which,  it  was  expected,  would  thereby  ac- 
quire the  properties  of  magnets,  and  retain  these  properties  so  long  as  the  cur- 
rent might  be  transmitted  through  them.  The  author  also  explained  his  theory 
of  magnets,  ascribing  their  attractive  and  directive  powers  to  currents  of  elec- 
tricity circulating  constantly  round  their  molecules,  in  planes  at  right  angles 
to  the  line  joining  their  poles  ;  the  position  of  the  poles  on  the  one  side  or  the 


ELECTRO-MAGNETISM. 


121 


Other  of  these  planes  of  revolution,  depending  on  the  direction  of  the  revolving 
current. 

On  the  25th  of  the  same  month,  Ampere  communicated  to  the  Academy 
another  paper.*  In  this  he  delivered  the  results  of  his  experiments  on  the 
reciprocal  attractions  and  repulsions  of  electric  currents  acting  on  each  other. 
He  showed  that  two  straight  wires,  along  which  currents  are  transmitted  will 
attract  or  repel  each  other,  according  to  the  direction  of  the  currents.  Let  a 
line  be  imagined  intersecting  both  wires  at  right  angles.  If  both  currents 
move  toward  this  perpendicular  or  both  from  it,  the  wires  will  attract  each 
other  ;  but  if,  while  one  of  the  currents  moves  toward  this  perpendicular,  the 
other  moves  from  it,  then  they  will  repel  each  other.  If  the  wires  be  parallel 
to  each  other,  they  will  attract  or  repel  each  other,  according  as  the  currents 
move  in  the  same  or  opposite  directions.  If  the  wires  be  in  the  same  plane, 
but  not  parallel,  their  directions  will  meet  if  produced  :  in  this  case  they  will 
attract  each  other,  if  the  currents  be'  both  directed  toward  or  from  the  point 
where  their  directions  meet ;  and  they  will  repel  each  other,  if  one  current  be 
directed  toward,  and  the  other  from,  that  point. 

In  the  same  paper  he  proposes  the  hypothesis  of  currents  of  electricity  cir- 
culating round  the  terrestrial  globe,  from  east  to  west,  in  planes  at  right  angles 
to  the  direction  of  the  dipping  needle,  to  account  for  the  phenomena  of  terres- 
trial magnetism. 

These  researches  proceeded  with  unusual  celerity.  On  the  9th  of  the  fol- 
lowing month  (October),  three  weeks  after  the  reading  of  the  last-mentioned 
paper,  he  presented  another  memoir  to  the  Academy,  in  which  he  investigated 
the  properties  of  currents  transmitted  through  wires  forming  closed  curves 
(courbes  fermees),  or  complete  geometrical  figures. 

While  Ampere  was  proceeding  with  these  researches,  Arago  directed  his 
inquiries  to  the  state  of  the  wire  through  which  the  current  was  transmitted, 
more  especially  with  a  view  to  determine  whether  every  part  of  its  surface 
was  endowed  with  the  same  magnetic  properties.  With  this  view  he  placed 
iron  filings  within  the  sphere  of  attraction  of  the  wire,  and  found  that  they  ad- 
hered to  it,  so  as  to  form  concentric  rings  upon  it.  The  moment  the  connexion 
of  the  wire  with  the  pile  was  broken,  and  the  current  was  no  longer  transmit- 
ted along  it,  the  filings  fell  off,  and  all  attraction  disappeared. 

By  a  process  inferred  from  the  theory  of  Ampere,  M.  Arago  succeeded  in 
imparting  permanent  magnetism  to  needles  and  bars  of  steel  by  means  of  the 
electric  current.  This  was  accomplished  by  making  a  spiral  of  wire,  through 
which  the  current  was  transmitted,  while  the  needle  or  bar  to  be  magnetized 
was  placed  within  its  coils.  The  position  of  the  poles  of  the  magnets  thus 
made  depended  on  the  direction  of  the  screw,  or  helix,  formed  by  the  conduct- 
ing wire.  If  the  wire  formed  a  right-handed  screw,  the  poles  were  placed  in 
one  direction  ;  and  if  it  made  a  left-handed  screw,  they  were  reversed.  When 
the  wire  was  made  to  form  a  succession  of  screws  alternately  right-handed  and 
left-handed,  the  bar  was  magnetized  with  a  corresponding  series  of  consequent 
points.  The  same  results  were  obtained  whether  the  electricity  transmitted 
through  the  wire  proceeded  from  a  Voltaic  apparatus  or  from  the  common  elec- 
trical machine.! 

At  the  same  time,  or  a  very  little  later,  and  before  the  information  of  Arago's 
,  experiments  reached  England,  Davy  succeeded  also  in  imparting  magnetism 
'  to  needles  by  the  Voltaic  current,  and  by  common  electricity  ;  and  also  showed 
,  the  effect  of  the  conducting  wire  on  iron  filings.^ 

'  *  Annales  de  Chimie  et  Physique,  torn,  xv.,  59-170. 

'  t  Annales  de  Chimie  et  Physique,  torn,  xv.,  p.  93. 

'  i  Letter  to  Wollaston,  12th  of  November,  1820,  Philosophical  Transactions,  1821. 


122 


ELECTRO-MAGNETISM. 


M.  Ampere,  with  the  view  of  more  completely  developing  the  action  of 
electric  currents  and  magnets  separately  and  on  each  other,  contrived  various 
methods  by  which  wires,  formed  into  parallelograms,  circles,  and  other  geo- 
metrical figures,  could  have  a  current  transmitted  round  them,  and  be  at  the 
same  time  so  supported  or  suspended  as  to  be  capable  of  assuming  any  posi- 
tion or  direction  to  which  their  mutual  attraction,  or  the  attraction  between 
them  and  magnets  placed  near  them,  or  the  influence  of  the  magnetism  of  the 
earth  upon  them,  might  dispose  them.  These  contrivances  afterward  became 
instruments  by  which  many  important  experiments  were  made  ;  the  first  of 
which  was  communicated  to  the  Academy  on  the  30th  of  October,  1820. 
This  was  the  fact,  that  a  wire  forming  a  plane  geometrical  figure  through 
which  the  electric  current  is  transmitted  will,  if  free  to  move,  dispose  itself  so 
that  its  plane  shall  be  at  right  angles  to  the  dipping  needle. 

On  the  same  day,  MM.  Biot  and  Savart  communicated  to  the  Academy  the 
results  of  experiments  made  with  the  view  to  determine  the  law  of  the  mutual 
attraction  and  repulsion  of  electric  currents.  The  results  of  these  experiments 
were  reduced  to  analytical  investigation  by  Laplace,  who  showed  that  the 
law  resulting  from  them  was,  that  the  attraction  or  repulsion  of  each  elementary 
part  of  the  current  diminishes  in  the  same  ratio  as  the  square  of  the  distance 
of  the  object  on  which  it  acts  increases  :  a  law  identical  with  that  of  all 
other  modes  of  electrical  attraction  and  repulsion.  The  effect  of  the  obli- 
quity of  the  current  to  the  direction  in  which  the  force  acted  was  also  deter- 
mined. 

On  the  4th  of  December  following,  M. ^Ampere  read  to  the  Academy  the 
memoir  which  contains  the  reduction  of  the  phenomena  of  electro-magnetism 
to  mathematical  analysis.  He  showed  that  all  the  various  phenomena  attend- 
ing the  action  of  magnets  on  each  other,  of  electric  currents  on  magnets,  and 
of  magnets  on  electric  currents,  and,  in  fine,  of  electric  currents  on  each  other, 
could  be  derived,  by  mathematical  calculation,  from  formulae  expressing  the 
action  of  two  infinitely  small  elements  of  electric  currents,  acting  on  each  other 
in  the  direction  of  the  line  joining  their  middle  points.  The  discussion  of  this 
subject  was  concluded  in  another  memoir,  read  to  the  Academy  on  the  8th  and 
15thof  January,  1821. 

This  year,  1821,  was  signalized  by  the  commencement  of  the  labors  of  Far- 
aday in  electro-magnetism.  This  philosopher,  who  has  since  attained  such 
well-raerked  celebrity,  realized  a  suggestion  which  originated  with  Dr.  Wol- 
laston.  A  magnet  being  placed  in  a  vertical  position,  a  wire  was  so  suspended 
that,  while  the  electric  current  was  passing  through  it,  it  was  capable  of  mo- 
ving round  the  axis  of  the  magnet  so  as  to  describe  a  conical  or  cylindrical 
surface.  While  the  current  was  maintained,  the  wire  took  spontaneously  this 
motion  ;  and  when  the  direction  of  the  current  along  it  was  reversed,  it  re- 
versed its  motion,  and  turned  round  the  magnet  the  contrary  way.  Reversing 
these  conditions,  and  instead  of  fixing  the  magnet  and  leaving  the  wire  free,  he 
fixed  the  wire,  and  so  adjusted  the  magnet  that  it  was  at  liberty  to  revolve 
round  the  wire  as  an  axis.  When  the  current  was  transmitted  through  the 
wire,  the  magnet  spontaneously  revolved  round  it ;  and  when  the  direction  of 
the  current  through  the  wire  was  changed,  the  motion  of  the  magnet  was  re- 
versed. 

Faraday  attempted,  without  success,  to  cause  a  magnet  to  revolve  on  its  own 
axis  ;  but,  the  memoir  containing  the  account  of  his  experiments  being  pub- 
lished in  France,  Ampere  succeeded  in  producing  rapid  rotatory  motion  of 
magnets  on  their  own  axes,  and  showed  that  this  and  the  two  former  results  of 
Faraday's  experiments  followed  as  necessary  consequences  of  his  own  mathe- 
matical principles..    He  also  showed  that  the  same  effects  could  be  produced 


ELECTRO-MAGNETISM. 


123 


!  with  helical  currents,  thus  affording  a  further  corroboration  of  his  theory  of 
magnetic  action. 

Immediately  after  the  publication  of  these  experiments  of  Faraday,  Davy 
thought  that  the  effect  of  the  magnet  on  the  current  might  be  obtained  in  a 
more  simple  state  by  transmitting  the  current  through  a  fluid  conductor,  and 
exposing  the  conductor  to  the  action  of  a  strong  magnet.  With  this  view,  two 
copper  wires,  about  a  sixth  of  an  inch  in  diameter,  coated  with  sealing-wax, 
and  flattened  and  polished  at  the  ends,  were  cemented  into  two  holes  three 
inches  apart  in  the  bottom  of  a  glass  dish,  so  that  the  direction  of  the  wires 
was  perpendicular  to  the  dish.  The  coating  of  sealing-wax  rendered  the  wires 
non-conductors,  except  at  their  flattened  and  polished  ends,  which  were  not 
coated.  Mercury  was  poured  into  the  dish  so  as  to  cover  the  ends  of  the  wires 
to  the  depth  of  the  tenth  or  twelfth  of  an  inch.  The  parts  of  the  wires  pro- 
ceeding from  the  bottom  of  the  dish  were  now  put  in  connexion  with  a  power- 
ful Voltaic  batter}',  the  positive  current  flowing  into  the  mercury  at  one  wire, 
and  passing  from  it  at  the  other.  The  moment  the  current  commenced,  the 
mercury  over  each  wire  was  thrown  into  a  state  of  violent  agitation.  Its  sur- 
face was  raised  into  the  form  of  two  small  cones,  one  over  each  wire ;  waves 
flowed  off  in  all  direction  from  these  cones.  On  holding  the  pole  of  a  power- 
ful bar  magnet  some  inches  above  one  of  the  cones,  its  vertex  was  lowered ; 
and  according  as  the  magnet  descended  toward  the  mercury  the  subsidence  of 
the  cone  continued,  and  the  propagation  of  waves  around  it  ceased,  until  at 
length  the  surface  of  the  mercury  became  perfectly  level,  and  a  slow  revolving 
motion  of  the  mercury  round  the  pole  of  the  magnet  began  to  be  manifested. 
As  the  magnet  was  brought  still  closer  to  the  mercury,  this  gyration  of  the 
fluid  became  more  rapid,  and  the  centre  round  which  the  gyration  took  place 
(which  was  directly  over  the  end  of  the  wire)  became  depressed.  The  rapid- 
ity of  the  rotation  of  the  mercury,  and  the  depression  of  the  centre  of  the  vor- 
tex, continued  to  increase  as  the  magnet  was  brought  nearer  to  the  mercury, 
until  no  more  mercury  remained  over  the  end  of  the  wire  than  was  barely  suf- 
ficient to  cover  it.  This  rotation  took  place  with  either  pole  of  the  magnet, 
and  over  either  wire,  changing  its  direction  when  either  the  pole  of  the  mag- 
net or  the  direction  of  the  current  was  changed.  It  is  evident  that  these  phe- 
nomena are  in  accordance  with,  and  referable  to,  the  same  general  law  as  those 
previously  discovered  by  Faraday.  The  same  effects  were  observed  when 
fused  tin  was  substituted  for  mercury,  and  when  steel  wires  were  used  to  con- 
duct the  current.  The  current  was  also  conducted  to  the  dish  by  tubes  filled 
with  mercury,  with  like  results.* 

In  order  to  determine  whether  the  matter  forming  the  conductor  along  which 
the  electric  current  passed  had  any  influence  on  the  electro-magnetic  phenom- 
ena which  at  this  time  engaged  the  attention  of  philosophers,  Davy  placed 
two  pieces  of  charcoal  in  connexion  with  the  wires  of  a  powerful  Voltaic  bat- 
tery, and,  by  presenting  their  points  toward  each  other,  at  a  distance  vary- 
ing from  one  to  four  inches,  according  to  the  density  of  the  air  in  which  the 
experiment  was  made,  he  obtained  a  column  of  electric  fluid  formed  by  the 
current  passing  through  the  space  between  the  charcoal  points.     This  current 
was  not  transmitted,  as  usual,  along  any  conductor,  but  merely  passed  through  \ 
the  air  between  the  points  ;  and  its  presence  was  rendered  manifest  by  the  i 
light  evolved.     When   a  powerful  magnet  was  presented  to  this  column,  with  J 
its  pole  at  a  very  acute  angle  to  it,  the  column  was  attracted  or  repelled  with  a  < 
rotatory  motion,  or  made  to  revolve  by  placing  the  poles  in  different  positions,  J 
in  the  same  manner  as  metallic  wire  conducting  the  current  would  have  been.  ( 
The  electric  column  was  more  easily  affected  by  the  magnet,  and  its  motion  | 

*  Phil.  Trans.,  1823 ;  also  Davy's  works,  vol.  vi.,  p.  258.  < 


124 


ELECTRO-MAGNETISM. 


was  more  rapid  when  it  passed  through  dense  than  through  rarefied  air  ;  and,  .' 
in  this  case,  the  conducting  medium,  or  chain  of  aeriform  particles,  was  much  , 
shorter.*  * 

While  these  investigations  were  proceeding  in  France  and  England,  the  dis-  , 
coveries  to  which  they  led  conducted  a  German  philosopher  to  the  invention  ' 
of  an  instrument  of  physical  inquiry  of  surpassing  beauty  and  utility,  and  , 
equalled  in  importance  by  none  which  had  appeared  since  the  balance  of  ' 
torsion.  i 

The  multiplier,  or,  as  it  has  been  called  in  England,  the  galvanometer,  in-  ' 
vented  by  Schweiger,  is  an  instrument  by  which  the  presence  of  an  electric  , 
current  is  detected,  and  its  intensity  measured.  It  is  based  upon  the  fact,  that  ' 
a  wire  through  which  a  current  passes  will  have  a  tendency  to  turn  a  magnetic 
needle  at  right  angles  to  it.  By  this  beautiful  instrument  the  most  feeble  cur- 
rents may  be  made  manifest,  and  their  intensities  compared.  It  is  various  in 
its  construction,  according  to  the  nature  and  power  of  the  electric  currents  in- 
tended to  be  observed,  but  generally  consists  of  a  rectangular  frame  of  wood, 
round  two  parallel  sides  of  which  a  copper  wire,  lapped  with  silk,  is  rolled,  so 
that  the  coils  of  wire  shall  be  close  beside  each  other,  and  parallel  in  their  gen- 
eral direction  to  the  other  two  sides  of  the  frame.  Within  the  frame,  and  be- 
tween the  two  surfaces  formed  by  the  coils  of  wire,  is  suspended  a  magnetic 
needle.  If  the  frame  be  so  placed  that  the  needle,  when  at  rest,  shall  be  par- 
allel to  the  coils  of  wire,  these  coils  will  be  parallel  to  the  magnetic  meridian. 
Matters  being  thus  disposed,  let  the  extremities  of  the  wire  be  put  in  connex- 
ion with  the  poles  of  a  Voltaic  pile.  The  current  passing  through  the  wire 
will  act  upon  the  needle,  and  each  coil  of  the  wire  will  affect  it  as  a  separate 
current,  so  that  the  total  effect  will  be  in  proportion  to  the  number  of  coils.  If 
the  current  in  the  lower  coils  moved  in  the  same  direction  as  the  upper,  it 
would  have  a  contrary  effect  on  the  needle  ;  but,  by  the  manner  in  which  the 
wire  is  carried  round  the  frame,  the  systems  of  inferior  currents  are  contrary 
in  their  direction  to  the  superior  currents,  and  they  have,  consequently,  the 
same  effect  on  the  needle.  If  the  effect  of  the  current  thus  multiplied  be  sufR- 
cient,  the  effects  of  the  earth's  magnetism  will  be  overcome,  and  the  needle 
will  be  turned  at  right  angles  to  the  wires,  and,  consequently,  will  take  the 
direction  of  magnetic  east  and  west ;  but  if  the  force  of  the  current  be  insuffi- 
cient for  this,  the  needle  will  be  deflected  at  some  definite  angle  with  the  mag- 
netic meridian,  the  magnitude  of  which  angle  will  supply  the  means  of  estima- 
ting the  force  of  the  current. 

It  is  evident  that  the  sensibility  of  this  instrument  will  be  augmented  in  pro- 
portion as  the  magnetism  of  the  needle  is  enfeebled,  and  the  number  of  coils  of 
wire  augmented. 

The  direction  of  the  current  is  indicated  by  the  direction  in  which  the  de- 
flection of  the  needle  takes  place.  If  the  north  pole  of  the  needle  be  deflected 
toward  the  east  when  the  current  passes  in  one  direction  through  the  wire  of 
the  multiplier,  it  will  be  equally  deflected  toward  the  west  when  the  same  cur- 
rent is  reversed. 

When  Ampere  had  demonstrated  the  reciprocal  action  of  electric  currents 
on  each  other,  and  on  magnets,  he  showed  that  the  terrestrial  globe  exerted  an 
influence  on  magnets  freely  suspended,  and  on  electric  currents  transmitted 
through  wires  so  supported  as  to  be  capable  of  obeying  any  forces  exerted  upon 
them,  identical  in  all  respects  with  the  influence  which  a  sphere  would  exert 
round  which  a  wire  coiled  so  that  its  coils  shall  nearly  coincide  with  the  paral- 
lels of  latitude,  through  which  wire  an  electric  current  is  transmitted,  running 
continually  from  east  to  west,  or  contrary  to  the  diurnal  motion  of  the  earth ; 

*  Pliil.  Trans.,  1821 ;  Davy's  works,  vol.  vi.,  p.  232. 


ELECTRO-MAGNETISM. 


125 


or,  since  the  wire  in  such  case  is  merely  necessary  to  conduct  the  electricity, 
the  phenomena  of  terrestrial  magnetism  only  require  the  admission  that  a 
series  of  electric  currents  continually  circulate  round  the  globe,  according  to 
lines  which  intersect  the  magnetic  meridians  perpendicularly. 

To  present  an  experimental  verification  of  this  theory,  M.  Ampere  construct- 
ed a  plane  geometrical  figure — a  circle,  for  example — of  wire,  and  suspended  it 
in  such  a  manner  that,  while  the  current  circulated  upon  it,  the  figure  was  ca- 
pable of  moving  on  a  vertical  axis  through  its  centre  of  gravity.  It  was  ob- 
served to  throw  its  plane  into  a  position  at  right  angles  to  the  magnetic  me- 
ridian. When  the  current  was  reversed,  it  turned  round  through  one  hundred 
and  eighty  degrees,  and  reversed  its  plane.  When  a  helix  was  suspended  on 
its  centre  of  gravity,  and  a  current  was  transmitted  through  the  wire,  it  exhib- 
ited all  the  properties  of  a  magnet ;  when  suspended  on  a  vertical  axis,  it  as- 
sumed the  direction  of  the  magnetic  meridian  ;  and  when  suspended  on  a  hori- 
zontal axis  at  right  angles  to  the  magnetic  meridian,  it  threw  itself  parallel  to 
the  dipping  needle.  *^ 

The  hypothesis  of  Davy,  that  the  nucleus  of  the  globe  consisted  of  the  me- 
callic  bases  of  the  alkalies  and  earths,  and  that  its  surface  was  oxydated,  sup- 
plied Ampere  with  strong  grounds  of  probability  in  support  of  these  theoretical 
ideas  of  terrestrial  magnetism.  It  was  easy  to  imagine  that,  at  the  surface  of 
contact  of  the  metallic  nucleus  and  the  surrounding  shell  of  oxydated  matter, 
there  were  constant  chemical  actions  in  progress,  which  might  produce  a  se- 
ries of  electric  currents  at  some  distance  below  the  surface  of  the  earth,  and 
that  these  currents,  acting  through  the  shell  of  oxides,  would  produce  the  phe- 
nomena of  terrestrial  magnetism. 

In  the  same  year,  M.  de  la  Rive,  of  Geneva,  published  a  memoir,  in  which 
he  showed  that  when  a  current  is  transmitted  through  a  closed  circuit  of  a  rec- 
tangular form,  for  example,  it  affected  only  the  sides  which  have  a  vertical  po- 
sition. He  established,  as  a  general  law,  that  a  vertical  current,  capable  of 
revolving  round  a  fixed  vertical  line  as  an  axis,  will  place  itself  so  that  the 
plane  passing  through  its  own  direction,  and  the  axis  round  which  it  revolves, 
shall  be  at  right  angles  to  the  magnetic  meridian,  the  side  on  which  the  cur- 
rent descends  being  on  the  east  of  the  axis,  and  the  side  on  which  it  ascends 
being  on  the  west. 

He  also  showed  that  a  horizontal  current,  though  not  susceptible  of  being 
influenced  by  the  magnetism  of  the  earth,  is  not  therefore  free  from  all  action  ; 
on  the  contrary,  he  proved  that  when  it  is  free  to  move  parallel  to  itself,  it  will 
move  in  this  manner  in  the  one  direction  or  the  other,  according  to  its  own  di- 
rection ;  and  that  this  motion  will  equally  ensue  in  all  positions  in  which  it 
may  be  placed,  whether  it  be  directed  north  and  south,  east  and  west,  or  in  any 
intermediate  azimuth. 

These  laws,  proved  experimentally  by  M.  de  la  Rive,  were  immediately 
shown  by  M.  Ampere  to  be  direct  consequences  of  his  theoretical  principles. 

In  the  year  1827,  M.  Savary  directed  his  labors  to  follow  out  the  researches 
on  the  power  of  the  Voltaic  current  to  impart  magnetism  to  iron,  which  had 
been  demonstrated  by  the  experiments  of  Davy  and  Arago.  M.  Savary  dis- 
charged a  Leyden  jar  through  a  metallic  wire,  needles  placed  near  which  were 
found  to  be  magnetized,  and  the  strength  of  the  magnetism  imparted  to  them 
was  observed  to  vary  with  their  distance  from  the  wire.  Being  placed  at  va- 
rious distances  from  it,  the  magnetizing  power  of  the  current  was  not  found 
either  continually  augmented,  or  continually  decreased  ;  but,  as  the  needle  re- 
ceded, it  first  increased,  and  then  diminished,  attaining  a  maximum  at  a  certain 
position.  He  also  showed  that  as  the  distance  varied,  not  only  the  intensity 
of  the  magnetic  force  passed  thus  successively  through  maxima  and  minima, 


126 


ELECTRO-MAGNETISM. 


but  the  polarity  was  reversed,  taking  aJternately  one  direction  or  the  other. 
These  alternations  of  intensity  and  polarity  appeared  to  be  determined  in  a 
great  measure  by  the  weight,  diameter,  and  conducting  power  of  the  wire,  and 
the  strength  of  the  electric  discharge. 

One  of  the  most  novel  and  unexpected  circumstances  attending  the  experi- 
ments of  M.  Savary,  was  the  manner  in  which  he  showed  that  the  magnetizing 
influence  of  the  current  was  modified  by  transmitting  it  through  other  metals. 
When  the  needle  to  be  magnetized  was  enveloped  in  metallic  leaf,  the  magnet- 
ism it  received  was  augmented.  By  gradually  increasing  the  thickness  of 
its  metallic  coating,  the  force  of  the  magnetism  it  received  increased  by  de- 
grees till  it  attained  a  maximum,  after  which  it  diminished ;  and,  by  further 
augmenting  the  thickness  of  its  coating,  it  was  diminished  so  as  to  be  equal  to 
the  magnetism  it  would  receive  without  any  coating.  Copper,  tin,  gold,  silver, 
and  mercury,  used  as  coating,  were  found  to  possess  this  property  in  different 
degrees.  The  force  of  the  electric  charge  transmitted  through  the  wire  was 
found  to  have  a  singular  influence  on  the  phenomenon  ;  for,  according  as  this 
force  was  increased  or  diminished,  different  thicknesses  of  the  same  coating 
were  necessary  to  produce  equal  effects.  These  considerations  also  affected 
the  direction  of  the  polarity. 

These  facts  appeared  to  M.  Savary  to  be  scarcely  compatible  with  any  hy- 
pothesis which  requires  the  admission  or  the  translation  of  electric  matter  by 
the  current ;  and  he  considered  that  they  indicated  rather  that  the  current  pro- 
ceeds from  a  system  of  undulations  propagated  along  the  wire,  and  transmitted 
by  it  laterally  to  adjacent  media. 


THERMO-ELECTRICITY. 


The  fact  that  a  derangement  of  the  equilibrium  of  temperature  was  attended 
with  the  evolution  of  electric  effects  was  observed  by  Volta,  and  subsequently 
by  Dessaignes.  Volta  found  that  a  plate  of  silver,  one  end  of  which  was  more 
heated  than  the  other,  produced  Galvanic  effects  ;  and  Dessaignes  observed 
that  convulsions  were  produced  in  the  frog,  when  the  muscles  and  nerves  were 
connected  by  a  silver  spoon  in  which  lighted  charcoal  was  placed.  These 
isolated  observations,  however,  led  to  no  conclusions  affecting  the  progress  of 
discovery. 

Immediately  after  the  discovery  of  Oersted  became  known  throughout  Eu- 
rope, Professor  Seebeck,  of  Berlin,  engaged  in  a  series  of  researches  on  the 
Voltaic  effects  produced  by  derangement  of  temperature  ;  and  communicated 
to  the  Academy  of  Sciences  of  Berlin,  during  the  years  1821  and  1822,  the 
results  of  his  experiments,  which  were  published  in  the  "  Transactions"  of  that 
body,  and  form  the  basis  of  whatever  has  since  been  collected  imder  the  title 
of  thermo-electricity . 

A  rod  of  copper  being  bent  into  a  semicircle,  a  bar  of  antimony  was  soldered 
to  it,  so  that  the  two  metals  had  the  form  of  a  stirrup.  The  temperature  of  one 
of  the  points  of  junction  of  the  metals  was  raised,  while  that  of  the  other  was 
unchanged.  An  electric  current  was  immediately  excited,  passing  from  the 
copper  at  the  heated  point  through  the  antimony.  The  intensity  of  the  current 
was  augmented  by  augmenting  the  difference  of  temperature  of  the  two  points 
of  connexion  of  the  metals,  and  the  direction  of  the  current  was  reversed 
when  the  source  of  heat  was  removed  from  one  point  of  junction  to  the  oth- 
er. The  current  was  rendered  manifest  by  its  power  to  deflect  a  magnetic 
needle. 

Seebeck  observed  similar  effects  by  combining  various  other  metals  in  pairs  ; 
and  found  that  the  relative  electric  state  of  the  metals  did  not  correspond  with 


ELECTRO-MAGNETISM. 


that  assigned  to  them  in  Volta's  theory  of  contact.  He  also  observed  that  cur- 
rents were  produced  by  inequality  of  temperature  in  bars  of  a  single  metal, 
when  they  have  a  crystalline  structure  ;  and  suggested  that  the  changes  of 
temperature  of  the  metallic  nucleus  supposed  by  Davy  to  be  within  the  exter- 
nal crust  of  the  earth,  might  produce  those  currents  circulating  round  the 
globe  to  the  influence  of  which  Ampere  ascribed  the  magnetism  of  the  globe. 

In  the  year  1823,  these  inquiries  were  prosecuted  by  the  chevalier  Yelin, 
and  MM.  Marsh  and  Gumming.*  The  first  investigated  the  influence  of  the 
nature  and  form  of  homogeneous  metals  on  the  direction  and  intensity  of  the 
electric  current.  The  two  latter  philosophers  produced  the  revolution  of  ther- 
mo-electric currents  round  magnets.  Soon  afterward,  MM.  Oersted  and  Fou- 
rier communicated  to  the  Academy  of  Sciences  a  series  of  experiments  on 
currents  obtained  by  thermo-electric  piles,  made  by  combining  bars  of  anti- 
mony and  bismuth  in  a  series.  The  alternate  points  of  junction  were  heated, 
and  the  current  was  manifested  by  the  deflection  of  a  magnetic  needle.  This 
deflection,  though  considerable,  was  not  observed  to  increase  in  proportion  to 
the  number  of  thermo-electric  elements  constituting  the  pile.  They  attempted, 
Avithout  success,  to  efl^ect  chemical  decompositions  by  the  current  thus  ob- 
tained. This  has,  however,  been  since  effected  by  Becquerel,  by  exposing  to 
the  action  of  the  thermo-electric  current  solutions  easily  decomposable.  M. 
Bottot,  of  Turin  has  also  succeeded  in  decomposing  water,  and  various  so- 
lutions, by  the  thermo-electric  current  obtained  from  a  pile  formed  of  a  series 
of  wires  of  platinum  and  iron. 

The  result  of  these  and  subsequent  investigations  of  Seebeck,  Becquerel, 
and  others,  has  led  to  the  establishment  of  the  following  series  of  metals  pos- 
sessing thermo-electric  excitability,  in  the  order  in  which  they  stand  :  — 

1.  Bismuth.  5.  Tin.  9.  Zinc. 

2.  Platinum.  6.  Gold.  10.  Iron. 

3.  Mercury.  7.  Silver.  11.  Antimony. 

4.  Lead.  8.  Copper. 

If  a  thermo-electric  couple  be  formed  by  any  two  metals  in  this  series,  the 
positive  electricity  at  the  heated  point  will  pass  from  that  metal  which  holds 
the  higher  to  that  which  holds  the  lower  place  in  the  series  ;  consequently, 
each  of  the  metals  in  the  series  is  thermo-electrically  positive  to  all  above  it, 
and  negative  to  all  below  it. 

The  intensity  of  many  thermo-electric  currents  increases  in  proportion  to 
the  temperature  up  to  40°  R.,  but  not  after  ;  and  at  a  certain  point  it  falls.  It 
appears,  too,  from  the  experiments  of  M.  Becquerel,  that  each  metal  has  for 
itself  a  proper  thermo-electric  power,  which  is  the  same  for  any  circuit.  He 
thus  expresses  it : — 

Metals.  Thermo-electric  power. 

P.  Iron 5 

P.  Silver 4-07 

P.  Gold 4-052 

P.  Zinc 4-035 

P.  Copper 4 

P-  Tin 3-89 

P.  Platinum 3-68 

M.  Nobili  obtained  thermo-electric  currents  by  the  contact  of  a  hot  and  a 
cold  cylinder  of  porcelain,  on  each  of  which  was  moist  cotton.  M.  Becquerel 
considers  that  the  water,  at  two  temperatures,  is  here  the  exciting  cause.  The 
rank  of  the  chief  metals,  in  the  thermometric  series,  beginning  with  the  posi- 

*  Bibl.  Univ.,  torn,  xxiv.,  xxv.,  xxvii.,  and  xxix. 


ELECTRO-MAGNETISM. 


live,  is,  according  to  Gumming — bismuth,  mercury,  platinum,  tin,  lead,  gold, 
copper,  silver,  zinc,  iron,  antimony.  When  heat  is  applied  to  the  junction  of 
any  pair  of  these,  the  current  passes  from  that  higher  in  the  list  to  that  lower. 
Thermo-electric  batteries  have  been  made  by  a  combination  of  pairs  in  series. 
Baron  Fourier  made  a  hexagon  of  three  pairs  of  bismuth  and  antimony  :  by 
heating  with  a  lamp  or  cooling  with  ice  three  junctions,  he  obtained  increased 
effects  ;  by  heating  and  cooling  the  alternate  junctions  at  the  same  time,  he  in- 
creased the  effect.  From  experiments  by  Oersted,  "  it  appears  that  the  thermo- 
electric current  produces  a  prodigious  quantity  of  electricity,  but  in  a  state  of 
very  feeble  intensity,  while  the  Voltaic  current  has  a  very  great  intensity  ;"  so 
that  short  elements  are  most  advantageous.  M.  Pouillet  found  that  if  the  elec- 
tro-motive power  of  a  constant  Voltaic  pair  were  95,  that  of  a  thermo-pair  of 
bismuth  and  antimony  would  be  1.  Mr.  Wheatstone,  by  his  admirable  appli- 
cation of  Ohm's  law,  found  the  proportion  1  :  94-6. 


THE    THERMOMETER. 


(    Advantages  of  mercurial  Thermometer. — Method  of  constracting  one. — To  purify  the  Mercury. — 
/       Formation  of  the   Tube. — To  fill  the  Tube. — Determination  of  the  freezing  and  boiling  Points. — 

i       Modes  of  Graduation. — Alcohol  Thermometers. — DifBculty  of  fixing  the  boiling  Point. — Useful- 
ness of  the  Thermometer. — History  of  its  Invention. — Methods  of  comparing  Scales  of  different  ( 
Thermometers. 


.J 


THE    THERMOMETER. 


Heat,  like  all  other  physical  agents,  can  only  be  measured  by  its  effects,  and 
these  effects  are  very  various.  The  dilatations  and  contractions  which  bodies 
undergo  by  change  of  temperature,  so  long  as  these  bodies  suffer  no  change  in 
their  physical  state  from  solid  to  liquid,  or  from  liquid  to  gas,  or  vice  versa,  form 
the  best  and  most  convenient  means  of  measuring  the  degrees  of  temperature. 
This  property  has,  therefore,  been  taken  as  a  principle  for  the  construction  of 
instruments  for  measuring  heat,  w^hich  have  been  called  thermometers  and  py- 
rometers ;  the  former  being  applied  to  the  measure  of  more  moderate  tempera- 
tures, while  the  latter  have  been  chiefly  applied  to  determine  the  more  fierce 
degrees  of  hea,t. 

Bodies  in  every  state  being  affected  with  a  change  of  dimension  by  every 
change  of  temperature,  all  are  adapted,  more  or  less,  to  form  measures  of  tem- 
perature. Solids  and  gases,  being  more  uniform  than  liquids  in  their  expan- 
sions, and  having  a  wider  range  of  temperature  without  attaining  the  limits  at 
which  they  change  their  physical  states,  would  appear  at  first  view  to  be  the 
best  suited  for  this  purpose.  There  are  other  considerations,  however,  to  be 
attended  to,  which  show  that,  on  the  other  hand,  liquids  are  best  adapted  for 
thermometric  indication.  The  changes  of  dimension  which  a  solid  undergoes 
by  change  of  temperature  are,  as  has  been  seen,  extremely  small,  and  not  easily 
observed.  To  appreciate  them,  it  is  necessary  that  their  effects  should  be  in- 
creased by  wheels  or  levers,  or  other  mechanical  means  ;  and  such  apparatus 
never  fail  to  introduce  error  into  the  result,  in  proportion  to  their  complexity. 
Bodies  in  the  aeriform  state  command,  it  is  true,  an  unlimited  range  of  temper- 
ature, without  changing  their  form  ;  but,  on  the  contrary,  their  high  suscepibil- 
ity  of  dilatation  and  contraction  renders  them  extremely  inconvenient  in  meas- 
uring any  considerable  variations  of  temperature.  The  changes  of  dimension 
of  liquids,  while  they  are  greater  and  more  easily  observed  than  those  of  solids, 
and  therefore  require  no  mechanical  contrivance  for  magnifying  them,  are,  on 
the  other  hand,  less  than  those  of  gases,  and  present  a  means  exempt  from  the 
inconveniences  of  either  of  the  other  methods. 


132 


THE  THERMOMETER. 


The  range  of  a  liquid  thermometer  must  not  only  be  confined  between  its  boil- 
ing and  freezing  points,  but  within  still  more  narrow  limits  ;  for  it  has  been 
proved  that  the  expansion  of  liquids,  as  they  approach  those  temperatures  at 
which  they  pass  into  the  solid  or  gaseous  state,  are  subject  to  irregularities, 
which  render  them  an  uncertain  measure  of  temperature.  In  the  choice  of  a 
liquid  for  a  thermometer  we  must  necessarily  be  directed  in  some  degree  by  the 
purpose  to  which  the  instrument  is  applied.  An  instrument  intended  to  meas- 
ure very  low  temperatures  may  be  constructed  with  a  liquid  which  itself  boils 
at  a  low  temperature  ;  while,  on  the  other  hand,  such  a  liquid  would  be  inap- 
plicable in  a  thermometer  designed  for  measuring  higher  degrees  of  heat. 
Thermometers  intended  only  to  measure  high  temperatures  might,  on  the  other 
hand,  be  constructed  of  a  liquid,  like  certain  oils,  which  solidifies  at  a  consid- 
erable temperature.  For  all  ordinary  purposes,  however,  that  liquid  will  be 
the  best  adapted  for  thermometers  in  which,  while  the  freezing  and  boiling 
points  are  separated  by  a  great  interval,  that  interval  shall  comprise  the  tem- 
perature of  the  most  ordinary  objects  of  domestic  or  scientific  inquiry. 

Among  liquids,  there  is  one  which  eminently  fulfils  these  conditions,  and 
which,  by  reason  of  its  various  physical  and  chemical  qualities,  is  otherwise 
well  adapted  for  the  purposes  of  the  thermometer.  This  liquid  is  mercury,  or 
quicksilver.  Mercury  boils  at  a  higher  temperature  than  any  other  liquid,  ex- 
cept certain  oils  ;  and,  on  the  other  hand,  it  freezes  at  a  lower  temperature  than 
all  other  liquids,  except  some  of  the  more  volatile,  such  as  alcohol,  or  ether. 
Thus  a  mercurial  thermometer  will  have  a  wider  range  than  any  other  liquid 
thermometer.  It  also  is  attended  with  this  convenience,  that  the  extent  of 
temperature  included  between  melting  ice  and  boiling  water  stands  at  a  con- 
siderable distance  from  the  limits  of  its  range.  Thus  it  happens  that  nearly  all 
the  temperatures  which  are  necessary  to  be  observed,  whether  for  domestic 
purposes  or  scientific  inquiry,  fall  within  the  range  of  a  mercurial  thermometer. 
It  is  attended  with  the  further  advantage  of  a  higher  susceptibility  to  the  action 
of  heat,  and  its  indications  are  therefore  more  immediate  tban  those  of  other 
liquids.  Its  expansibility  within  the  extent  of  temperature  of  the  phenomena 
most  commonly  observed  are  perfectly  regular,  and  proportional  to  those  of  sol- 
ids and  gases  at  the  same  temperatures.  These  properties  have  brought  mer- 
curial thermometers  into  general  use  in  all  parts  of  the  world. 

To  render  the  thermometer  practically  useful,  it  is  necessary  that  its  indica- 
tions should  be  steady  and  uniform,  and  capable  of  being  compared  one  with 
another  at  different  times  and  places.  To  accomplish  this,  it  is  chiefly  neces- 
sary that  the  mercury  which  is  used  in  different  thermometers  should  be  per- 
fectly the  same.  To  insure  this  identity,  it  is  necessary  that  the  mercury  used 
should  be  pure  and  free  from  any  admixture  of  foreign  matter.  Mercury,  how- 
ever, under  ordinary  circumstances,  is  never  found  in  this  state.  In  the  mine 
it  is  commonly  mixed  with  other  substances,  which  by  chemical  combination 
render  it  solid,  and  from  which  it  must  be  disengaged  by  the  process  of  metal- 
lurgy. Even  when  it  is  found  in  the  liquid  state,  it  is  commonly  mixed  with 
silver,  lead,  or  tin — metals  with  which  it  combines  with  great  facility.  In  order 
to  have  it  perfectly  pure,  it  is  necessary  first  to  disengage  it  from  the  grosser 
substances  with  which  it  may  be  mixed.  This  is  easily  accomplished  by 
straining  it  through  a  piece  of  chamois  leather  ;  the  subtle  parts  of  the  mercury 
will  pass  freely  through  the  pores  by  merely  squeezing  the  leather  between  the 
fingers,  and  the  solid  impurities  with  which  it  is  mixed  will  be  thus  intercepted 
and  separated. 

It  is  still  necessary,  however,  to  disengage  from  the  mercury  other  liquids 
which  may  be  combined  with  it.  This  is  easily  accomplished.  Let  a  boiler 
be  provided,  terminated  in  a  tube  at  the  top,  which  tube  is  conducted  into  a  re- 


THE  THERMOMETER. 


133 


}  ceiver,  placed  beyond  the  influence  of  the  fire,  so  as  to  be  capable  of  recon- 
verting the  vapor  of  mercury  into  liquid.  Let  the  impure  mercury  be  placed  in 
this  close  boiler  on  a  fire.  The  fact  that  mercury  boils  at  a  lower  temperature 
than  any  other  metal,  vvrill  cause  it  to  be  converted  into  vapor,  while  the  other 
metals  with  which  it  is  mixed  continue  in  the  liquid  or  solid  state.  The  mer- 
cury will  thus  pass  over  in  vapor  through  the  pipe  from  the  top  of  the  boiler 
into  the  cooler,  where  it  will  be  restored  to  the  liquid  state,  and  will  be  col- 
lected free  of  admixture,  with  other  metals.  This  process,  which  is  called  dis- 
tillation, will  be  more  fully  described  hereafter.  If  the  mercury  happen  to  hold 
in  combination  any  liquid  which  boils  at  a  lower  temperature  than  the  mercury 
itself,  such  a  liquid  may  be  dismissed  by  raising  the  mercury  in  the  boiler  to  a 
temperature  below  its  own  boiling  point.  The  liquids  combined  with  it  will 
then  pass  over  in  vapor,  and  will  be  collected  in  the  cooler  separate  from  the 
mercury. 

Having  now  obtained  pure  mercury,  unalloyed  by  admixture  with  any  other 
substance,  the  next  object  is  to  contrive  a  means  of  rendering  its  dilatations  and 
contractions  observable.  For  this  purpose,  let  a  glass  tube,  of  very  small  bore, 
hs  obtained  by  the  ordinary  process  of  glass-blowing ;  let  a  spherical  bulb  be 
blown  at  one  end  of  it,  of  a  magnitude  very  considerable  compared  with  the 
bore  of  the  tube.  As  the  tube  must  be  of  that  extremely  small  bore  which  is 
called  capillary,  the  bulb,  though  not  of  great  magnitude,  may  still  bear  a  very 
considerable  proportion  to  it.  When  the  bulb  is  filled,  a  very  slight  change  in 
the  volume  of  the  mercury  will  cause  a  considerable  rise  or  fall  in  the  tube  ; 
because  the  bulb  not  considerably  altering  its  dimensions,  an  increase  of  vol- 
ume in  the  mercury  must  necessarily  find  room  by  forcing  the  column  upward 
in  the  tube  ;  and  a  diminution  of  volume,  for  a  like  reason,  will  cause  the  col- 
umn in  the  tube  to  fall.  If  a  portion  of  the  bore  of  a  tube,  measuring  the  eighth 
of  an  inch  in  length,  contain  the  1000th  part  of  the  whole  quantity  of  mercury 
in  the  apparatus,  then  an  expansion,  amounting  to  one  part  in  1000,  will  cause 
the  column  of  mercury  to  rise  in  the  tube  the  eighth  of  an  inch,  a  space  which 
is  easily  observable  ;  and  if  the  bore  of  the  tube  be  everywhere  uniform, 
every  eighth  of  an  inch  which  the  column  of  mercury  rises  or  falls  will 
correspond  to  an  equal  increase  in  the  volume  of  mercury.  The  tube 
and  bulb,  thus  constructed,  are  attached  to  a  divided  scale,  by  which  the  rise 
or  fall  of  the  column  of  mercury  in  the  tube  may  be  accurately  measured  and 
observed. 

If  the  scale  by  which  the  variations  of  a  mercurial  column  are  measured  be 
divided  into  equal  parts,  it  is  obvious  that  the  bore  of  the  tube  should  be  uni- 
form, for  otherwise  equal  divisions  of  the  scale  would  not  correspond  to  equal 
dilatations  or  contractions  of  the  mercury.  If  one  part  of  the  bore  were  larger 
than  another,  a  division  at  that  part  would  correspond  to  a  greater  change  in 
the  volume  of  the  mercury  than  a  division  at  another  part  where  the  bore  is 
narrower.  As  it  is  a  matter  of  convenience  that  the  divisions  on  the  scale 
should  be  equal,  it  is  obviously  essential  that  the  bore  of  the  tube  should  be 
either  accurately  or  very  nearly  uniform.  There  is  a  very  simple  and  efi'ectual 
method  of  ascertaining  whether  the  bore  of  a  tube  fulfil  this  condition.  Before 
the  bulb  is  blown  on  the  tube,  let  a  drop  of  mercury  be  introduced  into  its  bore 
so  small  as  to  occupy  a  space  in  the  bore  not  exceeding  a  quarter  of  an  inch, 
or  even  less.  Let  this  mercury  be  gradually  moved  through  the  tube  from  end 
to  end,  causing  it  to  rest  at  different  points  by  holding  the  tube  horizontally, 
and  let  the  space  which  it  occupies  in  the  tube  at  different  places  be  measured 
by  some  accurate  measure.  If  the  mercury  occupies  the  same  length  of  the 
tube  in  every  part  of  its  bore,  it  is  evident  that  the  bore  will  be  everywhere 
uniform  ;  but  if  it  occupies  a  less  extent  of  the  bore  in  one  place  than  in  an-  i 


134 


THE  THEHMOMETER. 


Other,  then  that  part  where  it  occupies  a  less  extent  must  be  greater  in  diam-  ( 
etor  than  other  parts,  and  the  bore  is  consequently  not  uniform.  ' 

For  ordinary  domestic  purposes,  and  even  for  most  scientific  observations,  ( 
thermometer  tubes  can  be  easily  obtained  of  sufficiently  uniform  bore  ;  but  in  j 
scientific  experiments,  where  the  utmost  possible  accuracy  is  sought,  it  has  < 
been  thought  better  not  to  depend  on  the  uniformity  of  the  bore,  but  to  graduate  | 
the  scale  independently  of  this  condition.  Such  a  graduation  may  be  effected  i 
by  causing  a  drop  of  mercury  to  move  from  end  to  end  of  the  tube,  and  en-  J 
graving  on  the  glass  with  a  diamond  a  number  of  divisions  regulated  by  the  < 
space  which  the  drop  of  mercury  occupied  in  different  parts  of  the  bore.  These  \ 
divisions,  whether  equal  or  unequal,  would  evidently  contain  the  same  quantity  ' 
of  mercury,  and  correspond  to  equal  dilatations  or  contractions  of  the  fluid.* 

Let  us  suppose,  then,  that  a  tube  has  been  obtained  of  uniform  bore,  and  a 
bulb  blown  upon  its  extremity,  and  that  we  are  furnished  with  pure  mercury. 
The  next  object  is  to  fill  the  tube  with  the  mercury.  If  the  tube  had  not  been 
capillary,  but  had  a  bore  of  considerable  magnitude,  the  mercury  could  have 
been  easily  introduced  by  pouring  it  through  the  tube  into  the  bulb  ;  but  the 
bores  of  tubes  commonly  used  for  thermometers  are  much  too  small  to  admit 
of  this  process.  A  method  of  filling  the  tube  is  practised  which  depends  part- 
ly on  the  high  expansibility  of  atmospheric  air,  and  partly  on  the  atmospheric 
pressure.  The  bulb  of  the  tube  is  held  for  some  time  over  the  flame  of  a  spirit- 
lamp,  so  that  the  air  contained  in  it  becomes  intensely  heated.  This  air,  there- 
fore, expands,  and  becomes  highly  rarefied,  so  that  the  quantity  or  weight  of 
air  contained  in  the  bulb  and  tube  at  length  bears  a  very  inconsiderable  propor- 
tion to  that  which  was  contained  in  it  at  the  ordinary  temperature  of  the  atmo- 
sphere. At  the  same  time,  another  purpose  is  answered  by  this  process.  A 
thin  film  of  moisture,  attracted  from  the  atmosphere,  or  in  the  process  of  blow- 
ing the  bulb,  is  liable  to  attach  itself  to  the  interior  surface  of  the  bulb  and  bore  ; 
and  if  this  film  were  allowed  to  remain  on  the  tube,  it  would  disturb  the  indi- 
cations of  the  instrument,  by  becoming  mixed  with  the  mercury,  and  expanding 
with  it  in  different  degrees,  so  that  the  apparent  expansion  would  be  partly  de- 
pendant on  the  expansion  of  the  mercury,  and  partly  on  the  expansion  of  the 
vapor  arising  from  this  film  of  moisture.  By  the  process  of  heating  the  bulb,  and 
rarefying  the  air  contained  in  the  tube,  this  film  of  moisture  is  effectually  evapo- 
rated and  expelled,  and  nothing  remains  in  the  tube  but  a  very  small  quantity 
of  highly-rarefied  air.  In  this  state  the  tube  is  inverted,  placing  the  bulb  up- 
ward, and  the  open  end  of  the  tube  is  plunged  in  a  vessel  containing  pure  mer- 
cury. The  heat  by  which  the  air  contained  in  the  bulb  was  rarefied  being  now 
removed,  the  air  begins  to  resume  its  former  temperature,  and  all  communica- 
tion with  the  atmosphere  being  thus  cut  ofi"  by  the  open  end  of  the  tube  being 
immersed  in  the  mercury,  no  supply  of  air  is  admitted  to  fill  the  space  caused 
by  the  contraction  of  the  air  remaining  in  the  tube.  Meanwhile,  the  pressure 
of  the  atmosphere  acts  on  the  surface  of  the  mercury  in  the  cistern,  and  presses 
it  up  in  the  tube  in  the  same  manner,  and  from  the  same  cause  by  which  mer- 
cury is  sustained  in  the  barometer.  In  this  manner  the  mercury  will  be  found 
to  rise  in  the  thermometer  tube,  and  ultimately  to  pass  into  the  bulb,  the  greater 
part  of  which  will  be  filled.  The  small  quantity  of  rarefied  air,  now  contracted 
into  very  limited  dimensions,  will  occupy  the  upper  part  of  the  bulb.  Let  the 
tube  be  now  once  more  inverted,  placing  the  open  end  upward,  and  let  the  bulb 
containing  the  mercury  be  again  held  over  the  flame  of  a  lamp.  After  some 
time,  the  bubble  of  air  which  remains  intermixed  with  the  mercury  will  be  forced 
',  out  of  the  tube  by  the  expansion  caused  by  the  heat.     The  bulb  must  still  be 


*  This  method  of  graduation  was  practised  by  Gay-Lussac. 


THE  THERMOMETER. 


135 


held  over  the  lamp  till  the  mercury  boils.  The  vapor  of  the  mercury  then  rising 
from  its  surface  will  fill  the  unoccupied  part  of  the  bulb  and  tube,  and  will  alto- 
gether expel  the  atmospheric  air  from  them,  so  that  the  whole  bulb  and  tube 
will  be  filled  with  the  mercury  and  its  vapor.  The  instrument  must  now  be 
once  more  inverted  into  the  cistern  of  mercury,  and  immediately  the  mercurial 
vapor  in  the  tube  and  bulb  will  be  restored  to  the  liquid  form  by  being  removed 
from  the  lamp  which  sustained  it  in  the  state  of  vapor.  The  atmospheric  pres- 
sure will  force  mercury  into  the  tube  and  bulb  until  both  are  perfectly  filled. 
The  apparatus,  therefore,  is  now  filled  with  pure  mercury,  free  from  intermix- 
ture with  any  kind  of  foreign  matter,  whether  in  the  solid,  liquid,  or  gaseous 
form. 

Since  the  indications  of  the  thermometer  are  made  by  the  rise  and  fall  of  the 
column  of  mercury  in  the  tube,  it  follows  that,  when  adapted  for  use,  the  in- 
strument must  be  only  partially  filled  with  mercury.  It  is  evident  that  at  the 
lowest  temperature  which  the  instrument  is  intended  to  measure,  the  surface 
of  the  mercury  ought  to  be  above  the  point  where  the  tube  rises  from  the  bulb  ; 
for  any  contraction  of  the  mercury  which  would  cause  the  whole  of  that  fluid 
to  enter  into  the  bulb  could  not  be  estimated.  The  whole  quantity  of  mercury 
in  the  instrument  ought,  therefore,  to  exceed  the  contents  of  the  bulb  when  the 
mercury  is  at  the  lowest  temperature  to  which  the  instrument  is  intended  to  be 
exposed.  On  the  other  hand,  when  the  temperature  is  raised,  the  expansion 
of  the  mercury  causing  the  column  in  the  tube  to  ascend,  it  is  necessary  that 
the  length  of  the  tube  should  be  such  that  the  highest  temperature  to  which  it 
is  intended  to  expose  the  instrument  should  be  such,  that  the  tube  may  afford 
sufficient  room  for  the  increase  of  the  column  produced  by  the  corresponding 
expansion.  From  these  observations  it  will  be  apparent  that  the  quantity  of 
mercurj^  to  be  left  in  the  thermometer  must  depend  on  the  relative  magnitudes 
of  the  bulb  and  tube,  and  on  the  extremes  of  temperature  which  the  instrument 
is  intended  to. measure.  Let  us  suppose  that  the  range  of  the  instrument  shall 
be  confined  to  a  few  degrees  below  and  above  the  temperatures  of  melting  ice 
and  boiling  water.  If  too  much  mercury  be  left  in  the  tube,  on  plunging  the 
instrument  in  boiling  water,  the  mercury  would  rise  to  the  top  of  the  tube,  and 
by  its  expansion  overflow  if  it  were  open,  or  burst  it  if  closed.  If,  on  the 
other  hand,  too  little  mercury  were  left  in  the  instrument,  on  plunging  it  in 
melting  ice  a  contraction  of  the  mercury  by  the  cold  would  cause  it  to  fall  into 
the  bulb,  and  no  indication  could  be  obtained  of  that  part  of  the  contraction  of 
the  mercury  which  took  place  in  the  bulb.  The  law  by  which  the  dilatation 
of  mercury  is  regulated,  will  determine  the  length  which  it  is  necessary  the 
tube  of  the  thermometer  should  have,  provided  the  diameter  of  the  tube  and  the 
contents  of  the  bulb  are  known.  We  shall,  however,  for  the  present,  suppose 
that  the  proper  quantity  of  mercury  has  been  introduced  into  the  apparatus,  so 
that  the  extremes  of  heat  and  cold  shall  not  cause  either  of  the  effects  to  which 
we  have  just  referred. 

It  is  now  necessary  to  close  the  tube  at  the  top  by  melting  the  glass  with  the 
blowpipe  ;  but  in  performing  this  operation,  care  must  be  had  to  exclude  all  the 
air  which  may  remain  in  the  tube  above  the  column  of  mercury.  It  is  found 
that  if  this  air  were  suffered  to  remain  above  the  mercury  in  the  tube  of  the 
thermometer,  any  accidental  agitation  of  the  instrument  is  liable  to  cause  the 
bubbles  of  it  to  mix  with  the  mercury  so  as  to  break  the  column  ;  and  when  this 
happens,  it  is  extremely  difficult  to  disengage  it  from  the  mercury,  and  cause 
it  to  ascend  to  the  top  of  the  tube. 

In  closing  the  top  of  the  tube,  the  air  is  excluded  by  the  following  process  : 
The  bulb  of  the  thermometer  is  exposed  to  heat  until  the  mercury  has  dilated 
so  as  to  cause  the  column   to  rise  very  near  the  extremity  of  the  lube.     The  [ 


136  THE  THERMOMETEB. 


glass  at  the  extremity  is  then  suddenly  melted  by  the  blowpipe,  so  as  to  close 
the  aperture  immediately  above  the  surface  of  the  mercury,  leaving  no  space 
betvi^een  them.  In  this  state  the  sealed  instrument  is  completely  filled  with 
mercury  to  the  exclusion  of  air.  The  instrument  being  now  removed  from  the 
source  of  heat,  the  mercury  again  contracts,  leaving  the  space  between  the  top 
of  the  column  and  the  extremity  of  the  tube  a  vacuum. 

So  far  as  the  formation  of  the  tube  and  the  preparation  of  the  mercury  is  con- 
cerned, the  thermometer  is  now  complete,  and  by  exposure  to  any  variations 
of  temperature,  the  column  of  mercury  in  the  tube  may  be  seen  to  rise  and  fall ; 
but  it  is  necessary  to  provide  an  accurate  and  easy  means  of  measuring  the 
variations  of  this  column.  As  we  suppose  the  tube  to  be  uniformly  cylindrical, 
a  scale  of  equal  divisions  attached  to  it  would  accomplish  this  purpose  ;  but 
such  a  scale  would  merely  give  the  variations  of  temperature  relative  to  one 
thermometer,  and  would  not  be  capable  of  indications  by  which  observations  at 
different  times  and  places  might  be  compared  when  taken  with  instruments 
similarly  construcied.  To  render  the  results  of  different  thermometers,  thus 
constructed,  capable  of  being  compared  one  with  another,  it  will  be  necessary 
to  select  some  points  of  temperature,  by  reference  to  which  all  thermometers 
may  be  graduated. 

Let  us  suppose  that  the  instrument,  as  already  described,  is  plunged  in  a 
vessel  containing  melting  snow  or  ice.  It  will  be  observed  that  the  mercury 
in  the  tube  will  gradually  descend  until  it  arrives  at  a  certain  point,  at  which 
it  will  remain  stationary,  neither  ascending  nor  descending,  so  long  as  any 
portion  of  the  snow  or  ice  remains  to  be  dissolved.  When,  however,  the 
whole  of  the  ice  or  snow  is  liquefied,  and  the  contents  of  the  vessel  become 
pure  water,  then  the  thermometer  will  be  observed  gradually  to  rise  until  it 
attains  that  elevation  at  which  it  would  stand  if  it  were  placed  in  the  atmo- 
sphere of  the  apartment  in  vs'hich  the  experiment  takes  place.  The  inference 
from  this  experiment  is,  that  so  long  as  the  process  of  liquefaction  continues, 
the  temperature  remains  constant,  but  after  the  liquefaction  is  complete  the 
superior  temperature  of  the  apartment  causes  the  water  to  become  hotter ;  and 
this  increase  of  temperature  continues  until  the  w^ater  in  the  vessel  and  the  air 
in  the  apartment  acquire  the  same  temperature.  Now  it  is  found  that  the  point 
at  which  the  column  of  mercury  fixes  itself,  when  immersed  in  the  melting  ice, 
is  invariable  under  all  circumstances.  In  whatever  part  of  the  world  the  ex- 
periment be  tried,  and  at  whatever  season,  and  whatever  be  the  temperature  of 
the  apartment,  still  the  column  will  stand  at  the  same  height.  This,  therefore, 
furnishes  a  fixed  point  of  temperature,  which  can  be  ascertained  in  all  coun- 
tries, and  under  all  circumstances.  This  fixed  point  of  temperature,  being 
marked  in  the  scale  attached  to  the  tube,  is  called  the  freezing  point,  or  the  tem- 
perature of  meliing  ice. 

Let  a  vessel  of  pure  water  be  now  placed  on  a  fire,  and  let  the  thermoriieter 
be  immersed  in  it.  It  will  be  observed  that  the  column  of  mercury  in  the  tube 
will  gradually  rise,  according  as  the  water  receives  heat  from  the  fire,  and  this 
ascent  will  continue  until  ebullition  takes  place.  It  will  be  then  observed  that 
however  long  a  time  the  fire  continues  to  act  on  the  vessel,  the  mercury  will 
no  longer  rise,  nor  will  the  intensity  of  the  fire  cause  any  difference  in  this 
eflect.  The  mercury  will  remain  steadily  at  the  same  point  until  the  whole  of 
the  water  escapes  in  steam,  and  the  vessel  remains  empty.  From  this  experi- 
ment we  infer  that  there  is  a  temperature  beyond  which  water  is  incapable  of 
rising,  so  long  as  it  remains  in  the  liquid  state ;  and  that  the  whole  of  the  heat 
communicated  to  it,  after  it  has  attained  this  point,  is  carried  off  by  the  vapor 
into  which  the  water  is  converted.  If  this  experiment  be  repeated  under  like 
circumstances,  it  is  invariably  found  that  in  all   countries,  and  at  all  seasons, 


THE  THERMOMETER. 


137 


the  mercury,  when  the  thermometer  is  immersed  in  boiling  water,  will  always 
stand  at  the  same  point.  This,  then,  is  another  fixed  point  of  temperature, 
which  may  be  determined  at  all  times,  and  in  all  places,  and  is  called  the 
boiling  point.  Let  the  point  at  which  the  column  of  mercury  stands,  under 
these  circumstances,  be  marked  on  the  scale. 

The  interval  between  the  freezing  and  boiling  points,  thus  ascertained,  is  the 
portion  of  the  tube  which  corresponds  to  the  expansion  of  the  mercury  between 
these  two  points  of  temperature,  and  this  expansion  is  necessarily  always  the 
same  ;  consequently  the  proportion  which  the  capacity  of  the  tube  between  these 
two  points  bears  to  the  volume  of  mercury  contained  in  it  at  the  temperature 
of  melting  ice  must  always  be  the  same.  If  a  number  of  different  thermome- 
ters, prepared  in  a  manner  similar  to  that  already  described,  be  submitted  to 
this  process,  it  will  be  found  that  the  intervals  between  the  freezing  and  boiling 
points  in  them,  severally,  will  differ  in  length.  The  capacities  of  the  tubes, 
between  these  points,  however,  will  always  bear  the  same  proportions  to  the 
capacities  of  those  parts  of  the  instrument  below  the  freezing  point,  including 
the  bulb.  This  is  a  necessary  consequence  of  the  uniform  expansion  of  mer- 
cury when  submitted  to  the  same  limits  of  temperature.  It  is  ascertained  that 
between  the  boiling  and  freezing  points,  the  expansion  of  the  mercury  amounts 
to  one  sixty-third  part  of  its  bulk,  at  the  temperature  of  melting  ice  ;  conse- 
quently the  capacity  of  the  tube  between  the  temperature  of  melting  ice  and 
S  boiling  water,  must  always  be  equal  to  one  sixty-third  part  of  the  capacity  of 
{  the  bulb,  and  that  part  of  the  tube  below  the  mark  indicating  the  temperature 
)  of  melting  ice.  The  different  lengths  of  the  intervals  in  different  thermometers 
between  the  freezing  and  boiling  points  will,  therefore,  arise  from  the  different 
proportions  which  the  capacity  of  that  part  of  the  tube  bears  to  the  capacity  of 
the  bulb,  and  the  portion  of  the  tube  below  the  mark  indicating  the  freezing 
point. 

Thermometer  thus  constructed  would,  at  all  times  and  places,  determine  the 
temperatures  of  all  bodies  whatsoever,  whose  temperatures  were  equal  to  those 
particular  ones  which  have  been  marked  on  the  scale. 

Instruments  thus  constructed  woidd  determine  with  certainty  whether  the 
temperature  of  bodies  to  which  they  were  exposed  were  greater  or  less  than 
those  of  melting  ice  or  boiling  watery  but  could  two  philosophers,  instituting 
experiments  in  different  countries  corresponding  with  each  other,  declare  the 
exact  quantity  by  which  the  temperature  of  any  body  to  which  the  thermometer 
was  exposed  exceeded  or  fell  short  of  those  fixed  temperatures  ?  To  do  so,  he 
would  naturally  inquire  by  what  proportion  of  the  whole  interval  between  the 
freezing  and  boiling  points  the  column  stood  above  or  below  either  of  these 
fixed  terms.  Thus,  if  he  were  able  to  declare  that  the  column  stood  at  a  point 
between  the  fixed  terms  at  a  distance  above  the  freezing  point  equal  to  one 
third  of  the  whole  distance  between  the  freezing  and  boiling  points,  he  would 
enable  another  philosopher,  in  a  distant  country,  to  repeat  the  same  experiment, 
and  to  compare  the  results.  In  order,  therefore,  perfectly  to  estimate  these 
proportional  distances,  the  scale  attached  to  the  thermometer  is  further  divided, 
and  the  interval  between  the  temperatures  of  melting  ice  and  of  boiling  water 
is  divided  into  a  number  of  equal  parts  previously  agreed  upon  ;  and  that  being 
done,  the  same  divisions  are  continued  above  the  term  of  boiling  water  and 
below  the  term  of  melting  ice.  The  number  of  divisions  into  which  the  inter- 
val between  the  fixed  points  of  temperature  is  divided,  being  altogether  arbitrary, 
has  been  differently  determined  in  different  countries,  and  by  the  different  con- 
trivers of  thermometers.  The  thermometer  commonly  used  in  this  country, 
and  called  Fahrenheit'' s  thermomnter,  has  its  interval  divided  into  180  equal  parts, 
called  degrees  ;  and  these    divisions    are   continued  upward  and   downward. 


138 


THE  THERMOMETER. 


They  are  not,  howsA'er,  numerated   commencing  from  either  of  those  fixed  , 
points  of  temperature,  but  the  numeration  commences  at  the  thirty-second  di-  ' 
vision  below  the  freezing  point,  so  that  the  freezing  point  is  32°  and  the  boil-  , 
ing  point  212°.     The  origin  of  this  circumstance  will  be  stated  hereafter.     The  ' 
centigrade  thermometer,  used  in  France,  has  the  intervals  betw^een  the  fixed  ! 
terms  divided  into  100  equal  parts  called  degrees,  the  numeration  commencing 
at  the  freezing  point.     The  thermometer  of  Reaumur,  generally  used  in  other 
parts  of  Europe,  has  the  intervals  divided  into  80°,  the  numeration  commencing 
likewise  at  the  freezing  point.     In  all  thermometers,  the  degrees  below  that  at 
which  the  numeration  commences  upward  are  called  negative,  and  are  marked 
by  the  sign  —  prefixed  to  the  number.     Thus,  —  10°  means  10°  below  that  de- 
gree at  which  the  numeration  upward  commences. 

On  the  slightest  consideration  it  will  be  perceived  that  however  thermome- 
ters may  vary  in  the  intervals  between  the  freezing  and  boiling  points,  they 
must,  if  constructed  in  the  manner  just  described,  agree  in  their  indications  of 
temperature.  If  two  thermometers,  having  difl^erent  intervals  between  these 
points,  be  immersed  in  melting  ice,  they  will  both  stand  at  the  freezing  point. 
If  they  then  be  both  transferred  into  the  water  at  a  temperature  exactly  mid- 
way between  that  and  the  temperature  of  boiling  water,  the  mercury,  expand- 
ing in  the  same  proportion  in  both,  will  dilate  by  exactly  half  that  quantity  which 
it  would  dilate  were  it  exposed  to  the  temperature  of  boiling  water ;  conse- 
quently it  will  stand  at  the  middle  point  exactly  between  the  fixed  terms  of 
the  scale,  and,  consequently,  upon  Fahrenheit's  scale,  it  will  indicate  the 
temperature  of  122°,  being  90°  above  the  freezing  point,  and  90°  below  the 
boiling  point.  In  like  manner,  if  the  thermometer  were  immersed  in  water 
having  a  temperature  exceeding  the  temperature  of  melting  ice  by  one  third 
of  the  excess  of  the  temperature  of  boiling  water  above  that  of  melting  ice, 
it  is  evident  that  the  mercury  will  rise  in  both  through  one  third  of  the  in- 
tervals between  the  fixed  terms,  and,  consequently,  would  ascend  through 
a  space  equal  to  60°  of  Fahrenheit  above  the  freezing  point.  It  would,  there- 
fore, stand  in  both  at  the  temperature  of  92°.  This  reasoning  may  easily  be 
generalized  ;  and  it  will  be  sufficiently  apparent  that  the  indications  of  difl^er- 
ent  thermometers  will  be  the  same,  whatever  be  the  length  of  the  interval 
between  the  fixed  terms  of  their  scales. 

These  arrangements  being  made,  it  will  be  perceived  that  all  thermometers 
thus  constructed,  however  diff'erent  they  maybe  in  size,  in  the  capacity  of  their 
bulbs,  or  in  other  circumstances,  will  always  be  comparable  with  each  other. 
Experiments  performed  in  diff'erent  parts  of  the  world  may,  therefore,  be  com- 
municated from  place  to  place,  and  repeated,  with  the  certainty  of  an  exact 
correspondence  ;  and  all  the  advantages  arising  from  multiplied  experience  will 
thus  be  obtained. 

Various  other  liquids  besides  mercury  have  been  employed  in  the  construc- 
tion of  thermometers  ;  but  the  several  conditions  for  the  attainment  of  accuracy 
which  have  been  explained  in  reference  to  the  mercurial  thermometers  are,  for 
the  most  part,  generally  applicable  to  all  liquid  thermometers  whatever.  Alco- 
hol, or  spirits  of  wine,  is  a  liquid  not  uncommonly  used  for  thermometers.  Its 
inconvenience,  however,  for  ordinary  purposes,  is,  that  it  boils  at  a  tempera- 
ture below  that  of  boiling  water  ;  and,  consequently,  it  will  not  admit  of  a  scale 
so  high  as  this  temperature.  By  adopting  the  precaution  of  excluding  the  air 
from  the  tube  by  the  method  already  explained  in  the  mercurial  thermometers, 
the  spirits  of  wine  may,  however,  be  made  to  indicate  much  higher  tempera- 
tures than  is  commonly  supposed.  They  may  be  raised  to  the  temperature  of 
boiling  water,  or  even  above  it.  If  the  air  be  perfectly  excluded  i'rom  the  tube 
when  the  temperature  is  raised  above  the  boiling  point  of  alcohol,  the  upper 


THE  THERMOMETER. 


139 


I  part  of  the  tube  will  be  occupied  exclusively  by  the  vapor  of  alcohol,  which 
)  will  be  raised  by  the  heat.  The  pressure  of  this  will  prevent  the  remaining 
[  spirit  from  boiling  ;  and,  the  increase  of  temperature  not  being  limited  by  ebul- 
)  lition,  the  liquid  will  continue  to  be  indefinitely  dilated.  The  indications  of 
[  such  a  thermometer,  however,  at  a  higher  temperature,  are  not,  like  those  of 
»  mercury,  equable.  The  scale,  therefore,  if  intended  to  indicate  equal  varia- 
[  tions  of  temperature,  should  not  be  resolved  into  equal  divisions,  but  should  be 
I  divided  experimentally  by  comparison  with  a  mercurial  thermometer.  The 
[  cause  of  this  has  been  already  explained  in  our  chapter  on  the  dilatation  of 
I  liquids.  As  we  approach  the  boiling  point,  the  rate  of  their  dilatation  sensibly 
]  increases,  so  that  equal  changes  of  temperature  would  correspond  to  increasing 

divisions  on  the  scale. 
I  It  is  of  the  most  extreme  importance,  in  the  construction  of  mercurial  ther- 
mometers, that  the  fixed  terms  of  melting  ice  and  of  boiling  water,  which  are, 
in  fact,  the  foundation  of  the  accuracy  of  the  instrument,  should  be  determined 
with  great  care,  and  should  be  rendered  independent  of  all  causes  which  could 
produce  accidental  variation  in  them. 

In  determining  the  freezing  point,  care  should  be  taken  not  to  confound  the 
temperature  of  melting  ice  with  the  temperature  at  which  water  begins  to  freeze. 
It  will  be  explained  hereafter  that,  under  certain  circumstances,  water  may  be 
cooled  considerably  below  the  temperature  of  melting  ice  before  it  becomes 
solid ;  and,  consequently,  the  temperature  at  which  it  freezes  or  solidifies  can- 
not be  considered  as  fixed. 

The  temperature,  however,  at  which  ice  or  snow  melts  is  constantly  the 
same,  provided  the  water  of  which  the  snow  or  ice  is  formed  be  perfectly  pure. 
If  this  water,  however,  hold  salts  in  solution,  it  will  freeze  at  lower  tempera- 
tures, and,  consequently,  it  will  melt  at  lower  temperatures.  Rain-water  or 
pure  snow,  when  melted,  will,  however,  always  give  the  lower  term  of  the 
thermometric  scale,  without  any  liability  to  error. 

The  determination  of  the  higher  term  of  the  scale  is,  however,  attended  with 
more  difficulty,  and  with  more  numerous  causes  of  variation.  It  is,  in  the  first 
place,  necessary  that  the  water  should  be  pure  and  free  from  all  admixture  with 
ibreign  substances.  Thus,  water  charged  with  salts  will  boil  at  temperatures 
different  from  pure  water.  It  is  necessary,  therefore,  that  the  water  with 
which  the  experiment  is  made  should  be  either  rain-water  or  distilled  water. 

There  is,  however,  another  cause,  which  more  constantly  affects  the  temper- 
ature at  which  water  boils.  It  appears,  as  may  be  elsewhere  shown,  that  the 
pressure  exerted  on  the  surface  of  the  water,  whether  of  the  atmosphere  or 
from  condensed  or  rarefied  air,  will  affect  its  boiling  temperature.  If  this  tem- 
perature be  increased,  the  water  will  receive  a  higher  temperature  before  it 
will  boil ;  and  if  it  be  diminished,  it  will,  on  the  other  hand,  boil  at  a  lower 
temperature.  Thus,  water  in  an  exhausted  receiver  will  boil  at  a  much  lower 
temperature  than  when  exposed  to  the  atmosphere.  These  circumstances  may 
be  more  fully  detailed  in  another  lecture  ;  but,  for  the  present,  it  will  be  suf- 
ficient to  allude  to  them,  in  order  to  explain  why  the  pressure  of  the  atmo- 
sphere must  be  attended  to  in  determining  the  boiling  point  on  a  thermometric 
scale.  The  barometer,  from  day  to  day,  and  from  hour  to  hour,  is  subject  to 
fluctuation,  and  a  corresponding  change  takes  place  in  the  pressure  of  the  at- 
mosphere ;  consequently,  although  this  variation,  being  small,  cannot  affect  the 
temperature  at  which  water  boils  to  any  considerable  extent,  yet  it  does  affect 
it  so  much  as  to  render  it  an  object  of  important  calculation  in  determining  an 
element  such  as  that  now  under  consideration,  upon  which  the  accuracy  of  all 
thermometric  indications  must  depend.  To  determine  this  fixed  temperature, 
therefore,  it  will  be  necessary,  either  to  recur  to  some  phenomena  not  affected 


140 


THE  THERMOMETER. 


by  the  atmospheric  pressure,  or  to  select  some  determinate  pressure  of  the  at- 
mosphere, or  height  of  the  barometer,  at  which  the  fixed  temperature  must  be 
taken.  An  alloy  of  two  parts  of  lead,  three  of  tin,  and  five  of  bismuth,  was 
found  by  Newton  to  be  fused  at  a  fixed  temperature  nearly  equal  to  that  of  boil- 
ing water.  As  this  fusion  is  not  aff'ected  by  the  atmospheric  pressure,  it  might 
be  taken  as  the  means  of  determining  the  boiling  point  on  a  thermometer ;  but 
it  is  more  convenient  to  note  the  temperature  of  boiling  water,  and  at  the  same 
time  to  observe  the  height  of  the  barometer.  If  it  be  agreed  that  the  boiling 
point  be  taken  when  the  barometer  stands  at  a  given  altitude,  as  at  30  inches, 
then,  by  knowing  the  law  at  which  the  temperature  of  boiling  water  varies, 
with  reference  to  the  variation  in  the  pressure  of  the  atmosphere,  it  will  be 
easy  to  reduce  the  boiling  temperature  under  any  pressure  to  that  with  the 
pressure  agreed  upon.  The  pressure  recommended  in  the  directions  published 
by  the  Royal  Society  for  the  construction  of  thermometers,  is  that  of  the  atmo- 
sphere when  the  barometer  stands  at  29-8  inches. 

The  temperature  at  which  water  boils  is  varied,  in  some  degree,  according 
to  the  material  of  the  vessels  which  contain  it,  and  also  according  to  solid  sub- 
stances which  may  be  mixed  with  it,  though  they  may  not  be  held  in  solution. 
If  distilled  water  be  boiled  in  a  vessel  of  glass,  the  process  will  be  observed  to 
go  on  irregularly,  and  with  apparent  difficulty.  When  the  fire  is  removed,  and 
the  temperature  lowered,  it  may  be  restored  to  the  state  of  ebullition  by  throw- 
ing into  it  some  iron  filings.  Nevertheless,  though  it  thus  boils,  its  tempera- 
ture is  lower  than  that  which  it  had  when  boiled  in  the  glass  before  the  iron 
filings  were  introduced.  In  determining  the  boiling  point  on  the  thermometric 
scale,  the  water  should,  therefore,  be  free  from  any  solid  admixture,  and  should 
be  boiled  in  a  metallic  vessel. 

In  observing  these  fixed  points  of  temperature,  the  thermometer,  when  im- 
mersed in  melting  ice,  should  be  completely  submerged,  not  only  as  to  the 
bulb,  but  as  to  the  tube,  in  order  that  every  part  of  the  mercury  should  take  the 
same  temperature.  If  the  bulb  alone  were  immersed,  the  mercury  in  the  bulb 
would  have  the  temperature  of  the  melting  ice,  while  the  mercury  in  the 
tube  would  have  the  temperature  of  the  surrounding  air ;  consequently,  the 
column  would  stand  at  a  greater  altitude  than  that  which  it  would  have  were 
it  all  at  the  same  temperature.  It  is  possible,  by  calculation,  to  allow  for  this 
difference  ;  but  it  is  more  effectual,  and  more  conducive  to  accuracy,  to  im- 
merse the  whole  thermometer  in  the  fluid. 

The  accurate  determination  of  the  boiling  point  requires  still  further  precau- 
tions. 

When  water  contained  in  the  vessel  boils,  the  strata  at  different  depths  have 
different  temperatures  ;  and  if  the  instrument  be  immersed  vertically,  the  mer- 
cury in  the  bulb  will  have  a  higher  temperature  than  the  mercury  in  the  tube. 
It  is  necessary,  therefore,  if  the  thermometer  be  immersed  in  the  fluid,  that  it 
should  be  placed  in  a  horizontal  position,  and  not  immersed  to  a  greater  depth 
than  is  necessary  to  cover  the  bulb  and  tube.  This  position,  however,  is  one 
which  renders  it  extremely  difficult  to  observe  with  accuracy  the  height  of  the 
column.  The  fact,  which  will  be  proved  hereafter,  that  steam  raised  from  wa- 
ter has  the  same  temperature  with  the  water  from  which  it  proceeds,  furnishes 
an  easy  means  of  fixing  the  boiling  point.  Let  the  thermometer  tube  be  in- 
serted in  the  neck  of  a  vessel,  so  that  the  bulb  shall  reach  nearly  to  the  surface 
of  the  water,  and  let  another  orifice  be  provided  through  which  the  steam  may 
escape  into  the  atmosphere.  This  done,  let  the  water  be  boiled  until  the 
space  in  the  vessel  above  its  surface  is  completely  filled  with  steam,  as  will  be 
shown  by  the  rapid  escape  of  the  steam  from  the  orifice  provided  for  that  pur- 
pose.    The  thermometer,  including  the  tube   and  bulb,  is  now  surrounded  by 


THE  THERMOMETER. 


141 


an  atmosphere  of  steam  raised  from  the  water  under  a  pressure  equal  to  that 
of  the  atmosphere.  This  steam  has  the  true  temperature  of  the  boiling  water  ; 
and,  by  drawing  the  tube  upward  through  the  orifice  in  which  it  plays,  the 
height  of  the  mercurial  column  in  the  thermometer  may  be  marked  with  the 
utmost  accuracy,  and  thus  the  boiling  point  may  be  determined. 

The  variation  of  the  column  in  the  thermometric  tube,  strictly  speaking,  arises 
not  from  the  expansion  of  the  mercury  alone,  but  from  the  difference  between 
the  expansions  of  the  mercury  and  glass.  It  is  clear  that,  if  a  given  change 
of  temperature  dilated  equally  the  glass  of  the  tube  and  bulb,  and  the  mercury 
contained  in  it,  the  height  of  the  column  would  not  be  varied  ;  because,  in  the 
same  proportion  as  the  dimensions  of  the  mercury  would  be  increased,  the  ca- 
pacity of  the  tube  and  bulb  would  also  be  increased.  But,  in  fact,  although  the 
tube  and  bulb  undergo  an  increase  of  dimension  from  every  change  of  tempera- 
ture, that  increase  is  extremely  small  when  compared  with  the  dilatations  of 
the  mercury,  and  consequently,  notwithstanding  that  more  room  is  made  for 
the  fluid  by  the  dilatation  of  the  glass,  yet  still,  the  room  not  being  nearly  suf- 
ficient, the  mercury  rises.  Nevertheless,  although  the  variations  of  the  mer- 
curial column  are  not  absolute  indications  of  the  dilatation  or  contraction  of  the 
mercury,  yet  it  so  happens  that,  under  all  the  changes  of  temperature  to  which 
a  mercurial  thermometer  can  be  submitted,  the  dilatation  of  glass  is  in  the  same 
proportion  as  the  dilatation  of  mercury,  and  consequently  the  change  of  volume 
of  the  mercury  bears  a  fixed  proportion  to  the  change  of  the  capacity  of  the 
tube  ;  and  the  variation  in  the  height  of  the  column  contained  in  the  tube  bears 
also  the  same  proportion  to  the  variations  which  it  would  undergo  if  the  glass 
suffered  no  expansion  or  contraction.  The  apparent  dilatation  of  the  mercury, 
or  the  difference  between  the  dilatations  of  the  mercury  and  glass,  between 
the  freezing  and  boiling  points,  amounts  to  one  sixty-third  part  of  the  volume 
of  mercury  at  the  temperature  of  melting  ice  ;  and  the  actual  dilatation  of  the 
mercury  between  these  limits  of  temperature  is  somewhat  less  than  this, 
being  g'jYa  P^^^^s  of  the  volume  of  the  mercury  at  the  temperature  of  melt- 
ing ice. 

The  fact  that  the  indications  of  the  thermometer  are  independent  of  the  ab- 
solute expansion  of  the  glass  which  forms  it  is  a  matter  of  great  importance, 
because  it  shows  that  the  accuracy  of  thermometers  does  not  depend  upon  the 
species  of  glass  of  which  they  are  formed.  Had  it  been  otherwise,  one  of  the 
conditions  necessary  in  the  construction  of  a  thermometer  would  be,  that  the 
glass  should  be  manufactured  of  elements  precisely  alike  in  all  cases.  That, 
however,  is  by  no  means  necessary.  Different  kinds  of  glass  undergo  different 
degrees  of  expansion  by  change  of  temperature  ;  but  they  will  expand  propor- 
tionally to  each  other,  and  proportionally  to  the  expansion  of  mercury  within 
those  limits  of  temperature  to  which  mercurial  thermometers  are  applied. 

It  will  be  perceived,  from  the  reasoning  that  has  been  pursued  upon  this  sub- 
ject, that  the  indications  of  all  thermometers  whatever  would  necessarily  cor- 
respond, even  though  the  fluid  from  which  they  are  formed  were  different,  pro- 
vided only  that  the  rate  of  its  expansion  correspond  with  that  of  mercury.  A 
thermometer  of  spirits  of  wine,  within  that  part  of  the  scale  through  which  the 
dilatation  of  that  fluid  is  uniform,  would  necessarily  correspond  with  the  mer- 
curial thermometer.  The  difference  would  only  be  in  the  length  of  the  scale, 
or,  in  other  words,  in  the  distances  between  the  freezing  and  boiling  points. 
In  the  case  of  spirits  of  wine,  however,  the  rate  of  dilatation  approaching  the  ' 
boiling  point  of  water  is  not  uniform,  as  has  been  already  stated.  \ 

It  may  possibly  be  thought  that  the  preceding  details  respecting  the  con-  * 
struction  and  use  of  thermometers  may  be  elaborately  minute,  and  that  an  in-  < 
strument  apparently  so  trifling  as  a  glass  bulb  blown  on  the  extremity  of  a  tube,  j 


142 


THE  THERMOMETER. 


and  partially  filled  with  quicksilver,  could  be  described,  and  have  its  properties 
explained,  in  a  much  more  limited  space.  It  should,  however,  be  remembered 
that,  trifling  as  this  instrument  may  appear,  its  uses  are,  perhaps,  more  exten- 
sive, and  certainly  not  less  important,  than  any  other  means  of  experimental 
investigation  by  which  we  are  enabled  to  scrutinise  the  laws  of  nature.  There 
is  no  department  of  natural  science  where  experiment  and  observation  are  the 
means  of  knowledge,  in  which  the  indications  of  this  instrument  are  not  abso- 
lutely indispensable  ;  and  this  must  be  apparent,  if  it  be  considered  how  essen- 
tially the  states  of  all  bodies,  whether  those  contemplated  in  mechanical  sci- 
ence, in  chemistry,  nay,  even  in  medicine  and  the  natural  sciences,  are  affected 
both  by  the  external  application  of  heat  and  its  internal  development.  Without 
the  thermometer,  we  should  possess  no  means  of  determining  those  changes  of 
effects  better  than  the  very  fallible  and  inaccurate  perceptions  of  the  senses ; 
perceptions  which,  as  it  will  hereafter  appear,  depend  much  more  upon  cir- 
cumstances in  our  ever-changing  states  of  body,  than  on  the  states  of  the  bod- 
ies around  us.  In  physics,  the  thermometer  is  indispensable  in  almost  every 
experiment.  In  the  laboratory,  the  chemist  can  scarcely  conduct  a  process 
with  any  degree  of  philosophical  accuracy  without  an  observation  of  tempera- 
tures. In  the  observatory,  the  astronomer  who  is  ignorant  what  effects  chan- 
ges of  temperature  produce  on  the  indications  of  the  large  metallic  instruments 
which  he  uses — instruments  so  highly  susceptible  of  dilatation  and  contraction 
— would  be  surrounded  with  sources  of  error,  of  which  it  would  be  impossible 
for  him  to  estimate  the  amount,  or  even  to  detect  the  existence.  Even  the  as- 
pect of  the  heavens  changes  its  appearance  in  obedience  to  the  fluctuating  tem- 
peratures of  air  ;  nor  is  there  a  single  object  in  the  firmament  seen  in  the  same 
position  for  two  successive  hours,  and  never  in  the  true  position  which  it 
would  have  independently  of  the  effects  of  heat.  The  vicissitudes  of  heat  and 
cold,  to  which  the  atmosphere  is  subject,  must,  therefore,  be  appreciated  before 
the  observer  can  pronounce  on  the  position  of  any  celestial  object ;  and  to  this 
there  is  no  guide  but  the  thermometric  tube.  The  naturalist,  in  investigating 
the  properties  of  the  various  classes  of  organized  bodies,  bases  many  of  his 
generalizations  on  their  temperatures  discovered  by  this  instrument.  In  inves- 
tigating the  qualities  of  different  parts  of  our  planet,  the  variations  of  climate 
corresponding  with  changes  of  latitude,  the  phenomena  peculiar  to  land  and 
sea,  the  various  meteorological  facts  essential  to  all  knowledge  of  climate  and 
to  all  investigation  in  physical  geography,  depend  on  the  indications  of  the 
thermometer.  The  measurement  of  the  heights  of  mountains,  of  the  position 
of  balloons  in  the  atmosphere,  are  estimated  by  combined  observations  on  this 
instrument  and  the  barometer.  When  these  and  numerous  other  considerations 
are  called  to  mind,  it  will  scarcely  be  deemed  inappropriate,  even  in  a  work 
of  a  popular  nature,  to  enter  into  the  details  which  have  been  here  given  re- 
specting the  construction  and  use  of  this  instrument.  For  the  same  reasons,  it 
may  not  be  uninteresting  to  the  general  reader  shortly  to  trace  the  history  of 
the  invention  and  improvement  of  thermometers  before  we  conclude  this  lec- 
ture. 

Like  other  inventions  of  very  extensive  utility  and  remote  date,  that  of  the 
thermometer  is  disputed  by  many  contending  claimants  ;  and,  like  other  inven- 
tions, the  merit  is  not  to  be  ascribed  to  one  person,  but  to  be  distributed  among 
many.  The  several  arrangements  which  render  the  instrument  useful  and  ac- 
curate as  a  measure  of  a  degree  of  temperature  were  suggested  successively, 
and  adopted  through  a  long  period  of  time,  and  some  of  the  latest  of  them  have 
not  been  of  very  remote  date. 

The  notion  of  using  the  expansion  of  a  liquid  contained  in  a  bulb  and  tube 
of  glass,  as  a  means  of  indicating  changes  of  temperature,  is  said  by  some  to 


THE  THERMOMETER. 


143 


j  have  been  first  suggested  by  Cornelius  Drebbel,  a  resident  at  Alkmaer,  in  Hol- 
(  land.  He  is  said  by  Boerhaave  and  Muschenbroek  to  have  invented  thermom- 
)  eters  about  the  year  1600.  Some  Italian  writers  also  assign  this  honor  to 
(  Drebbel,  but  others  give  the  credit  of  the  invention  to  Galileo  ;  while  it  is  as- 
)  sorted  by  other  Italian  authorities,  including  Borelli  and  Malpighi,  that  the 
(  merit  of  the  invention  is  due  to  Sanctorio,  a  well-known  medical  professor  at 
)  Padua.  Sanctorio,  indeed,  claims  the  invention  himself,  and  the  Florentine  aca- 
(  demicians,  Borelli  and  Malpighi,  are  witnesses  not  likely  to  be  biased  in  favor 
)  of  the  Patavinian  professor. 

<  The  thermometer  of  Sanctorio  was  formed  of  a  glass  bulb  and  tube,  in  which 
)  the  air  was  first  rarefied  in  a  slight  degree  by  the  application  of  heat.  The  end 
(  of  the  tube  was  then  plunged  in  a  colored  liquid,  which,  when  the  air  contract- 
)  ed  by  cooling,  was  forced  up  into  the  tube  by  the  atmospheric  pressure.  The 
I  tube  was  divided  into  a  number  of  equal  parts,  called  degrees.  When  the  tem- 
)  perature  of  the  medium  surrounding  the  bulb  was  raised,  the  air  included  in 
(  it  expanded,  and  the  colored  liquid  was  forced  downward  in  the  tube.  When 
)  the  temperature  surrounding  the  bulb,  on  the  other  hand,  was  lowered,  the  air 
(  losing,  some  of  its  elasticity,  the  liquid  was  forced  higher  in  the  tube  by  the 
)  atmospheric  pressure.  The  number  of  degrees  on  the  tube  through  which  the 
(  colored  liquid  moved  were  taken  as  the  indication  of  the  changes  of  tempera- 
)  ture.  Thus  the  thermometer  of  Sanctorio  was,  in  fact,  an  air  thermometer.  Its 
(  indications,  however,  were  necessarily  affected  by  the  changes  in  the  atmo- 
)  spheric  pressure,  as  well  as  by  change  of  temperature.  At  the  same  tempera- 
(  ture,  an  increase  in  the  atmospheric  pressure  would  cause  the  column  to  rise 
)  in  the  tube,  and  a  decrease  would  cause  it  to  fall.     Such  an  instrument,  there- 

<  fore,  when  used  as  an  indicator  of  the  variations  of  temperature,  should  always 
)  be  corrected  with  reference  to  the  changes  in  the  thermometric  column.  This 
(  thermometer  has  no  fixed  points  of  temperature,  nor  could  the  indications  of 
)  one  instrument  be  compared  with  those  of  another,  nor  with  itself,  after  any  de- 
(  rangement  or  change  of  circumstances. 

)  About  fifty  years  subsequently  to  this,  the  Florentine  professors  constructed 
(  thermometers  of  spirits  of  wine,  and  excluded  from  them  the  air  in  the  upper 
)  part  of  the  tube  by  the  manner  already  explained  with  reference  to  the  mercu- 
(  rial  thermometer.  The  tube  was  divided  into  one  hundred  parts,  called  degrees  ; 
)  but  still  no  fixed  points  of  temperature  were  adopted. 

S  About  the  year  1725,  Fahrenheit,  a  thermometer-maker  of  Amsterdam,  first 
)  substituted  mercury  for  spirits  of  wine  in  thermometers,  and  by  this  means  con- 
s  siderably  reduced  their  magnitude.  The  instrument  was  thus  capable  of  meas- 
)  uring  much  higher  degrees  of  temperature  than  thermometers  of  spirits  of  wine, 
s  because  mercury  does  not  boil  until  it  attains  a  very  high  temperature.  Still, 
/  however,  thermometers  labored  under  defects  arising  from  the  want  of  fixed 
S  points  of  temperature,  the  nature  of  which  have  been  already  fully  explained. 
)  Various  attempts  were  made  to  insure  the  correspondence  of  the  scale  of  differ- 
s  ent  thermometers  employed  in  different  parts  of  the  world,  but  as  yet  no  efFect- 
)  ual  method  was  suggested. 

S  Late  in  the  seventeenth  century  Dr.  Hook  discovered  the  fact,  that  water 
}  during  its  conversion  into  ice,  and  ice  during  its  conversion  into  water,  main- 
^  tained  a  fixed  temperature  ;  and  also  that  water,  during  the  process  of  boiling 
}  under  the  same  circumstances,  retains  the  same  temperature.  These  two  tem- 
)  peratures,  depending  upon  fixed  phenomena  not  affected  by  change  of  time  or 
(  place,  furnished  convenient  star^dards  by  which  the  fixed  points  upon  thermom- 
)  eters  might  be  determined  ;  and  as  such  they  were  first  recommended  and  . 
(  adopted  by  Newton.  As  the  process  of  fusion  and  evaporation  of  all  bodies 
)  are  attended  with  the  same  peculiar  effects  as  those  of  water,  their  temperatures  ' 


THE  THERMOMETER. 


during  these  states  of  transition  might  with  equal  convenience  be  taken  as  the 
standards  for  the  fixed  points  of  thermometers  ;  but  water,  being  a  substance 
always  attainable  and  easily  reduced  to  a  pure  state,  has  been  selected  by  com- 
mon consent,  in  preference  to  other  bodies. 

The  same  unanimity  has  not  prevailed  respecting  the  division  of  the  scale. 
It  would  have  been  a  matter  of  great  convenience,  had  all  nations  agreed  to  di- 
vide the  interval  between  the  boiling  and  freezing  points  of  thermometers  into 
the  same  number  of  equal  parts  ;  but  such  a  convention  was  scarcely  to  be  ex- 
pected. When  Fahrenheit  adopted  the  fixed  points  suggested  by  Newton,  it 
was  supposed  that  the  greatest  degree  of  cold  which  was  attainable  was  that 
of  a  mixture  of  snow  and  common  salt,  or  snow  and  sal  ammoniac.  A  ther- 
mometer, when  plunged  in  such  a  mixture,  was  observed  to  fall  considerably 
below  the  point  at  which  it  stood  in  melting  ice,  and  at  which  temperature  Fah- 
renheit determined  to  commence  his  scale  of  numeration  upward.  The  inter- 
val between  this  and  the  temperature  of  melting  ice  is  divided  into  32  equal 
parts  or  degrees ;  so  that  upon  this  scale  the  temperature  produced  by  mixing 
snow  and  common  salt  is  0°,  while  the  temperature  of  melting  ice  is  32°.  He 
continued  these  equal  divisions  upward,  and  found  that  when  the  thermometer 
was  immersed  in  the  steam  of  boiling  water,  the  barometer  standing  at  about 
30  inches,  the  mercury  in  the  thermometer  stood  at  212°.  Thus  the  interval 
between  the  freezing  and  boiling  points  was  180°.  Temperatures  have  since 
been  experienced  much  lower  than  that  obtained  by  the  mixture  of  snow  and 
common  salt,  and  hence  it  has  been  necessary  to  continue  the  scale  below  the 
0°  of  Fahrenheit.  Degrees  below  this  point  are  called  negative  degrees,  as 
already  explained. 

The  scale  as  adopted  by  Fahrenheit  has  continued  in  general  use  in  this 
country  to  the  present  day ;  and  in  all  English  works  on  science,  as  Avell  as  in 
the  arts,  manufactures,  and  medical  practice,  the  thermometer  used  is  Fah- 
renheit's thermometer,  and  the  freezing  and  boiling  points  are  32°  and  212°. 
The  thermometer  generally  used  in  France  before  the  revolution,  and  still  used 
in  many  parts  of  Europe,  was  constructed  by  Reaumur  early  in  the  1 8th  cen- 
tury. The  liquid  used  by  him  was  spirit  of  wine  ;  but,  subsequently,  mercury 
was  substituted  for  this  by  De  Luc.  The  fixed  points  on  this  instrument  were 
likewise  the  freezing  and  boiling  points  of  water,  the  scale  proceeding  up- 
ward. The  internal  between  the  fixed  points  was  divided  into  80  equal  parts, 
called  degrees.  Thus,  the  freezing  point  of  water  was  0°,  and  its  boiling 
point  80°.  The  degrees  in  this  thermometer  were  longer  than  those  in  Fah- 
renheit, in  the  proportion  of  2^  to  1.  To  convert  a  temperature  indicated  upon 
Reaumur  into  the  corresponding  temperature  upon  Fahrenheit,  it  would,  there- 
fore, be  necessary  to  multiply  the  degrees  upon  Reaumur  by  2^,  and  to  add  to 
the  product  32°,  to  allow  for  the  distance  of  the  points  at  which  the  scale  com- 
mences. On  the  other  hand,  to  reduce  Fahrenheit's  degree  to  Reaumur,  it 
would  be  necessary  to  subtract  32,  and  to  diminish  the  remainder  in  the  pro- 
portion of  2J  to  1. 

About  the  middle  of  the  eighteenth  century,  Celsius,  a  Swedish  astronomer, 
constructed  thermometers,  in  which  he  commenced  the  scale,  like  Reaumur, 
at  the  freezing  point  of  water,  and  divided  the  interval  between  the  freezing  and 
boiling  points  into  100°.  This  thermometer  was  adopted,  after  the  revolution, 
in  France,  under  the  name  of  the  centigrade  thermometer.  It  harmonized  with 
the  uniform  decimal  system  of  weights  and  measures,  adopted  in  that  coun- 
try, and  has  been  since  that  time  in  general  use  there.  100°  of  the  centigrade 
are  equal  in  length  to  180°  of  Fahrenheit.  To'convert  the  temperature  on  the 
centigrade  into  the  corresponding  temperature  on  Fahrenheit,  it  would  then  be 
necessary,  first,  to  increase  the  number  of  degrees  in  the  proportion  of  100  to 


r 


THE  THERMOMETER. 


145 


180,  or,  what  is  the  same,  5  to  9,  and  to  add  to  the  result  32°,  to  allow  for  the 
difference  between  the  points  at  which  the  scale  commences.  To  convert  a 
temperature  on  Fahrenheit  into  the  corresponding  temperature  on  the  centi- 
grade thermometer,  it  would  be  necessary  to  subtract  32°,  and  to  diminish  the 
remainder  in  the  proportion  of  9  to  5. 

Thermometers  are  sometimes  constructed  for  scientific  purposes,  to  which 
all  the  three  scales  are  annexed.  The  reduction,  however,  of  equivalent  tem- 
peratures, one  to  the  other,  is  a  measure  of  easy  arithmetical  calculation. 

Like  all  thermometers  whose  indications  depend  upon  the  dilatation  or  con- 
traction of  a  liquid,  the  range  of  the  mercurial  thermometer  is  limited  to  the 
points  at  which  mercury  freezes  and  boils.  These  points,  however,  as  has 
been  already  said,  include  between  them  a  range  of  very  great  extent,  through- 
out, nearly  the  whole  of  which  the  indications  of  the  thermometer  are  uniform. 
The  freezing  point  of  mercury  is  placed  at  about — 39°  of  Fahrenheit,  or  72° 
below  the  freezing  point. 

Mercury  boils  at  660°.  Thus  the  range  of  the  thermometer  includes  about 
700°  of  Fahrenheit.  The  dilatations  of  the  mercury,  as  it  approaches  its  boil- 
ing point,  go  on  at  a  slowly-increasing  rate  ;  but  this  increase  is  compensated 
for  by  the  expansion  of  the  glass  in  which  the  mercury  is  contained,  in  such 
a  manner  that  the  apparent  dilatation  shown  by  the  actual  ascent  of  the  col- 
umn in  the  tube  is  really  uniform,  and  the  same  which  would  take  place  if  the 
glass  did  not  expand  at  all,  and  the  dilatation  of  the  mercury  were  absolutely 
uniform.  A  thermometer  intended  to  measure  temperatures  below  the  freez- 
ing point  of  mercury  may  be  constructed  of  spirits  of  wine  or  alcohol.  No 
attainable  degree  of  cold  has  ever  yet  reduced  this  liquid  to  the  solid  state,  and 
a  thermometer  filled  with  it  may  be  graduated,  by  comparison  with  a  mercurial 
thermometer,  above  the  freezing  point  of  mercury  ;  and  its  indications  below 
the  freezing  point  will  thus  be  rendered  capable  of  comparison  with  the  indi- 
cations of  a  mercurial  thermometer. 

Thermometers  whose  indications  depend  on  the  dilatation  of  air  are  rarely 
used,  except  for  peculiar  purposes  in  which  minute  variations  of  temperature 
only  are  required  to  be  obtained. 

Since  mercury  boils  at  a  higher  temperature  than  any  known  liquid,  it  fol- 
lows that  no  liquid  thermometer  can  indicate  higher  temperatures  than  that  of 
660°  Fahrenheit.  To  determine  temperatures  above  this,  the  dilatation  of 
solids  has  generally  been  used ;  and  instruments  founded  upon  this  principle 
are  commonly  called  pyrometers.  The  changes  of  temperature  are  indicated 
by  the  difference  of  the  expansions  of  two  metals.  Such  an  instrument 
would  indicate  all  temperatures  below  that  at  which  the  more  fusible  metal 
melts. 

In  the  use  of  the  thermometer,  and  in  the  inferences  drawn  from  its  indica- 
tions, care  should  be  taken  not  to  assume  that  the  quantity  of  caloric  introduced 
into  the  bodies  is  represented  by  the  degrees  of  the  thermometer.  We  shall 
hereafter  show  that  caloric  may  be  introduced  into  a  body  without  affecting  the 
thermometer  at  all,  and  also  that  different  quantities  of  caloric  introduced  into 
different  bodies  affect  the  thermometer  equally.  "  Degrees  of  temperature" 
are,  therefore,  to  be  carefully  distinguished  from  the  "  quantity  of  heat ;"  and 
the  thermometer  must  be  understood  as  a  measure  of  temperature,  and  not  as  a 
measure  of  heat.  When  two  bodies  are  said  to  undergo  the  same  increase  of 
temperature,  it  is  not  meant  that  these  two  bodies  receive  the  same  increase  of 
heat,  but  merely  that  they  undergo  such  a  change,  with  respect  to  heat,  that 
they  are  capable  of  causing  a  thermometer  exposed  to  them  to  undergo  the 
same  degree  of  expansion.  Again,  if  a  thermometer  be  immersed  in  melting 
ice,  and  observed  to  stand  at  the  temperature  of  32°,  and  the  same  thermome- 

VOL.  II.— 1() 


ter  be  surrounded  by  the  steam  of  boiling  water,  and  be  observed  to  stand  at 
212°,  we  declare  that  the  temperature  of  boiling  water  exceeds  the  tempera- 
ture of  melting  ice  by  180°  ;  the  meaning  of  which  is,  that  the  state,  with  re- 
spect to  heat,  of  boiling  water  compared  with  melting  ice,  is  such  as  to  cause 
a  quantity  of  mercury  transferred  from  the  one  to  the  other  to  increase  its 
dimensions  by  about  one  sixty-third  part  of  its  whole  bulk  at  the  lower  tem- 
perature. 


ATMOSPHEEIC  ELECTRICITY. 


On  the  Electncity  of  the  Atmosphere  in  clear  Weather. — Connexion  between  Electricity  and  Me- 
teorology.— Apparatus  for  observing  the  Electricity  of  the  Atmosphere.—  lnsxiXsled.  elevated  Rod. — 
Portable  Apparatus  made  of  a  fishing'  Rod. — Saussure's  Electroscope  and  his  Mode  of  estimating 
the  Value  of  the  Divergences. — Occasional  Use  of  the  Galvanometer. — Theordinary  State  of  the 
Atmosphere. — Volta's  Theory  of  the  Origin  of  Atmospheric  Electricity. — Inadequacy  of  the  The- 
ory of  Chemical  Origin. — The  Author's  Suggestion  of  the  probable  Influence  of  Friction. — Diur- 
nal Vaiiation  of  the  Electricity. — Periodical  hourly  Variation. — Representation  of  the  Rate  of  Va- 
riation.— Maxima  and  Minima  at  a  given  Parallel. — Schubler's  Observations. — Annual  Variation 
of  the  Electricity. — Variation  of  the  daily  Maxima  and  Minima. — Arago's  Repetition  of  Schiibler's 
Observations. — Local  Variations  of  the  Electricity. — Influence  of  particular  Localities,.  Buildings, 
&c.^ — No  satisfactory  Explanation  yet  given  of  the  Variations. — Correspondence  belvs^een  Electric 
and  Magnetic  Variations. — Becquerel's  Explanation  of  the  Phenomena  of  Variation. — Distribution 
of  Electricity  of  the  Air. — Negative  State  of  the  Earth. — Character  of  the  lower  Stratum  of  Air. — 
Increase  of  Electric  Charge  in  the  higher  Strata  of  Air. — Decrease  in  the  lower  Strata. — Compar- 
ative Electric  Character  of  different  Strata. — Formulae  for  the  comparative  Electricity  of  two 
Strata. — Electricity  of  the  Air  in  clouded  Weather. — Preliminary. — Schiibler's  Observations. — Ta- 
ble  of  Observations  explained. 


ATMOSPHERIC  ELECTRICITY, 


Among  the  innumerable  relations  of  the  electric  fluid  with  the  phenomena 
of  nature,  there  are  none  which  present  so  many  circumstances  of  general  in- 
terest as  its  connexion  with  the  various  states  and  appearances  of  the  atmo- 
sphere. Indeed,  it  were  difficuU  to  name  any  atmospheric  change  which  is 
not  directly  or  indirectly  connected  with  electric  agency.  It  is  true  that  these 
phenomena,  fugitive  and  transitory  as  most  of  them  are,  have  not  been,  in  every 
case,  traced  to  their  causes ;  that  the  relation  of  many  of  them  to  the  agency 
of  electricity  is  rendered  probable  from  general  appearances,  rather  than  dis- 
tinctly and  satisfactorily  demonstrated  ;  that  some  of  them,  which  are  evidently 
of  electric  origin,  nevertheless  have  not  been  explained  by  or  reduced  to  any 
of  the  known  laws  which  govern  that  physical  agent ;  still,  there  is  much  that 
falls  under  the  general  principles  of  electrical  science  ;  and  those  phenomena 
which  remain  without  any,  or  without  satisfactory  explanation,  require  to  be 
stated,  that  those  who  pursue  this  part  of  physical  science,  with  the  view  to 
extend  its  limits,  may  be  guided  to  the  proper  subjects  of  observation  and  in- 
vestigation. 

We  shall  first,  then,  state  generally  the  apparatus  used  for  observing  the 
electric  state  of  the  air,  and  shall  next  proceed  to  explain  the  results  at  which 
those  philosophers  have  arrived  whose  attention  has  been  directed  to  atmo- 
spheric electricity. 


APPARATUS    FOR    OBSERVING    THE    ELECTRICITY    OF    THE    ATMOSPHERE. 

To  construct  a  stationary  apparatus  for  observing  the  electric  state  of  the 
air,  let  a  rod  of  iron,  from  twenty  to  twenty-five  feet  in  length,  be  erected  at  the 
top  of  the  building  in  which  the  observatory  is  placed,  and  let  it  be  carefully 
insulated  at  the  points  where  it  meets  the  roof  and  other  parts  of  the  build- 
ing. The  lower  parts  of  this  rod  should  be  in  metallic  communication  with  an 
electroscope  placed  in  the  observatory,  by  means  of  a  chain  or  bar  capable  of 


150 


ATMOSPHERIC  ELECTRICITY. 


being  removed  at  pleasure.  A  moveable  commimication  should  also  be  provi- 
ded betvi^een  the  pointed  rod  and  a  metallic  bar  continued  to  the  ground,  so 
that  in  cases  of  thunder-storms,  or  at  any  other  time  vs^hen  the  electricity  of  the 
air  is  so  strong  as  to  be  attended  with  danger,  it  may  be  allowed  to  escape  to 
the  earth  by  putting  the  pointed  rod  in  communication  with  this  conductor.  If 
it  be  desired  to  observe  the  electric  state  of  the  air  when  it  is  strongly  charged, 
the  bar  connecting  the  pointed  rod  with  the  conductor  may  be  brought  so  near 
the  latter  as  to  allow  the  chief  part  of  the  electricity  to  pass  through  it  to  the 
ground ;  and,  at  the  same  time,  the  connexion  of  the  electroscope  with  the 
pointed  rod  being  preserved,  a  sufficient  quantity  of  electricity  will  affect  it  to 
indicate  the  species  of  electricity  with  which  the  air  is  charged. 

For  occasional  observations  a  convenient  and  portable  apparatus  may  be 
formed  with  a  common  fishing-rod,  which  is  divided  into  several  pieces  capa- 
ble of  being  united  at  pleasure,  so  as  to  form  a  single  rod  of  considerable 
length.  To  the  extreme  piece  of  this  let  a  rod  of  glass,  terminated  by  a  fine 
metallic  point,  be  attached ;  a  metallic  wire  attached  to  this  point  is  carried  to 
the  electroscope,  which  will  thus  receive  the  electricity  collected  by  the  point 
of  the  rod.  This  rod  may  be  elevated  in  any  situation  in  which  it  is  desired 
to  examine  the  electric  state  of  the  air. 

Various  forms  of  electroscopes  are  used  to  observe  atmospheric  electricity. 
Saussure  used  two  fine  metallic  wires,  each  having  a  small  pith-ball  suspended 
at  its  lower  extremity,  and  having  its  upper  end  attached  to  a  rod  of  metal  in- 
serted in  the  top  of  a  square  tube  of  glass  about  two  inches  in  the  side.  The 
two  balls  were  suspended  in  contact  in  the  interior  of  this  tube,  and  the  extent 
of  their  divergence  was  measured  by  a  scale  drawn  on  one  of  the  sides  of  the 
tube.  To  the  upper  extremity  of  the  rod  supporting  the  wires  was  screwed  a 
pointed  conductor,  composed  of  three  parts  fitting  into  each  other,  each  meas- 
uring from  three  to  four  inches  in  length. 

This  conductor,  being  elevated  in  the  air,  collected  the  electricity.  To  pre- 
serve the  electroscope  from  the  effect  of  the  weather,  a  brass  cup  was  provi- 
ded, which  was  screwed  upon  the  rod  supporting  the  wires  at  the  foot  of  the 
conductor. 

This  apparatus  is  usually  affected  sensibly  by  the  electricity  of  the  air,  when 
raised  in  the  atmosphere  to  the  height  of  ten  or  twelve  feet  above  the  head  of 
the  observer.  In  order  to  compare  numerically  the  intensity  of  the  electricity 
jvhich  produces  different  degrees  of  divergence  of  the  wires,  Saussure  adopted 
the  following  ingenious  method.  Having  constructed  two  electroscopes  as 
similar  to  each  other  in  all  respects  as  possible,  and  removed  the  conductors 
from  them,  he  electrified  one  of  them  so  as  to  produce  a  certain  divergence,  six 
lines,  for  example,  of  the  balls.  He  then  brought  into  contact  the  metal  rods 
of  the  two  instruments,  so  as  to  share  equally  between  them  the  electricity 
with  which  the  first  was  charged.  The  divergence  was  now  reduced  to  four 
lines.  Hence  electric  charges  in  the  ratio  of  1  to  2,  correspond  to  divergences 
of  the  balls  in  the  ratio  of  2  to  3. 

The  second  electrometer  being  discharged,  and  again  put  in  communication 
with  the  first,  the  remaining  charge  of  the  latter  was  again  shared,  equally  be- 
tween them,  so  that  the  first  remained  charged  with  only  a  fourth  of  its  original 
electricity.  The  separation  of  the  balls  was  now  found  to  be  2-8  lines.  By 
continuing  this  process,  a  table  was  constructed  by  which  the  ratio  of  the  in- 
tensities of  the  electricity  could  always  be  approximatively  inferred  from  the 
extent  to  which  the  balls  were  separated.  It  is  evident  that  such  a  table  will 
not  be  the  same  for  all  electroscopes.  Each  observer  must,  therefore,  con- 
struct, from  immediate  observation,  a  table  suitable  to  the  individual  electro- 
scope which  he  uses. 


ATMOSPHERIC  ELECTRICITY. 


151 


Volta  used,  for  a  like  purpose,  an  apparatus  similar  to  that  of  Saussure,  but 
adopted  the  straw  electroscope.  He  assumed  that  the  angles  of  divergence  of 
the  blades  of  straw  within  the  limits  of  26°  are  sensibly  proportional  to  the  in- 
tensities of  the  electric  charges,  and  that,  provided  the  blades  exceed  an  inch 
or  two  in  length,  the  results  are  not  affected  by  any  small  variation  of  length. 
It  is  safer,  however,  to  construct  a  table  according  to  the  method  explained 
above,  whatever  be  the  form  of  the  electroscope. 

To  augment  the  sensibility  of  the  instrument,  Volta  also  fixed  a  lamp  to  the 
point  of  the  conductor,  and  interposed  a  condenser  between  the  conductor  and 
the  electroscope.  Both  of  these  expedients,  however,  render  the  indications 
of  the  instrument  uncertain.  In  the  process  of  combustion  electricity  will  be 
liberated,  the  effects  of  which  will  combine  with  those  of  the  atmosphere  in  af- 
fecting the  electroscope  ;  and  unless  the  plates  of  the  condenser  be  formed  of 
gold  or  platinum,  or  be  coated  with  these  metals,  their  oxydation,  by  the  depo- 
sition of  moisture  upon  them,  would  produce  disturbing  effects. 

In  some  cases  the  mulliplur,  or  galvanometer,  is  advantageously  applicable 
for  meteorological  purposes.  Since,  however,  the  electric  current  transmitted 
through  it  in  such  applications  has  greater  intensity  than  that  which  is  pro- 
duced in  Voltaic  arrangements,  greater  precautions  must  be  taken  to  insulate 
the  wire.  For  this  purpose  the  wire,  wrapped  in  the  usual  manner  with  silk, 
may  be  immersed  in  a  concentrated  solution  of  gum  lac  in  alcohol.  When  well 
coated  with  this  varnish,  the  electricity  will  not  escape  from  one  convolution 
to  another. 

In  the  application  of  the  multiplier  to  detect  the  electricity  of  the  air,  one 
extremity  of  the  wire  is  attached  to  the  foot  of  a  pointed  insulated  conductor, 
elevated  to  the  proper  height  in  the  atmosphere,  and  the  other  extremity  com- 
municates with  the  ground.  The  air  and  the  earth  being  in  opposite  electrical 
states,  a  current  will  pass  through  the  wire,  the  intensity  of  which  will  be 
indicated  in  the  usual  manner,  by  the  deviation  of  the  magnetic  needle. 


OF    THE    ORDINARY    STATE    OF    THE    ATMOSPHERE. 


One  of  the  earliest  results  of  the  observation  of  the  electrical  state  of  the 
air  was  the  discovery  of  the  fact  that  in  clear  weather,  when  the  natural  state 
of  the  atmosphere  is  undisturbed  by  clouds,  it  is  always  charged  with  positive 
electricity,  and  the  surface  of  the  earth  is,  on  the  contrary,  charged  with  negative 
electricity.  Volta  explained  this  fact  by  stating  that  in  the  evaporation  of  wa- 
ter the  natural  electricity  of  the  liquid  is  decomposed,  the  positive  fluid  esca- 
ping with  the  vapor,  and  the  negative  fluid  remaining  on  the  vessel  in  which 
the  liquor  is  evaporated  ;  and  this  process  going  on  upon  a  large  scale  in  the 
oceans,  seas,  and  other  large  collections  of  water,  might  charge  the  atmo- 
sphere with  free  positive  electricity.  But  we  have  seen  from  Peltier's  experi- 
ment, that  mere  evaporation  without  chemical  decomposition  is  not  enough  ;  we 
have  seen,  too,  from  Armstrong's  and  Faraday's  experiments,  that  mere  evapo- 
ration without  friction  is  not  enough  ;  we  are  hence  led  to  modify  our  views, 
and  consider  how  far  chemical  effects  and  friction  can  be  included  as  operating 
causes  in  the  electrization  of  the  atmosphere. 

It  is  certain  that  such  essential  chemical  effects  as  the  liberation  of  particles 
of  water  of  crystallization  from  combination  with  salts,  do  not  exist  in  the 
evaporation  to  which  common  consent  has  ascribed  the  electricity  of  the  at- 
mosphere ;  and  philosophers  have  felt  that  the  cause  here  assigned  is  inade- 
quate to  the  effect.  If  they  tacitly  accept  the  theory,  it  is  rather  for  want  of 
a  better  than  from  any  feeling  of  conviction.     They  cannot  imagine  the  con- 


152 


ATMOSPHERIC  ELECTRICITY. 


necting  links  between  its  assumed  chemical  origin  and  its  ultimate  conversion 
into  the  lightning  flash. 

As  the  friction  of  watery  particles  is  a  discovery  only  just  matured,  the  idea 
has  not  yet  occurred  of  including  it  in  the  investigation  of  atmospheric  electri- 
city. Though  the  present  state  of  our  knowledge  does  not  justify  us  to  haz- 
ard an  answer,  yet  we  are  called  on  to  propose  the  question — Do  the  watery 
particles  with  which  the  atmosphere  is  charged  acquire  positive  electricity  as 
they  are  rubbed  by  the  wind  against  the  earth,  and  all  it  sustains,  as  hills, 
rocks,  trees,  &c.,  in  the  same  manner  as  the  stream  of  steam  and  water  be- 
comes positive  by  rubbing  against  the  jet  ?  If  so,  what  connexion  may  not  be 
traced  between  the  hurricane  winds  of  the  tropics  and  the  prevailing  lightning- 
storms  with  which  those  regions  abound  1  Does  the  friction  together  of  two 
currents  of  air,  charged  to  different  degrees  with  moisture,  develop  the  two 
electrical  states  1 

Throwing  out  these  hints,  we  come  to  consider  the  actual  conditions  pre- 
sented by  the  atmosphere.  The  first  fact  which  presents  itself  is  the  extreme 
irregularity  in  the  distribution  of  the  electricity  ;  and  this  would  necessarily 
ensue  from  either  theory,  for  local  variation  is  an  essential  element  in  any  view 
which  we  mayije  induced  to  adopt.  Each  theory  includes  evaporation,  either 
as  producing  the  electricity,  or  as  prxjviding  the  rubbing  particles  ;  so  that,  in 
the  sequel,  we  may  safely  adopt  the  current  language,  without  pledging  our- 
selves against  conviction  to  either  theory,  in  the  present  undecided  state  of  the 
question. 

If  the  evaporation  or  other  processes  by  which  positive  electricity  is  sup- 
plied to  the  atmosphere  were  uniform  over  the  surface  of  the  globe,  the  spher- 
ical shell  of  air  by  which  the  globe  is  enclosed  would  be  uniformly  charged 
with  positive  electricity,  and,  being  a  nonconductor,  it  would  be  related  to  the 
crust  of  the  globe  on  which  it  rests  in  the  same  manner  as  the  cake  of  an  elec- 
trophorus  is  related  to  the  metallic  disk  in  contact  with  it.  The  positive  elec- 
tricity of  the  atmosphere  will  then  act  by  induction  on  the  natural  electricities 
of  the  superior  parts  of  the  earth  ;  and  if  we  suppose  them  to  possess  conduct- 
ing power  in  the  same  degree  throughout  the  surface,  the  positive  fluid  result- 
ing from  the  decomposition  would  be  driven  downward,  while  the  negative  fluid 
would  be  drawn  toward  the  surface,  and  would  augment  the  intensity  of  the 
negative  fluid  already  collected  there  from  other  causes. 

Thus  the  atmosphere  over  different  parts  of  the  surface  of  the  earth  will  re- 
ceive different  quantities  of  electricity,  and,  since  air  is  a  nonconductor,  the 
inequality  of  the  electric  state  thus  produced  will  continue,  except  so  far  as  it 
may  be  modified  by  the  effects  of  atmospheric  currents. 


DIURNAL    VARIATION    OF    THE    ELECTRICITY. 

The  electric  state  of  the  air  depending,  then,  on  the  results  of  the  evapora- 
tion of  water  on  the  surface,  that  state  may  naturally  be  expected  to  be  subject 
to  periodical  changes  corresponding  in  some  definite  manner  to  the  changes 
incidental  to  the  process  of  general  evaporation  ;  and,  as  these  latter  changes 
must  be  related  to  the  influence  of  the  sun  on  the  atmosphere,  a  series  of  vicis- 
situdes in  the  electricity  of  the  air  may  be  looked  for,  having  some  correspond- 
ence with  the  rising  and  setting  of  the  sun  and  the  epochs  of  noon  and  raid- 
night.     Observation,  accordingly,  sanctions  this  anticipation. 

If  the  electricity  of  the  air  be  examined  by  proper  electroscopic  instruments 
at  and  immediately  after  midnight,  its  intensity  will  be  found  to  be  gradually 
decreasing,  and  this  decrease  will  continue  till  a  little  before  sunrise,  when  the 
intensity,  becoming  stationary  for  a  short  time,  will  afterward  begin  to  increase 


ATMOSPHERIC  ELECTRICITY. 


153 


at  a  slow  rate.  This  increase  will  continue,  becoming  more  rapid  for  some 
hours  after  sunrise,  when  it  will  attain  a  maximum ;  after  which  it  will  again 
decrease,  at  first  slowly,  and  afterward  more  rapidly.  This  gradual  decrease 
will  continue  for  some  time  after  the  sun  passes  the  meridian,  when  it  will 
cease,  the  electrical  intensity  again  attaining  a  minimum.  It  will  then  begin 
to  increase,  at  first  slowly,  and  afterward  more  rapidly,  until  it  attains  another 
maximum  sometime  after  sunset.  It  will  then  begin  to  decrease,  and  continue 
to  decrease  until  midnight. 

If  the  line  M  N  M,  fig.  1,  be  imagined  to  represent  the  interval  of  time  be- 
tween midnight  and  midnight,  its  middle  point,  N,  representing  the  intermedi- 


ate noon,  and  the  other  points  the  various  hours  before  and  after  noon,  and  if 
from  each  point,  such  as  P,  a  perpendicular  be  drawn,  representing  the  inten- 
sity of  the  atmospheric  electricity  at  the  hour  corresponding  to  P,  a  curve 
would  be  formed,  the  distances  of  which  from  the  line  M  N  M  would  represent 
the  electric  state  of  the  atmosphere. 

The  undulating  line  X  b  B  b'  B'  X  then  represents,  in  its  general  character, 
the  diurnal  variation  of  the  electricity  of  the  atmosphere  when  the  weather  is 
clear  and  no  extraordinary  disturbing  influence  intervenes  to  modify  the  com- 
mon effects.  The  points  a  and  a'  represent  the  times  of  the  morning  and  eve- 
ning minima,  and  the  perpendiculars  a  b  and  a'  b'  the  values  of  these  minima ; 
and  the  points  A  and  A'  represent  the  morning  and  evening  maxima,  and  the 
perpendiculars  A  B  and  A'  B'  the  values  of  these  maxima. 

If,  throughout  the  same  parallel  of  latitude,  no  disturbing  cause  be  supposed 
to  be  in  operation,  and  the  production  of  electricity  in  the  same  position  of  the 
sun  be  everywhere  the  same,  the  state  of  the  electricity  of  the  air  around  the 
parallel  may  be  represented  in  a  similar  way.     Let  E  N  W  M,  fig.  2,  repre- 


sent the  parallel ;  E  N  W  the  enlightened,  and  E  M  W  the  dark  part ;  C  S 
the  direction  of  the  meridian  passing  through  the  sun. 

At  the  point  N  the  time  will  be  noon,  and  at  M  it  will  be  midnight ;  at  E  it 
will  be  sunset,  and  at  W  sunrise.  The  point  a  represents  the  place  where  the 
electricity  is  at  the  morning  minimum,  and  a'  where  it  is  at  the  evening  mini- 


154 


ATMOSPHERIC  ELECTRICITY. 


mum.  In  like  manner  A  and  A'  represent  the  places  where  the  electricity  is 
at  the  morning  and  evening  maximum.  The  curve  of  electric  intensity  has, 
therefore,  the  form  of  an  oval ;  the  longer  axis,  B  B',  being  inclined  at  a  small 
angle  to  the  direction  of  the  sun,  and  the  lesser  axis,  a  a',  being  at  right  angles 
to  it.  As  the  position  of  the  sun  is  gradually  changed  by  its  apparent  motion 
from  east  to  west,  these  axes  of  the  oval  follow  it,  always  keeping  the  same 
relative  position  with  respect  to  it  in  the  absence  of  disturbing  causes. 

The  first  philosopher  who  presented  a  complete  and  connected  series  of  ob- 
servations on  the  electricity  of  the  air  was  Schiibler,  who  observed  at  Stutt- 
gard,  and  published  his  observations,  taken  at  various  hours,  daily,  from  May, 
1811,  to  June,  1812.  As  an  example  of  the  actual  succession  of  changes  ex- 
hibited in  a  single  day,  the  following  table  of  the  observations  taken  on  the 
11th  of  May,  1811,  will  serve  :— 


Hour. 

Electrometer. 

Hygrometer. 

Thermometer. 

4  A.M. 

5 

88 

9-3 

5 

6| 

88 

9-5 

6 

8 

87 

10-5 

7 

11 

86 

12-1 

8 

12 

84 

13-5 

9 

10 

76 

15-5 

10 

8 

70 

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Perfectly  dear.  After  a  short 
time  the  heavens  became  vapor- 
ous, and  dews  began  to  fall. 


The  heavens  clear  to  the  hori- 
zon ;  the  tint  of  the  firmament  a 
pure  blue. 


Vapors  begin  to  be  formed,  and 
dew  falls. 


Heavens  perfectly  clear. 


ANNUAL    VARIATION    OF    THE    ELECTRICITY. 


As  the  diurnal  change  in  the  position  of  the  sun,  relatively  to  a  given  place, 
produces  a  periodical  variation  in  the  electric  state  of  the  air,  the  change  of  its 
declination  from  month  to  month  may  be  expected  to  be  followed  by  some  cor- 
responding periodical  effect  on  the  mean  amount  of  the  maxima  and  minima 
values  of  the  electricity.  On  comparing  the  mean  values  from  month  to  month, 
it  is  accordingly  found  that  the  values  of  the  two  daily  maxima  and  minima 
undergo  a  progressive  decrease  from  January  to  July,  and  a  progressive  in- 
crease from  July  to  January.  It  is  found,  also,  that  during  the  winter  the  elec- 
tricity of  the  air  increases  as  the  thermometer  falls. 

On  comparing  the  mean  values  of  the  maxima  and  minima  throughout  the 
year,  it  is  found  that  the  morning  values  of  each  are  a  little  less  than  the  eve- 
ning values. 

The  hours  at  which  the  electricity  attains  its  maxima  and  minima  values  are 
likewise  subject  to  variation  from  month  to  month.  The  hour  of  the  morning 
minimum  and  maximum  continually  advances  toward  noon  from  winter  to  sum- 
mer, and  undergoes  the  contrary  change  in  the  latter  part  of  the  year. 

The  observations  of  Schiibler  indicate  that  the  hour  of  the  evening  minimum 
is  invariable.     From  June,  1811,  to  June,  1812,  it  took  place  at  Stuttgard  al- 


ATMOSPHERIC  ELECTRICITY. 


155 


ways  at  2  P.  M.  The  hour  of  the  second  maximum  also  gradually  approached 
nearer  to  noon  from  summer  to  winter,  and  receded  from  it  again  from  winter 
to  summer. 

The  series  of  observations  on  the  diurnal  changes  of  atmospheric  electricity, 
which  Schubler  made,  in  1811-12,  were  repeated  by  M.  Arago  at  Paris,  in 
1830,  who  obtained  similar  results.  Thus,  in  the  month  of  March,  1811, 
Schubler  found  that  the  mean  time  of  the  morning  maximum,  was  8  hs.  30  m., 
and  M.  Arago  found  the  mean  time  for  the  same  month,  8  hs.  48  m. 

LOCAL    VARIATIONS    OF    THE    ELECTRICITY. 


[       In  all  the  preceding  observations,  the  sources  which  supply  positive  electri- 
I  city  to  the  air,  are  supposed  to  be  uniformly  distributed  on  the  surface  of  the 
I  earth.     A  great   variety  of  local   causes,   however,   interrupt  this  uniformity. 
1  Saussure's  observations  show  that  the  positive  electricity  of  the  air  has  greatest 
[  intensity  in  the  most  elevated  places,  and  in  those   which   are  best  insulated. 
I  In  the  interior  of  buildings,  under  trees,  in  the   streets,   courts,   and  other  en- 
I  closed  and  sheltered  parts  of  towns,  no  free  electricity  is  found  in  the  air.     In 
'  the  midst  of  squares,  and  other  open  places  in  cities,   on   the   quays,  but  more 
I  specially  on  bridges,  it  is  even  more  intense  than  in  an  open,  flat  country.     In 
'  particular  localities,  such  as   Geneva,  where  fogs   prevail,  which  lie  low,  and 
!  are  not  converted  into  rain,  the  positive  electricity  of  the  air  is  most  intense. 
Although  the  general  correspondence  between  the  diurnal  and  annual  variations 
of  the  normal  electric  state  of  the  air  indicates,  unequivocally,  its  dependance 
on  the  variation  of  the  sun's  declination,  and  the  diurnal  motion  of  that  body, 
and  the  local  variations  accord   with  the   hypothesis,   that   evaporation   is  the 
chief  source  of  the  electricity  of  the  air  ;  still,  no   complete   and  satisfactory 
explanation  has  yet  been  proposed  for  the  diurnal  and  annual  electric  periods. 
Schubler  observed  that  some  correspondence  may  be  perceived  between  the 
diurnal  variation  of  the  magnetic  needle,  and  the  diurnal  variation  of  the  elec- 
tricity of  the  air,  and  that,  if  such  correspondence  be  admitted,  it  would  follow 
that  both  these  phenomena  must  be  ascribed  to  the  same  cause.     But  this  cor- 
respondence is  far  from  being  so  exact  as  to  justify  even  a  probable  conjecture 
as  to  their  identity  of  cause.     The  maximum  variation  of  the  needle  east  takes 
place  at  half  past  eight  in  the  forenoon,  from  which  time  till  a  quarter  past  one 
in  the  afternoon,  it  turns  gradually  round  toward  the   west,   attaining  its  maxi- 
mum western  variation  at  the  latter  hour.     From  that  time  till  half-past  eight 
the  following  morning,  it  returns  gradually   eastward.     The  times   of  greatest 
eastward  and  westward  variation  correspond  nearly  to  the  times  of  the  morn- 
ing maximum,  and  evening  minimum,  but  there  are  no  effects  exhibited  by  the 
needle  corresponding  to  the  other  maximum  and  minimum.  ' 

Becquerel  proposes  the  following  explanation  of  the  diurnal  variations  of  the  i 
electricity  of  the  air.  Toward  the  morning  the  electricity  ought  to  have  a  * 
feeble  intensity,  because  the  humidity  of  the  evening  and  night  has  restored  to  ( 
the  earth  a  part  of  the  electricity  which  had  been  accumulated  in  the  air.  * 
When  the  sun,  at  its  rising,  begins  to  warm  the  earth,  evaporation  is  promoted  < 
and  positive  electricity  supplied  to  the  air.  Hence,  after  sunrise,  for  some  j 
hours,  the  intensity  of  the  electricity  of  the  air  will  be  augmented.  When  the  5 
sun  has  attained  a  certain  elevation,  and  the  heat  has  increased,  the  air  is  dried,  ) 
and  transmits  with  less  facility  the  electric  fluid,  accumulated  in  the  higher  re-  ) 
gions  of  the  air  ;  electrometric  instruments,  therefore,  placed  near  the  surface  ) 
of  the  earth,  will  indicate  a  diminution  of  electricity,  even  though  the  electric  ) 
fluid  should  continue  to  be  augmented  in  the  higher  parts  of  the  air.  As  sun-  ) 
set  approaches,  the  air  is  cooled,  becomes  humid,  and  begins  to  transmit  the  < 


156 


ATMOSPHERIC  ELECTRICITY. 


electric  fluid,  accumulated  in  the  higher  regions,  more  abundantly  to  the  earth. 
The  electric  intensity  would,  therefore,  increase  with  the  humidity  and  the 
dew  until  two  or  three  hours  after  sunset.  Finally,  when  the  air  begins  to  be 
exliausted,  the  electricity  again  diminishes,  and  continues  to  decrease  till  the 
next  morning.  According  to  the  same  principles,  the  annual  variation  of  the 
electricity  is  explained.  In  clear  weather,  the  mean  intensity  of  the  electrici- 
ty of  the  air  will  be  much  less  in  summer  than  in  winter ;  for  the  air  in  sum- 
mer, being  warm  and  dry,  resists  more  strongly  the  transmission  of  the  elec- 
tric fluid  accumulated  in  the  higher  regions  of  the  atmosphere,  while  in  winter 
the  air,  beiug  more  humid,  produces  a  contrary  effect. 

DISTRIBUTION    OF    ELECTRICITY    OF    THE    AIR. 


Although  the  negative  electricity  of  the  surface  of  the  globe  be  a  conse- 
quence of  the  ascertained  fact,  that  positive  electricity  is  supplied  by  it  to  the 
air,  it  is  necessary,  nevertheless,  that  it  be  ascertained  by  immediate  observa- 
tion. This  has,  accordingly,  been  done  by  different  observers,  at  diff"erent 
times,  and  in  diff"erent  places.  Among  the  more  recent  observations  of  this 
kind,  are  those  of  M.  Peltier.  To  ascertain  the  electricity  of  the  ground,  this 
philosopher  used  a  multiplier,  placing  one  extremity  of  the  platinum  wire  in  a 
humid  part  of  the  soil,  and  attaching  the  other  end  to  a  pointed  metallic  con- 
ductor, raised  in  the  air.  When  the  air  was  sufficiently  humid  to  give  it  a 
conducting  power,  a  current  was  established  through  the  wire,  by  which  the 
needle  was  sensibly  affected,  and  the  deflection  of  the  needle  proved  that  the 
negative  current  came  from  the  ground,  and  the  positive  from  the  air. 

The  negative  electricity  of  the  ground,  and  the  positive  electricity  of  the 
stratum  of  air  contiguous  to  it,  have  a  continual  tendency  to  re-combine  and 
neutralize  each  other.  From  this  cause,  the  lowest  stratum  of  air  in  clear 
weather,  apart  from  disturbing  causes,  is  found  to  be  in  its  natural  state.  This 
effect  extends  to  the  height  of  three  or  four  feet  from  the  ground,  above  which 
height  the  positive  electricity  begins  to  be  perceivable,  and  increases  in  its  in- 
tensity in  ascending,  according  to  some  definite  law,  which  observation  has 
not  yet  disclosed. 

To  ascertain  the  increase  of  electricity  in  the  ascending  strata  of  air,  Bec- 
querel  and  Breschet  made  some  experiments  on  the  Great  Saint  Bernard,  ac- 
cording to  a  method  suggested  by  Saussure.  These  electricians  selected  a 
convenient  platform  of  ground  near  the  monastery,  extended  upon  it  a  piece  of 
gummed  sarcenet,  about  ten  feet  long  and  seven  feet  wide,  upon  which  they 
unrolled  a  silk  cord,  interlaced  with  metallic  wire,  measuring  about  250  feet  in 
length.  They  attached  one  end  of  this  cord  to  the  hook  or  rod,  which  commu- 
nicated with  the  straws  of  an  electrometer,  by  means  of  a  loose  knot,  in  such  a 
manner  that  when  drawn  upward,  it  would  be  detached  from  the  electrometer 
without  disturbing  the  instrument.  The  other  extremity  of  the  cord  was  tied 
to  the  tail  of  an  iron  arrow,  which  was  projected  upward  by  means  of  a  bow 
with  such  force  that,  attaining  a  height  of  more  than  250  feet,  it  detached  the 
lower  end  of  the  cord  from  the  electrometer.  As  the  arrow  ascended,  the 
electrometer  showed  a  gradually  increasing  divergence,  which  soon  became  so 
considerable  that  the  straws  struck  the  sides  of  the  case  enclosing  them. 
When  the  cord  was  detached,  the  instrument  retained  the  electricity  it  had  re- 
ceived, which,  on  examination,  proved  to  be  positive. 

Hence,  it  appears  that  from  three  feet  above  the  ground,  to  the  height  of  250 
feet,  the  air  is  charged  with  positive  electricity,  constantly  increasing  in  inten- 
sity, at  least,  in  localities  like  that  in  which  this  experiment  was  made. 

Lest  it  might  be  supposed  that  the  electricity  obtained,  was  produced  by  the 


ATMOSPHERIC  ELECTRICITY. 


157 


friction  of  the  arrovr  against  the  air,  the  experiment  was  repeated,  projecting  i 
the  arrow  horizontally,  at  the  height  of  three  feet  from  the  ground.  In  this  \ 
case  no  effect  was  produced  on  the  electrometer. 

Becquerel  made  experiments  with  a  like  object  in  clear  weather,  on  the 
summit  of  the  rock  called  Sanadoire,  near  the  Mont  d'Or.  This  summit,  sep- 
arated from  the  surrounding  mountains,  is  terminated  by  a  platform  of  the  ex- 
tent of  several  square  yards,  at  the  height  of  about  4,600  feet  above  the  level 
of  the  sea.  The  electrometer  of  Saussure  was  surmounted  by  a  pointed  con- 
ductor, about  twenty  inches  long.  A  divergence  of  the  straws,  amounting  to 
an  eighth  of  an  inch,  was  produced,  when  the  apparatus  was  raised  about  three 
feet  above  the  head.  The  divergence  was  doubled,  when  a  wire,  attached  to 
the  electrometer,  was  projected  upward  by  means  of  a  stone,  to  the  height  of 
about  thirteen  feet,  and  when  projected  to  greater  heights,  the  divergence  con- 
tinued to  augment. 

When  the  apparatus,  elevated  to  a  certain  height  above  the  head,  and  show- 
ing a  certain  divergence,  was  carried  down  the  side  of  the  hill,  the  divergence 
gradually  diminished,  and  disappeared  altogether,  before  attaining  the  foot  of 
the  hill. 

In  the  ascent  made  in  a  balloon  by  MM.  Gay-Lussac  and  Biot,  the  increase 
of  positive  electricity  in  the  ascending  strata'  of  air  was  also  rendered  mani- 
fest. These  philosophers  attached  a  metallic  ball  to  a  wire,  about  170  feet 
long,  and  suspended  it  from  the  car  of  the  balloon,  the  upper  end  of  the  wire 
being  attached  to  an  electrometer.  The  weather  being  perfectly  clear,  the  in- 
strument diverged  with  negative  electricity.  This  result,  which  was  in  appa- 
rent discordance  with  the  results  of  observations  in  general,  was,  however, 
easily  shown  to  be  consistent  with  them.  The  wire,  in  this  case,  supplied  a 
conducting  communication  between  two  strata  of  air,  one  170  feet  above  the 
other.  If  they  were  equally  charged  with  the  same  species  of  electricity,  the 
electrometer  would  not  have  been  affected;  for  the  natural  electricities  of  the 
wire  being  placed  between  two  equal  and  contrary  decomposing  influences,  no 
decomposition  would  take  place,  and  the  wire  would  remain  in  its  natural  state. 
If,  however,  the  two  strata  at  the  ends  of  the  wire  were  electrified  positively, 
in  different  df^grees,  a  decomposition  of  the  electricities  of  the  wire  would  en- 
sue, the  positive  fluid  being  repelled  toward  that  stratum  having  the  weaker 
positive  charge,  and  the  negative  fluid  being  attracted  toward  that  stratum  hav- 
ing the  stronger  charge.  Since,  then,  the  electrometer  at  the  upper  extremity 
of  the  wire  showed  negative  electricity,  it  follows  that  the  higher  stratum  was 
more  intensely  positive  than  the  lower. 

In  a  similar  experiment  made  by  Saussure,  the  electrometer  was  placed  at 
the  lower  end  of  the  wire,  and,  in  accordance  with  what  has  been  just  ex- 
plained, the  instrument  diverged  with  positive  electricity. 

The  method  of  explaining  the  apparently  inconsistent  results  of  the  experi- 
ments of  Biot  and  Saussure,  proposed  by  the  former,  is  imperfect,  unless  it  be 
admitted  that  the  two  strata  of  air  are  both  eleclrijied  positively  ;  for  if  they 
were  both  elecLrijied  negatively,  the  lower  stratum  having  the  stronger  charge,  the 

\  same  effects   would  ensue ;  or  even   if  they  were   differently  electrified,  the 

'  upper  stratum  being  positive  and  the  lower  negative,  the  effects  would  be  the 

I  same. 

Strictly   speaking,  therefore,  the  consequence  which  legitimately  follows, 

!  from  all  observations  made  on  the  electricity  of  the  air  at  different  heights,  by 
means  of  a  vertical  conducting-rod  or  wire  extending  from  the  electroscope  to 
the  stratum  of  which  it  is  desired  to  ascertain  the  electric  state,  is,  not  that  the 
electricity  with  which  the  instrument  diverges,  is  that  of  the  air  in  which  the 
remote  extremity  of  the  conductor  is  placed,  but  that  if  E'  be  the  electricity  of 


>  158 

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160 


ATMOSPHERIC  ELECTRICITY. 


the  Stratum  in  which  the  electrometer  is  placed,  and  E  that  of  the  stratum  in 
which  the  remote  end  of  the  conductor  is  placed,  then,  when  the  instrument 
diverges  with  positive  electricity,  E — E',  will  be  positive,  and  when  it  diverges 
with  negative  electricity.  E — E',  will  be  negative.  If  the  species  of  electricity 
of  either  stratum  be  otherwise  known,  such  an  observation  will  indicate  the 
species  of  the  other  stratum ;  but  if  not,  it  will  only  give  a  different  result. 

ELECTRICITV    OF    THE    AIR    IN    CLOUDED    WEATHER. 

The  electric  state  of  the  atmosphere  in  clear  and  unclouded  weather  only 
has  been  hitherto  explained.  We  shall  nowr  proceed  to  state  the  observations 
which  have  been  made  when  the  heavens  are  more  or  less  charged  with  clouds, 
whether  attended  or  not  with  rain,  snow,  hail,  or  other  phenomena  of  storms. 

From  the  month  of  June,  1811,  to  May,  1812,  both  inclusive,  M.  Schiibler 
observed  the  electricity  of  the  atmosphere  in  clouded  weather  and  in  times  of 
rain,  hail,  and  snow.  In  the  table  on  pp.  158,  159,  a  synopsis  is  given  of  the 
results  of  his  observations.  An  examination  of  the  results  registered  in  this 
table  will  establish  the  following  conclusions  :  — 

1.  That  in  stormy  weather,  in  rain,  hail,  or  snow,  the  electricity  of  the  air 
is  much  more  intense  than  at  other  times. 

2.  That  in  such  weather  the  electricity  is  sometimes  positive  and  sometimes 
negative,  and  nearly  as  often  the  one  as  the  other. 

3.  That  in  such  weather  the  electricity  often  undergoes  sudden  changes 
from  positive  to  negative,  and  vice  versa.  ' 

4.  That  in  clouded  weather,  unattended  by  storms,  rain,  hail,  or  snow,  the 
free  electricity  of  the  air  is  positive. 

5.  That  the  intensity  of  this  electricity  is  greater  in  winter  than  in  summer. 


EVAPORATION. 


EiToneously  ascribed  to  Chemical  Combination. — Takes  place  from  tbe  Surface. — Law  discovered 
by  Dalton  extended  to  all  Liquids. — Limit  of  Evaporation  conjectured  by  Faraday. — Hygrome- 
ters.— Various  Phenomena  explained  by  Evaporation. — Leslie's  Method  of  freezing. — Examples 
in  the  useful  Arts. — Methods  of  Cooling  by  Evaporation. — Dangerous  Eifects  of  Dampness. — 
WoUaston's  Cryophorus. — Pneumatic  Ink-Bottle.— rClouds. — Dew. 


VOL.  II.— 11 


r 


EVAPORATION. 


163 


EYAPORATIOI. 


It  was  long  supposed  that  the  vapor  produced  from  the  surface  of  liquids 
exposed  to  the  atmosphere,  was  the  consequence  of  an  affinity  between  the 
particles  of  air  and  the  particles  of  the  liquid,  by  virtue  of  which  a  combination 
was  formed,  and  consequently  a  constant  absorption  took  place  by  the  air,  of 
liquids  exposed  to  it.  The  properties  of  vapor,  however,  which  have  beea 
discovered  by  the  labors  of  modern  philosophers,  and  above  all,  by  those  of 
Dalton,  have  proved  the  fallacy  of  this  supposition,  and  have  shown  that  all 
the  phenomena  of  evaporation  may  be  accounted  for  without  supposing  any 
affinity  whatever,  or  other  attraction,  to  exist  between  the  particles  of  atmo- 
spheric air,  and  those  of  liquids. 

The  explanation  of  evaporation  on  the  principle  of  chemical  combination  of 
the  vapors  with  air,  was  first  suggested  by  Halley,  and  supported  by  many  suc- 
ceeding philosophers.  According  to  this  theory,  air  was  considered  as  having 
the  same  effect  on  water,  as  water  would  have  on  salt,  or  any  other  substance 
which  it  might  hold  in  solution.  The  theory  was  rendered  plausible  by  the 
facility  which  it  offered  in  explaining  some  of  the  most  obvious  phenomena  of 
evaporation,  such  as  the  circumstance  of  its  being  promoted  by  winds,  and  by 
increase  of  temperature.  Currents  of  air  removing  the  solvent  as  fast  as  it 
became  saturated,  brought  a  fresh  portion  of  it  to  receive  vapor,  and  so  the  pro- 
cess was  continued  and  stimulated.  Heat,  also,  was  supposed  to  increase  the 
solvent  power  of  the  air  on  water,  in  a  manner  analogous  to  that  by  which  it 
was  known  to  increase  the  solvent  power  of  water  on  other  substances. 

Vapor,  however,  at  low  temperatures,  was  considered  to  possess  no  elas- 
ticity, and  the  discovery  of  the  falsehood  of  this  supposition  was  the  first  step 
toward  removing  the  hypothesis  of  Halley ;  but  this  theory  received  its  death- 
blow from  the  fact  that  vapor  is  not  only  formed  in  a  space  where  no  air  is 
present,  but  that  in  that  space  it  possesses  the  same  elasticity,  and  occupies  the 
same  volume,  as  if  the  same  space  were  filled  with  the  supposed  solvent ;  nay 
more,  that  it  is  not  only  produced  in  such  a  space,  but  that  it  is  produced  in- 


164 


EVAPORATION. 


stantaneously ;  whereas,  if  the  supposed  solvent  were  present,  its  production 
would  be  considerably  retarded.  Thus  it  appeared  that  the  solution  would 
proceed  with  greater  facility  in  the  absence  of  the  solvent  than  in  its  pres- 
ence. 

It  has  been  already  shown,  that  liquids  dismiss  vapor,  whether  the  space 
above  their  surface  be  an  actual  vacuum,  or  be  filled  with  air  or  other  gas, 
and  that  if  such  space  be  confined  within  certain  limits,  it  will  be  capable  of 
receiving  from  the  liquids  a  difllerent  quantity  of  vapor,  depending  solely  on  the 
temperature  of  the  liquid,  and  that  the  quantity  which  will  saturate  a  given 
space  will  be  the  same,  whether  that  space  be  a  vacuum,  or  be  occupied  by 
atmospheric,  air,  or  other  aeriform  bodies.  The  difference  in  the  phenomena 
in  the  two  cases  will  only  consist  in  the  rate  at  which  the  saturating  vapor  is 
produced  from  the  liquid.  In  the  case  of  a  vacuum,  it  is  produced  almost  in- 
stantaneously ;  but  if  air  be  present,  its  production  is  retarded,  and  a  consider- 
able time  may  elapse  before  the  space  above  the  liquid  is  saturated. 

All  masses  of  water  placed  on  the  surface  of  the  globe,  have  above  them  a 
mass  of  atmospheric  air,  which  at  all  times  maintains  suspended  in  it  a  quantity 
of  aqueous  vapor,  raised  by  the  process  of  evaporation  from  the  surfaces  of  this 
liquid.  If  the  quantity  sustained  in  the  atmosphere  be  such  as  to  saturate  the 
air,  then  it  is  obvious  that  no  further  evaporation  whatever  can  take  place  at 
the  surface  of  the  water.  This,  however,  does  not  usually  occur.  Most  com- 
monly the  vapor  suspended  in  the  atmosphere  is  insuflicient  for  its  saturation  ; 
and  in  this  case  evaporation  will  take  place.  It  is  the  object  of  the  present 
lecture  to  explain  the  laws  which  attend  this  process  of  evaporation  in  the 
open  air. 

Dalton,  to  whose  labors  we  are  indebted  in  this,  as  in  every  other  part  of  the 
theory  of  vapors,  investigated  this  subject,  and  may  be  said  to  have  nearly  ex- 
hausted it.  He  commenced  by  determining  the  circumstances  which  attend  the 
evaporation  of  water  at  high  temperatures.  In  such  cases,  the  tension  of  the  va- 
por actually  suspended  in  the  air  would  produce  an  inappreciable  effect  on  the 
phenomena,  because  its  tension  would  be  inconsiderable,  when  compared  with 
that  of  the  vapor  of  water  at  high  temperatures.  In  this  first  experiment,  there- 
fore, he  regarded  the  atmosphere  as  perfectly  dry,  and  considered  the  phenom- 
ena to  proceed  as  they  would  in  a  receiver  subject  to  the  presence  and  pressure 
of  perfectly  dry  air.  A  small  vessel,  containing  boiling  water,  was  suspended 
from  the  arm  of  a  balance,  and  accurately  poised.  A  lamp  was  placed  under 
it,  which  maintained  it  at  the  boiling  point,  and  its  loss  of  weight  in  a  given 
time  by  evaporation  was  accurately  determined.  The  same  experiment  was 
repeated  with  the  same  vessel,  at  various  temperatures,  from  212°  to  138°,  and 
the  following  results  were  obtained : — 


Temperature  in 

Degrees 

of 

Elastic  force  of  Vapor  in 

Evaporation  per  Minute  in 

Fahrenheit. 

Inches. 

Grains. 

212° 

30-00 

30 

180 

15-15 

15 

164 

10-41 

10 

152 

7-81 

8-5 

144 

6-37 

6 

138 

5-44 

5 

From  this  table  it  is  apparent  that  at  each  temperature  between  the  above 
limits,  the  rate  of  evaporation  is  proportional  to  the  tension  of  the  vapor.  It 
will  easily  be  conceived,  however,  that  the  same  law  cannot  extend  to  evapora^ 
tion    at  low    temperatures,  because,  as   the  temperature   of  the   evaporating 


EVAPORATION. 


165 


liquid  approaches  the  temperature  of  the  vapor  suspended  in  the  air,  the  ten- 
sions will  approach  more  nearly  to  equality,  and  the  resistance  of  the  vapor 
already  suspended  in  the  air  will  speedily  begin  to  produce  a  sensible  eflect 
on  the  rate  of  evaporation.  In  order,  therefore,  to  detect  the  law  by  which 
evaporation  took  place  at  lower  temperatures,  it  became  necessary  first  to  de- 
termine the  actual  tension  of  the  aqueous  vapor  suspended  in  the  atmosphere 
at  the  time  of  the  experiment.  The  properties  of  vapor  previously  discovered 
by  Dalton,  led  him  to  an  elegant  and  simple  solution  of  this  problem.  The 
aqueous  vapor  suspended  in  the  atmosphere,  not  being  in  a  state  of  saturation, 
must  be  regarded  as  having  received  a  quantity  of  heat  which  dilated  it  and 
raised  its  temperature,  according  to  the  laws  for  the  dilatation  of  the  permanent 
gases  after  it  had  passed  from  the  liquid  to  the  vaporous  state.  Now  if  all  the 
heat  which  has  been  imparted  to  it  after  it  had  passed  into  the  vaporous  state 
be  taken  from  it,  it  will  undergo  a  diminution  of  temperature,  but  will  not  pass 
from  the  vaporous  to  the  liquid  form.  The  smallest  abstraction  of  heat  beyond 
this  point  will,  however,  cause  a  deposition  of  moisture,  and  a  partial  condensa- 
tion of  the  vapor.  If,  therefore,  a  body  at  a  temperature  considerably  lower 
thf.n  that  of  the  atmosphere  be  exposed  to  the  air,  it  will  first  by  abstract- 
ing heat  from  the  vapor  in  contact  with  it,  lower  its  temperature  until  it 
arrives  at  that  temperature  which  it  had  w^hen  it  passed  from  the  liquid 
to  the  vaporous  state.  If  the  body  be  at  a  lower  temperature,  then,  though  it 
can  no  longer  lower  the  temperature  of  the  vapor,  it  will  condense  it,  and  the 
vapor  will  deposite  itself  in  the  form  of  dew  on  the  sides  of  the  body.  If  the 
body  be  actually  or  nearly  at  that  temperature  at  which  the  vapor  passed  from 
the  liquid  to  the  aeriform  state,  then  the  commencement  of  the  condensation 
will  be  just  indicated  by  a  slight  dulness  produced  on  the  surface  of  the  body 
by  the  condensation  of  the  smallest  possible  quantity  of  vapor.  Led  by  such 
reasoning,  Dalton  adopted  the  following  means  of  determining  the  temperature 
at  which  the  vapor  suspended  in  the  atmosphere  had  passed  from  the  liquid  to 
the  aeriform  state  :  He  poured  water,  at  a  temperature  below  that  of  the  atmo- 
sphere, into  a  thin  glass  tumbler,  and  exposed  it  to  the  air.  If  he  observed  an 
immediate  and  rapid  deposition  of  dew  upon  its  surface,  he  then  wiped  the 
vessel  dry,  and  exposed  it  at  a  somewhat  higher  temperature.  He  thus  con- 
tinued to  expose  the  vessel  at  increasing  temperatures,  until  he  found  that  tem- 
perature at  which  a  deposition  of  moisture  would  just  take  place  on  its  surface, 
and  such  that  one  degree  higher  in  temperature  would  prevent  such  a  con- 
densation of  vapor.  This,  then,  he  assumed  to  be  the  temperature  at,  which 
the  vapor  suspended  in  the  atmosphere  had  passed  from  the  liquid  to  the 
aeriform  state,  and  the  elasticity  or  tension  corresponding  to  this  temperature 
was  found  from  the  table  of  elasticity  resulting  from  his  former  experiments. 
Now  the  vapor  actually  suspended  in  the  air  had  a  higher  temperature  than 
this,  and  was  raised  to  that  temperature  by  heat  communicated  to  it  after  it  had 
assumed  the  vaporous  form.  The  additional  tension  imparted  by  this  increase 
of  temperature  was  easily  computed  by  the  rules  for  the  dilatation  of  gases  and 
vapors  by  heat.  Hence  he  computed  the  actual  tension  of  the  vapor  suspended 
in  the  atmosphere. 

The  water  used  by  Dalton  in  this  experiment  was  taken  from  deep  wells  at 
Manchester,  the  temperature  of  which  was  from  10°  to  15°  colder  than  the 
atmosphere.  This  served  the  purpose  when  the  temperature  of  the  air  was  not 
very  low,  but  in  winter,  when  the  temperature  was  near  the  freezing  point,  it 
became  necessary  to  cool  the  water  by  means  of  ice,  or  a  mixture  of  snow  and 
salt,  or  other  freezing  mixtures. 

The  deposition  of  condensed  vapor  with  the  appearance  of  dew,  on  the  exter- 
nal surface  of  a  glass  vessel  containing  iced  water,  is  a  fact,  of  familiar  occur- 


166 


EVAPORATION. 


rence.  A  decanter  of  iced  water  placed  on  a  table  always  exHbits  this  effect ; 
and  in  summer,  a  decanter  of  fresh  spring-water  will  be  observed  to  have  a 
similar  deposition  on  its  surface. 

He  now  exposed  to  the  air  a  vessel  of  water  at  various  low  temperatures,  and 
noted  its  rate  of  evaporation ;  using,  however,  a  larger  surface,  in  order  to  ob- 
tain a  quicker  evaporation  than  in  the  former  case.  Upon  examining  the  rates 
of  evaporation  resulting  from  these  experiments,  he  found  that  they  were  accu- 
rately proportional  to  the  difference  between  the  tension  of  vapor  which  would 
saturate  the  atmosphere  at  the  temperature  of  the  water,  and  the  tension  of  the 
vapor  actually  suspended  in  it. 

It  thus  follows,  that  the  rate  of  evaporation  from  the  surface  of  water,  in  all 
stales  of  the  atmosphere,  will  be  proportional  to  the  tension  of  vapor  which 
would  saturate  the  air,  diminished  by  the  tension  of  the  vapor  which  is  actually 
contained  in  the  air. 

The  investigations  of  Dalton  were  next  extended  to  other  liquids,  and,  as  the 
portion  of  the  vapors  of  these  which  would  be  suspended  in  the  atmosphere 
would  be  altogether  insignificant,  the  problem  became  somewhat  more  simple. 
The  atmosphere  was  regarded  as  perfectly  dry  with  respect  to  these  liquids  ; 
and  it  was  found  that  their  rates  of  evaporation  were,  in  conformity  with  the 
law  already  obtained  for  water  in  a  dry  atmosphere,  always  proportional  to  the 
tension  of  the  vapor  of  the  liquid  which  would  saturate  an  empty  space  at  the 
proposed  temperature. 

AH  the  preceding  results  have  been  obtained  on  the  supposition  that  the  air 
above  the  surface  of  the  evaporating  liquid  is  perfectly  calm,  so  that  the  same 
stratum  shall  always  remain  in  contact  with  the  air,  and  the  successive  strata 
above  it  shall  continue  undisturbed. 

When  this  is  not  the  case,  the  rate  of  evaporation  must  needs  undergo  a  cor- 
responding change,  and  this  change  is  generally  one  which  accelerates  it.  As 
the  liquid  imparts  its  vapor  to  the  stratum  immediately  above  it,  and  that  vapor 
passes  from  stratum  to  stratum  upward,  the  evaporation  will  be  slower  in  pro- 
portion to  the  quantity  of  vapor  suspended  in  its  strata ;  but,  if  the  air  be  agi- 
tated, and  especially  if  a  current  of  wind  pass  across  the  surface  of  the  liquid, 
then,  as  fast  as  the  vapor  is  deposited  in  the  strata,  it  is  carried  off,  and  fresh 
portions  of  air,  not  impregnated  with  vapor,  take  their  place.  The  evaporation 
may,  in  this  case,  be  as  rapid  as  it  would  be  in  perfectly  dry  air,  inasmuch  as 
the  air  above  the  liquid  is  never  allowed  to  accumulate  in  it  any  quantity  of 
vapor.  It  may  therefore  be  assumed,  as  a  general  principle,  that  a  draught  main- 
tained across  the  surface,  or  winds,  or  any  agitation  of  the  air,  has  a  tendency 
to  accelerate  the  process  of  evaporation. 

In  the  experiments  of  Dalton,  on  the  vaporization  of  boiling  water,  he  found 
that  the  rate  of  vaporization  in  a  space  perfectly  sheltered  from  currents  was 
slower  than  when  exposed  to  a  draught  produced  by  open  windows  and  doors, 
in  the  proportion  of  two  to  three.  The  evaporation  in  still  air  was  at  the  rate 
of  thirty  grains  of  water  per  minute,  and  in  a  draught  forty-five  grains  per 
minute. 

Since  the  evaporation  of  different  liquids  is  proportional  to  the  tension  of 
their  vapors,  it  follows  that  liquids  which  boil  at  high  temperatures  must  eA^ap- 
orate  very  slowly  at  ordinary  temperatures,  for  the  tension  of  the  vapors  of  such 
liquids  are  insensible  at  all  ordinary  pressures.  Indeed,  sulphuric  acid,  mer- 
cury, and  other  like  liquids,  which  boil  at  very  high  temperatures,  may  be  re- 
garded as  fixed,  or  having  no  evaporation  whatever. 

The  evaporation  of  bodies  whose  boiling  point  is  high  on  the  thermometric 
scale  being  inappreciable  at  all  moderate  temperatures,  a  question  arises,  wheth- 
er the  vaporizing  principle  is  subject  to  any  limit  whatever.    As  the  diminution 


EVAPORATION. 


167 


in  the  rate  of  evaporation  is  subject  to  the  law  of  continuity,  or  undergoes  a 
slow,  gradual,  and  co'ntinued  diminution,  the  determination  of  its  actual  limit, 
if  it  has  one,  by  experiment  or  observation  must  obviously  be  exceedingly  diffi- 
cult, if,  indeed,  it  be  within  the  bounds  of  possibility.  Such  a  limit,  therefore, 
if  it  exist,  must  rather  be  sought  for  by  the  operation  of  the  reason  on  facts 
known,  than  by  the  operation  of  the  senses  on  facts  to  be  observed.  A  system 
of  reasoning,  applied  with  great  ingenuity  by  Dr.  Wollaston  to  fix  the  limits  of 
the  atmosphere,  has  been  applied  by  Faraday  to  show  that  an  actual  limit  must 
exist,  for  a  similar  reason,  to  the  operation  of  the  evaporating  principle.  Dr. 
Wollaston  argued  that  the  tendency  of  the  molecules  of  the  atmospheric  air  to 
repel  each  other  being  known  by  direct  observation  to  be  subject  to  a  continual 
diminution,  in  proportion  as  the  distances  between  the  molecules  increased,  or, 
in  other  words,  in  proportion  to  the  rarefaction  of  the  air,  and  the  same  mole- 
cules being  admitted,  in  common  with  all  other  matter,  to  be  subject  to  the  laws 
of  gravitation,  it  follows  inevitably  that,  when  the  actual  weight  of  the  mole- 
cules becomes  equal  to  their  mutual  repulsion,  then,  these  two  forces  balancing 
one  another,  the  molecules  will  rest  altogether  like  the  particles  of  a  liquid.  This 
must  happen,  therefore,  on  the  top  of  the  atmosphere,  where  it  is  possible  to  con- 
ceive a  body  whose  specific  gravity  is  less  than  the  specific  gravity  of  air  in  that 
state  of  rarefaction  in  which  the  repulsion  of  its  molecules  equals  their  weight  to 
float  on  the  surface  exactly  in  the  same  manner,  and  for  the  same  reason,  as  a  ship 
floats  on  water,  or,  to  come  to  a  closer  analogy,  for  the  same  reason  that  we  see  a 
balloon  float  between  two  strata  of  air  when,  bulk  for  bulk,  it  is  lighter  than  that 
on  which  it  presses,  and  heavier  than  that  immediately  above  it.  Now,  admitting 
that  the  tendency  to  evaporation  depends  on  the  energy  of  the  repelling  force 
produced  by  the  presence  of  heat  having  a  tendency  to  drive  off  the  stratum  of 
particles  which  rest  on  the  surface  of  the  liquiid,  it  will  follow  that  gravity  will, 
at  length,  balance  or  prevail  over  the  repulsive  force,  and  will  prevent  the  par- 
ticles from  flying  off  or  evaporating.  Immediately  before  the  liquid  attains  this 
state,  the  repulsive  principle  exceeds  the  gravitating  one  by  so  exceedingly 
small  an  amount,  that  the  quantity  of  evaporation,  though  not  exactly  nothing, 
may  be  conceived  to  be  so  extremely  small  as  to  be  utterly  inappreciable  by 
any  direct  sensible  observation.  Such  is  Faraday's  reasoning,  to  prove  that 
there  exists  a  limit  in  all  bodies  to  the  action  of  the  evaporating  principle,  and 
that  this  limit  is  very  low  in  those  bodies  that  fuse  at  low  temperatures,  and 
that  it  may  be  high  in  bodies  which  fuse  at  very  high  temperatures. 

If  it  be  admitted  that  the  evaporating  principle  has  no  limit  of  this  nature,  it 
will  follow  that  the  atmosphere  must  always  be  impregnated  with  the  vapors 
of  all  bodies,  whether  solid  or  liquid.  It  is  difficult  to  imagine  this  to  be  the 
case,  without  supposing  a  great  variety  of  chemical  effects  to  be  produced  by 
such  a  confusion  of  substances,  having  such  an  indefinite  variety  of  physical 
relations  one  to  another.  It  seems  much  more  probable  that  the  less  vaporiza- 
ble  substances  at  common  temperatures  are  below  the  vaporizing  limit,  and 
that  the  atmosphere  contains  suspended  in  it  chiefly  the  vapor  of  water,  with 
slight  and  occasional  admixtures  of  the  vapors  of  the  more  volatile  bodies. 

The  elevation  of  the  average  temperature  of  the  air  has  a  double  effect  on 
the  rate  of  evaporation.  By  raising  the  temperature  of  water,  it  has  a  tendency 
to  increase  the  rate  ;  but  by  causing  an  increased  quantity  of  vapor  to  be  sus- 
pended in  the  air,  it  has,  on  the  other  hand,  a  contrary  effect.  The  difference 
between  the  extreme  tension  due  to  the  temperature  and  the  tension  of  the  va- 
por actually  suspended  is,  perhaps,  greater  in  warm  than  in  cold  weather,  be- 
cause in  cold  weather  the  atmosphere  is  nearer  its  point  of  saturation  than  in 
warm  weather.  Hence  the  rate  of  evaporation  is  probably  greater  in  summer  [ 
than  in  winter.  ^  i 


168 


EVAPORATION. 


The  method  adopted  by  Dalton  for  determining  the  tension  of  vapor  sus- 
pended in  the  atmosphere  at  any  given  time  is,  perhaps,  in  skilful  hands,  more 
exact  than  any  which  has  since  been  discovered,  especially  if  the  glass  vessel 
used  be  sufficiently  thin.  Dr.  Thompson  states  that  he  has  submitted  to  ex- 
periment other  instruments  for  the  same  purpose,  and  this  simple  one,  and  that 
he  is  satisfied  that  the  results  obtained  by  the  last  are  susceptible  of  the  high- 
est degree  of  accuracy. 

Other  instruments,  however,  have  been  contrived  for  determining  the  quan- 
tity of  vapor  suspended  in  the  atmosphere,  and  are  called  hygrometers,  or  meas- 
urers of  the  moistness  of  the  air.  Such  instruments  are  generally  constructed 
from  some  substance  which  has  a  power  of  absorbing  moisture,  and  which 
gives  some  external  indication  of  the  quantity  which  it  absorbs. 

The  hygrometer  of  M.  De  Luc  consists  of  an  extremely  thin  piece  of  whale- 
bone, which  is  stretched  between  two  points  and  acts  on  the  shorter  arm  of  an 
index  or  hand,  which  plays  on  a  graduated  scale,  like  the  hand  of  a  clock.  The 
effect  of  the  whalebone  absorbing  moisture  is  to  cause  it  to  swell,  and  its  length 
increases  ;  and,  on  the  contrary,  when  it  dries,  its  length  is  contracted.  The 
index  is  moved  in  the  one  direction  or  the  other  by  these  effects,  and  the  space 
it  moves  over  gives  the  change  in  the  hygrometric  state  of  the  atmosphere. 

The  hygrometer  of  M.  Saussure  consists  of  a  human  hair,  previously  pre- 
pared by  boiling  it  in  a  caustic  ley.  It  then  becomes  a  highly  sensible  absorb- 
ent of  moisture.  One  extremity  is  suspended  from  a  hook,  and  the  other  ex- 
tremity carries  a  small  weight  which  keeps  it  stretched.  It  is  turned  once 
round  a  grooved  wheel,  which  moves  an  index  playing  on  a  graduated  arch. 
As  the  hair  contracts  and  expands  by  the  effect  of  absorbing  moisture,  the 
wheel  is  turned  in  the  one  direction  or  the  other,  and  the  index  shows  the  effect 
by  moving  through  a  corresponding  portion  of  the  arch. 

That  this  instrument  may  indicate  the  absolute  quantity  of  vapor  suspended 
in  the  air,  it  was  necessary  that  some  fixed  points  upon  it  should  be  determin- 
ed, analogous  to  the  boiling  and  freezing  points  of  water  on  a  common  ther- 
mometer. To  effect  this  is,  however,  more  difficult  in  the  present  case,  inas- 
much as  the  instrument  is  inffuenced  at  once  by  two  causes,  namely :  by  heat, 
and  by  the  quantity  of  vapor  suspended  in  the  air.  M.  Saussure  first  consid- 
ered the  application  of  the  instrument  when  exposed  to  an  invariable  tempera- 
ture. He  placed  it  in  a  vessel  which  contained  perfectly  dry  air  at  the  pro- 
posed temperature.  He  thus  obtained  the  point  of  extreme  dryness.  He  then 
successively  introduced  into  the  receiver  several  small  known  quantities  of  wa- 
ter. This  he  accomplished  by  depositing  the  liquid  on  small  pieces  of  linen, 
which  he  weighed  exactly,  and  determined  the  quantity  of  liquid  thus  intro- 
duced. When  each  successive  portion  of  the  liquid  was  vaporized,  he  observed 
and  marked  the  indication  of  the  hygrometer.  He  then  withdrew  them  and 
weighed  them  again,  thus  determining  exactly  the  quantity  of  liquid  evaporated 
on  each  occasion. 

Having  repeated  very  often  the  experiment  at  the  same  temperature,  he  found 
that  whatever  variation  the  hygrometer  had  previously  undergone,  it  always 
returned  to  the  same  point  when  the  quantities  of  water  vaporized  in  the  re- 
ceiver were  equal.  He  found  the  same  result  at  various  temperatures,  the  in- 
dications at  the  same  temperature  being  always  the  same  ;  but  the  absolute 
quantity  of  water  necessary  to  be  vaporized  in  the  space,  in  order  to  move  the 
hygrometer  through  the  same  number  of  degrees,  was  different  at  different  tem- 
peratures. To  obtain,  therefore,  the  actual  quantity  of  water  suspended  in  the 
form  of  vapor,  it  is  necessary  at  the  same  time  to  observe  the  indications  of  the 
thermometer  and  hygrometer.  These  two  indications  are  always  sufficient  for 
the  exact  solution  of  the  question. 


EVAPORATION. 


169 


The  hygrometer  of  Leslie  is  an  instrument  by  which  the  hygrometric  state 
ot"  the  air  is  indicated  by  the  rate  at  which  water  evaporates.  The  bulb  of  an 
air  thermometer  is  covered  with  silk  or  bibulous  paper,  which  is  moistened. 
The  moisture  evaporating  produces  cold  in  the  bulb,  and  immediately  affects 
the  thermometer.  The  rapidity  of  the  evaporation  thus  indicated  depends  on 
the  temperature  of  the  air,  and  the  quantity  of  moisture  it  contains.  This  in- 
strument, however,  is  a  very  imperfect  indicator  of  the  hygrometric  state  of  the 
atmosphere. 

The  beautiful  theory  of  evaporation,  the  details  of  which  we  have  attempted 
to  explain  in  the  present,  and  in  other  lectures,  and  for  the  principal  part  of 
which  the  world  is  indebted  to  the  genius  of  Dalton,  affords  a  full  and  satis- 
factory elucidation  of  innumerable  phenomena  which  present  themselves  in 
atmospheric  and  meteorological  effects,  and  in  all  the  processes  of  science  and 
art. 

It  has  been  already  explained,  that  when  two  liquids,  such  as  water  and  al- 
cohol, which  combine  with  a  weak  affinity,  are  mixed  together,  their  combina- 
tion is  destroyed  by  the  process  of  vaporization,  and  each  liquid  vaporizes  at  a 
given  temperature,  in  the  same  manner  that  it  would  do  if  it  were  vaporized 
independently  of  the  other.  The  process  of  the  distillation  of  spirits  depends 
on  this  principle.  Let  us  suppose  that  a  liquid,  composed  principally  of  water 
and  alcohol,  is  placed  in  a  boiler  or  still,  which  communicates  by  a  tube  with 
a  refrigeratory  or  coo/er,  which  is  capable  of  condensing  into  a  liquid  the  vapor 
which  passes  from  the  still  through  it.  If  this  mixture  be  raised  to  a  temper- 
ature nearly  as  high  as  that  at  which  the  alcohol  would  boil,  a  vapor  will  rise 
composed  of  the  vapor  of  water  and  the  vapor  of  alcohol,  mixed  mechanically. 
Now  it  will  be  recollected,  that  the  specific  gravity  or  density  of  the  vapor  of 
alcohol  at  its  boiling  point,  is  about  three  and  a  half  limes  that  of  the  vapor  of 
water  at  212'^  ;  and  again,  the  density  of  the  vapor  of  water  at  212°  is  double 
the  density  of  the  vapor  of  water  at  180°.  Hence  it  follows,  that  the  density 
of  the  vapor  of  alcohol  at  its  boiling  temperature,  180°,  will  be  about  seven 
times  the  density  of  the  vapor  of  water  at  the  same  temperature. 

Thus  in  the  steam  produced  from  the  mixture  of  equal  parts  of  water  and 
alcohol,  we  shall  have  the  proportion  of  alcohol  to  water  in  the  ratio  of  7  to  1. 
This,  when  condensed  in  the  refrigeratory,  will  give  a  strong  spirit.  By  re- 
peating the  process  of  distillation,  the  mixture  may  be  more  and  more  sepa- 
rated from  the  water  which  it  contains. 

If  the  distillation  be  conducted  under  a  diminished  pressure,  or  in  a  vacuum, 
the  liquid  will  boil  at  a  much  lower  temperature  ;  and  the  portion  of  aqueous 
vapor  which  will  be  disengaged  will  be  of  such  a  small  degree  of  density  as  at 
length  to  become  insensible. 

The  principle  on  which  the  process  of  distillation  in  general,  therefore,  de- 
pends, is,  that  the  constituent  parts  of  the  mixture  boil  at  different  temper- 
atures ;  and  that,  if  the  mixture  be  caused  to  vaporize  by  heat,  that  part  of  it 
which  boils  at  the  lower  temperature  will  vaporize  in  greater  quantities  than 
that  which  boils  at  the  higher.  When  the  vapor  is  condensed  in  the  refrigera- 
tory, a  new  mixture  will  then  be  obtained,  containing  a  much  greater  quantity 
of  that  constituent  part  which  boils  at  the  lower  temperature  ;  and,  on  the  other 
hand,  the  liquid  which  remains  in  the  boiler  will  contain  a  greater  portion  of 
that  which  boils  at  the  higher  temperature.  In  general,  by  conducting  the  pro- 
ces,s  in  vacuo,  or  under  diminished  pressure,  this  object  is  more  effectually  at- 
tained, because  less  in  proportion  of  the  liquid  which  boils  at  the  higher  pres- 
sure will  be  vaporized  in  the  process. 

In  some  cases  it  happens  that  the  temperature  necessary  to  boil  the  liquid 
under  ordinary  pressure  may  be  such  as  to  decompose,  or  otherwise  injure,  ! 


170 


EVAPORATION. 


some  constituent  part  of  the  mixture  which  it  is  important  to  preserve.  For  ] 
this  reason,  the  above  method  is  said  to  have  been  adopted  with  advantage  in  i 
the  distillation  of  vinegar,  which  it  is  impossible  to  distil  in  the  ordinary  way  | 
without  giving  it  a  peculiar  burnt  flavor  ;  but  by  distilling  it  in  vacuo,  the  vapor  . 
is  raised  at  the  temperature  of  130°,  and  this  effect  is  avoided. 

In  the  process  of  sugar  refining  it  was  found  that  by  raising  the  syrup  to 
the  necessary  temperature,  a  risk  was  incurred  of  burning  or  decomposing  it 
by  too  much  heat.  The  method  of  boiling  in  vacuo  was  adopted  by  Mr.  Ed- 
ward Howard  to  remove  this  inconvenience.  The  syrup  is  thus  concentra- 
ted to  the  granulating  point  without  risk  of  decomposition.  This  method  is 
now  generally  followed. 

When  vapor  was  produced  from  a  liquid  by  ebullition,  we  have  observed 
that  a  large  quantity  of  heat  was  absorbed  in  the  transition  from  the  liquid  to 
the  gaseous  form.  The  same  effect  attends  the  production  of  vapor  from  the 
surface,  and,  in  fact,  it  is  an  indispensable  consequence  of  the  transition  of  a 
body  into  the  vaporous  form,  at  whatever  temperature  that  transition  takes 
place.  In  the  formation  of  vapor,  therefore,  a  quantity  of  heat  mUst  be  supplied 
to  the  vapor  formed,  and  must  become  latent  in  it ;  and  this  heat  must  be  sup- 
plied either  by  the  body  itself  or  by  surrounding  objects.  By  whatever  means 
it  is  supplied,  the  object  which  communicates  it  must  undergo  a  corresponding 
depression  of  temperature ;  and  hence  vaporization  becomes  a  means  for  the 
production  of  cold,  on  a  principle  precisely  analogous  to  that  of  freezing  mix- 
tures. 

This  principle  is  illustrated  by  the  method  used  to  cool  water  for  domestic 
purposes  in  hot  countries.  The  water  is  placed  in  certain  porous  vessels, 
called  in  the  East  alcarrazas,  and  these  are  suspended  in  a  current  of  air :  as, 
for  example,  between  two  open  doors.  The  vessel  allows  the  water  to  pene- 
trate it,  and  thus  exposes  it  more  effectually  to  evaporation,  as  well  from  the 
surface  of  the  liquid  itself,  as  from  the  exterior  surface  of  the  vessel  containing 
it.  As  the  vapor  is  formed,  a  quantity  of  latent  heat  is  necessary  for  it ;  and 
this  latent  heat  is  supplied  from  the  water  contained  in  the  vessel,  which  un- 
dergoes a  corresponding  depression  of  temperature. 

The  same  effect  can  be  made  manifest  by  surrounding  the  bulb  of  a  ther- 
mometer by  a  moist  sponge,  and  exposing  it  to  the  sun.  Let  another  ther- 
mometer be  at  the  same  time  placed  near  it  in  the  shade,  and  the  thermometer 
surrounded  by  the  sponge  will  be  observed  rapidly  to  fall,  while  the  thermom- 
eter in  its  immediate  neighborhood  is  stationary.  This  effect  is  evidently  pro- 
duced by  the  rapid  evaporation  of  the  water  with  which  the  sponge  is  saturated, 
and  a  corresponding  depression  of  temperature  produced  in  the  liquid  remain- 
ing in  the  sponge,  arising  from  the  heat  supplied  by  it  to  the  vapor. 

The  depression  of  temperature  produced  by  evaporation  will  be  more  per- 
ceptible the  more  rapid  is  the  evaporation,  because  then  the  body  from  which 
the  heat  is  abstracted  has  not  time  to  receive  a  supply  of  heat  from  surrounding 
objects  to  replace  that  which  it  has  given  out.  Hence,  by  conducting  the  pro- 
cess of  evaporation  in  a  vacuum,  where  the  evaporation  is  almost  instantaneous, 
the  cooling  effect  is  more  conspicuous.  If  a  quantity  of  water  included  in  the 
bulb  of  a  thermometer  tube  be  surroimded  with  a  sponge  moistened  with  ether, 
and  placed  under  the  receiver  of  an  air-pump,  the  moment  the  air  is  withdrawn 
the  ether  suddenly  evaporates  ;  and  if  a  sufficient  quantity  of  ether  be  supplied, 
the  water  in  the  bulb  will  be  frozen. 

The  same  fact  may  be  exhibited  in  a  still  more  striking  manner,  by  pouring 
some  ether  on  the  surface  of  water  in  a  flat  vessel.  When  the  receiver  placed 
over  these  is  exhausted,  the  ether  will  boil  in  consequence  of  the  removal  of 
the  atmospheric  pressure,  and  its   rapid  evaporation  will  presently  cause  the 


EVAPORATION. 


171 


water  under  it  to  freeze.  We  shall  thus  have  the  singular  exhibition  of  two 
liquids,  one  resting  upon  the  other,  the  one  boiling  and  the  other  freezing  at 
the  same  moment ;  and,  after  the  lapse  of  a  few  minutes,  one  altogether  disap- 
pearing in  the  form  of  vapor,  Vvhile  the  other  solidifies  in  the  form  of  ice. 

A  beautiful  experiment  was  contrived  by  Leslie,  in  which  water  is  frozen  on 
this  principle.  A  shallow  vessel  containing  water  is  placed  under  the  receiver 
of  an  air-pump.  Under  the  same  receiver  is  placed  a  large  flat  dish,  contain- 
ing strong  sulphuric  acid.  The  receiver  is  now  exhausted  as  rapidly  as  pos- 
sible by  the  pump,  and  immediately  the  evaporation  of  the  water  takes  place. 
If  the  sulphuric  acid  were  not  present,  the  space  within  the  receiver  would  be 
saturated  almost  instantaneously  with  the  vapor  of  the  water,  and  all  further 
evaporation  would  be  stopped ;  but  the  sulphuric  acid  not  being  itself  subject 
to  sensible  evaporation,  has  besides  a  strong  affinity  for  water,  by  virtue  of 
which  it  attracts  the  aqueous  vapor,  and  causes  it  to  be  condensed  on  its  sur- 
)  face.  As  fast,  therefore,  as  the  water  evaporates,  its  vapor  is  seized  upon  by 
the  sulphuric  acid  in  the  large  dish,  and  the  space  within  the  receiver  is  still 
maintained  a  vacuum  ;  so  that  the  evaporation  of  the  water  continues  as  rap- 
idly as  in  the  first  instance.  Now  the  heat  necessary  to  give  the  vaporous  form 
to  the  water  can  only  be  received  from  the  water  itself  which  remains  in  the 
dish,  and  therefore  it  must  undergo  a  rapid  depression  of  temperature.  It  will 
speedily  fall  to  the  temperature  of  32°,  and  in  a  few  minutes  will  be  frozen. 
By  this  process,  conducted  under  favorable  circumstances,  Leslie  was  not  only 
able  to  freeze  water,  but  to  congeal  mercury ;  and  it  is  said  that  he  even  pro- 
duced a  cold  of — 120°.  The  property  on  which  this  beautiful  experiment  is 
founded  is  not  recommended  alone  by  the  surprise  and  pleasure  which  its  re- 
sult always  produces  ;  it  is  susceptible  of  useful  application  in  chemistry  when 
it  is  necessary  to  separate  water  from  liquids  which  heat  would  decompose  ; 
and  to  dry  animal  and  vegetable  substances  without  exposing  them  to  disorgan- 
ization. 

By  the  same  method,  the  fact  that  ice  itself,  at  all  temperatures,  is  subject 
to  evaporation,  may  be  made  manifest.  If  a  few  ounces  of  ice  be  placed  im- 
der  the  receiver  of  an  air-pump  over  a  similar  dish  containing  concentrated 
sulphuric  acid,  and  the  receiver  be  exhausted,  the  ice  will  altogether  disappear 
in  about  twenty-four  hours.  During  the  whole  of  this  time  the  temperature 
will  be  considerably  below  32°.  After  the  ice  has  disappeared  the  sulphuric 
acid  will  be  found  to  be  combined  with  water  and  to  have  increased  its  weight 
by  the  exact  weight  of  the  ice. 

In  climates  where  the  temperature  of  the  air  never  falls  so  low  as  the  freez- 
ing point,  and,  therefore,  where  no  natural  ice  ever  exists,  ice  is  obtained  arti- 
ficially by  a  cold  produced  by  evaporation.  In  India  it  is  obtained  by  making 
extensive  shallow  excavations  in  large  open  plains.  In  these  water  is  exposed 
to  evaporation  in  small  earthern  pots,  unglazed,  so  as  to  be  porous,  and  pene- 
trable by  water.  Soft  water,  previously  boiled,  is  placed  in  these  vessels  in 
the  evening,  in  the  months  of  December,  January,  and  February.  A  part  of  it 
is  usually  frozen  in  the  morning,  when  the  ice  is  collected  and  deposited  in 
pits,  surrounded  by  straw  and  other  bodies  which  exclude  heat.  Radiation,  ' 
also,  has  a  part  in  producing  this  effect  as  will  be  explained  hereafter.  i 

Evaporation  being  extensively  used  in  the  arts  and  manufactures,  it  has  be-  ] 
come  a  matter  of  considerable  importance  to  conduct  it  with  as  much  economy  < 
and  expedition  as  possible.  The  circumstances  which  principally  promote  it  j 
being  increase  of  temperature  and  a  constant  change  in  the  air  which  is  imme- 
diately above  the  evaporating  surface,  these  two  objects  have  received  special  ) 
attention.  In  factories  where  evaporation  is  used,  the  vessels  containing  the  ( 
liquid  to  be  evaporated,  are  usually  placed  where  they  shall  be  exposed  to  a  J 


172 


EVAPORATION. 


current  of  air  passing  over  their  surface.  In  cases  >where  it  has  been  found  i 
convenient  to  promote  the  evaporation  by  heating  the  liquid,  the  heat  is  fre-  ' 
quently  applied  only  to  the  surface,  instead  of  being  communicated  by  fire  at  i 
the  bottom  of  the  vessel.  In  fact,  the  current  of  air  which  is  made  to  pass  ' 
over  the  surface  of  the  evaporating  liquid,  is  previously  heated  by  forcing  it  ! 
through  afire.  The  flame  of  the  fire  is  also,  sometimes,  made  to  play  over  the 
evaporating  surface. 

The  coolers  in  breweries  are  large  shallow  vessels,  exposing  a  considerable 
surface  with  a  small  depth  of  the  liquid.  They  are  commonly  placed  at  the 
top  of  the  building,  and  are  open  on  every  side  to  the  air,  so  that  in  whatever- 
direction  a  wind  blows  a  current  of  air  must  pass  over  them.  There  are  also 
provided  a  number  of  revolving  fans,  by  which  the  stream  of  air  in  immediate 
contact  with  the  evaporating  surface,  is  continually  kept  in  a  state  of  agitation. 
The  evaporation  has  a  continual  tendency  to  saturate  the  stratum  of  air  im- 
mediately over  the  liquid,  and  by  these  expedients  this  stratum  is  caused  to 
undergo  a  constant  change  ;  the  air  saturated  with  vapor  being  driven  away,  and 
a  fresh  portion  supplying  its  place. 

When  salt  is  held  in  solution  by  water,  the  process  of  evaporation  affects 
only  the  water,  and  loosens  the  connexion  produced  by  the  affinity  of  its  par- 
ticles for  the  molecules  of  the  salt.  If  the  solution,  ia  this  case,  be  what  is 
called  a  saturated  solution,  that  is,  if  it  contain  as  much  salt  as  the  water  at  the 
given  temperature  is  capable  of  sustaining,  then  the  least  quantity  of  evapora- 
tion must  be  attended  with  a  deposition  of  crystals  of  salt  in  the  liquid  :  and, 
if  the  evaporation  be  continued,  the  water  will,  at  length,  altogether  disappear, 
and  nothing  but  a  mass  of  crystallized  salt  will  remain. 

This  principle  forms  the  basis  of  the  method  by  which  salt  is  obtained  from 
sea-water.  The  water  is  received  into  a  number  of  large  shallow  ponds,  lined 
with  clay,  and  prepared  on  the  seashore.  The  water,  being  received  into 
these,  and  dammed  in,  is  left  exposed  to  the  weather  in  the  heat  of  summer. 
If  the  weather  be  dry,  the  quantity  of  evaporation  will  considerably  exceed 
the  quantity  of  rain,  and  large  surfaces  being  exposed  in  proportion  to  the 
depth  of  water  in  the  pits,  the  water  will  be  gradually  dissipated,  and  will 
at  length,  altogether  disappear,  and  a  quantity  of  what  is  called  bay  salt  will 
remain  behind.  This  salt  is  said  to  be  the  fittest  for  the  purpose  of  curing 
fish. 

When  ice  cannot  be  obtained,  wine  may  be  cooled  in  various  ways  by  the 
process  of  evaporation.     If  a  moist  towel   be   wrapped  round  a  decanter  of 
wine  and  exposed  to  the  sun,  the  towel  in  the  process  of  drying  will  cool  the 
wine  ;  for  the  wine  must  supply  a  part  of  the  latent  heat  carried  off  by  the  va- 
por in  the  process  of  drying  the  towel.     Wine-coolers   constructed  of  porous 
earthern  ware  act  on  a  similar  principle.     The   evaporation  of  water  from  the 
porous  material  reduces  the  temperature  of  the  liquid  immediately  surrounding 
i  the  wine.     Travellers  in  the  Arabian  deserts  keep  the  water  cool  by  wrapping 
'  the  jars  with  linen  cloths  which  are  kept  constantly  moist. 
I       Historians   mention  that  the  Egyptians  applied   the  same   principle   to  cool 
'  water  for  domestic  purposes.     Pitchers  containing  the   water  were  kept  con- 
I  stantly  wet  on  the  exterior  surface  during  the  night,  and  in  the  morning  were 
f  surrounded  by  straw  to  intercept  the  communication  of  heat  from  the  external 

>  air. 

'       In  India  the  curtains  which  surround  beds  are  sprinkled  with  water,  by  the 

>  evaporation  of  which  the  air  within  the  curtains  is  cooled. 

r  The  absorption  of  heat  in  evaporation  will  enable  us  easily  to  comprehend 
)  the  danger  arising  from  wearing  damp  clothes,  or  from  sleeping  in  a  damp  bed. 
\  In  the  animal  economy  there  is  a  source,  the  nature  and  operation  of  which  is 


EVAPORATION. 


173 


not  understood  by  us,  by  which  heat  is  generated  in  the  system,  and  is  con-  s 
tinually  given  out  by  the  body.  If  any  cause  withdraws  heat  faster  from  the  ) 
body  than  it  is  thus  produced,  a  sensation  of  cold  is  felt ;  and  if,  on  the  con-  \ 
trary,  the  heat  be  not  withdrawn  as  fast  as  it  is  generated,  the  body  becomes  j 
unduly  warm.  A  balance  should,  therefore,  as  much  as  possible,  be  maintain-  ' 
ed  between  the  natural  power  of  the  body  in  the  production  of  heat,  and  the  ( 
;  faculty  of  receiving  that  heat  in  surrounding  objects.     In  cold  weather  all  sur-   ' 

<  rounding  objects  being  at  a  much  lower  temperature   than  the  body,  have  a  , 
)  tendency  to  receive  heat  faster  than  the  body  can  supply  it,   and  in  this  case, 

<  artificial  sources  of  external  heat  are  sought,  by  which  the  temperature  of  sur- 
)  rounding  objects  may  be  raised,  so  as  to  accommodate  themselves  to  the  ani- 
(  mal  system.  In  very  hot  weather,  on  the  contrary,  the  temperature  of  surround- 
)  ing  objects  is  so  near  the  temperature  of  the  body,  that  the  heat  produced  in  the 

<  system  is  not  received  with  sufficient  facility  to  keep  the  body  sufficiently  cool. 
)  In  this  case,  artificial  means  of  keeping  down  the  temperature  of  the  body  are 
(  necessarily  resorted  to. 

)  If  the  clothes  which  cover  the  body  are  damp,  the  moisture  which  they 
I  contain  has  a  tendency  to  evaporate  by  the  heat  communicated  to  it  by  the 
)  body. 

<  In  fact,  the  body,  in  this  case,  is  circumstanced  exactly  in  the  same  manner 
)  as  the  bulb  of  a  thermometer,  already  described,  surrounded  by  a  damp  sponge, 
(  in  which  case  we  saw  that  the  mercury  rapidly  fell.  The  heat  absorbed  in 
)  the  evaporation  of  the  moisture  contained  in  the  clothes  must  be,  in  part,  sup- 

<  plied  by  the  body,  and  will  have  a  tendency  to  reduce  the  temperature  of  the 
)  body  in  an  undue  degree,  and  thereby  to  produce  cold.  The  effect  of  violent 
l  labor  or  exercise  is  to  cause  the  body  to  generate  heat  much  faster  than  it  would 
)  do  in  a  state  of  rest.     Hence  we  see  why,  when  the  clothes  have   been   ren- 

<  dered  wet  by  rain,  or  by  perspiration,  the  taking  of  cold  may  be  avoided  by 
J  keeping  the  body  in  a  state  of  exercise  or  labor  until  the  clothes  can  be  changed, 

<  or  till  ihey  dry  on  the  person  ;  for  in  this  case,  the  heat  carried  off  by  the 
;  moisture  in  evaporating  is  amply  supplied  by  the  redundant  heat  generated  by 
I  labor  or  exercise. 

;       A  damp  bed,  however,  is  an  evil  which  cannot  be  remedied  by  this  means, 

<  the  object  of  bed-clothes  being  to  check  the  escape  of  heat  from  the  body,  so 
;  as  to  supply  at  night  that  warmth  which  may  be  obtained  by  exercise  or  labor 

<  during  the  day.  This  end  is  not  only  defeated,  but  the  contrary  effect  produ- 
;  ced,  when  the  clothes  by  which  the  body  is   surrounded,  contain  moisture  in 

<  them.  The  heat  supplied  by  the  body  is  immediately  absorbed  by  this  mois- 
}  ture,  and  passes  off  in  vapor ;  and  this  effect  would  continue  until  the  clothes 

<  were  actually  dried  by  the  heat  of  the  body. 

)  A  damp  bed  may  be  frequently  detected  by  the  use  of  a  warming-pan. 
\  The  introduction  of  the  hot  metal  causes  the  moisture  of  the  bed-clothes  to  be 
/  immediately  converted  into  steam,  which  issues  into  the  open  space  in  which 
i  the  warming-pan  is  introduced.  When  the  warming-pan  is  withdrawn,  this 
/  vapor  is  again  partially  condensed,  and  deposited  on  the  surface  of  the  sheets, 
\  the  dampness  will  be  then  distinctly  felt,  a  film  of  water  being,  in  fact,  deposited 
)  on  their  surface. 

s  The  danger  of  leaving  damp  or  wet  clothes  to  dry  in  an  inhabited  apartment, 
)  and  more  especially  in  a  sleeping-room,  will  be  readily  understood  from  what 
(  has  been  just  explained.  The  evaporation  which  takes  place  in  the  process 
)  of  drying  causes  an  absorption  of  heat,  and  produces  a  corresponding  depression 
S  of  temperature  in  the  apartment. 

}  A  striking  example  of  the  effects  of  cold  produced  by  evaporation  is  exhibited  in 
S  an  experiment  contrived  by  Dr.  Wollaston,  and  made  with  an  instrument  which 


174 


EVAPORATION. 


he  called  a  cryophorus.     This  instrument  consisted  of  a  glass  tube,  A  B,  fig.  1, 
furnished  with  two  bulbs,  C  D,  placed  on  short  branches  at  right  angles  to  it. 

Fig-  1- 


A  small  quantity  of  water  is  introduced  through  a  short  tube,  which  proceeds 
from  the  bottom  of  the  bulb  D  at  0.  It  is  boiled  in  C  until  the  space  above  C, 
and  tube  A  B,  and  the  bulb  D,  is  completely  filled  with  aqueous  vapor  to  the 
exclusion  of  atmospheric  air.  The  tube  O  is  then  closed  by  melting  it  with  a 
blowpipe,  so  that  the  interior  of  the  apparatus  now  contains  nothing  but  water. 
When  the  instrument  cools,  the  vapor  is  condensed,  and  such  a  vapor  only 
subsists  in  the  instrument  as  corresponds  to  the  temperature  of  the  water  in  C. 
If  the  bulb  D  be  now  surrounded  by  a  freezing  mixture,  or  exposed  to  any  in- 
tense cold,  the  vapor  produced  from  the  water  in  C  will  be  condensed  in  it,  so 
that  the  space  above  the  water  in  C,  and  in  the  tube  A  B,  will  be  constantly 
prevented  from  attaining  the  state  of  saturation.  The  evaporation  will  then  be 
continued,  and  the  latent  heat  of  the  steam  must  be  chiefly  derived  from  the 
sensible  heat  of  the  water  remaining  in  C.  The  temperature,  therefore,  of  this 
water  will  be  rapidly  depressed  until  it  reaches  the  freezing  point,  when  it  will 
be  solidified. 

When  an  ink  bottle  has  a  large  mouth,  the  surface  of  the  liquid  in  it  will  be 
exposed  to  a  rapid  evaporation  ;  and,  as  this  evaporation  affects  only  the  aque- 
ous part  of  the  liquid,  the  effect  will  be,  that  the  ink  will  first  become  thick, 
and,  if  exposed  a  longer  time,  the  whole  of  the  liquid  portion  of  it  will  pass  off, 
and  nothing  but  the  hard  coloring  matter  will  remain.  If,  however,  the  mouth 
of  the  bottle  be  contracted  to  a  small  aperture,  sufficient  to  receive  a  pen,  the 
rate  of  evaporation  will  be  considerably  diminished  ;  for,  although  the  surface 
of  ink  in  the  bottle  may  be  large,  yet  the  evaporation  having,  in  the  first  in- 
stance, saturated  the  space  between  the  surface  of  the  ink  and  the  mouth  of  the 
bottle,  no  farther  evaporation  could  take  place,  if  that  mouth  were  stopped  ;  but, 
if  it  be  opened,  then  a  portion  of  the  vapor,  contained  in  the  bottle  above  the 
surface  of  the  liquid,  will  escape  from  it  into  the  strata  of  air  immediately  above  ; 
but  this  portion  will  be  less  in  proportion  as  the  mouth  of  the  bottle  is  small. 
It  will,  therefore,  be  found  that  ink  will  be  less  liable  to  thicken  in  ink-bottles 
having  a  small  aperture  than  in  those  which  have  a  large  aperture ;  but  the 
thickening  of  ink  may  be  altogether  avoided  by  the  use  of  ink-bottles  which, 
while  they  are  capable  of  containing  a  considerable  quantity  of  ink,  expose  a 
very  small  surface  to  evaporation.  Such  bottles  are  constructed  like  bird-cage 
fountains.     A  B,  fig.  2,  is  a  glass  bottle,  completely  closed  at  the  top,  and  hav- 

Fig.  2. 


ing  a  tube,  C,  proceeding  laterally  from  the  bottom  turned  upward,  where  there 
is  a  small  mouth  large  enough  to  receive  a  pen.    The  bottle  is  filled  by  inch- 


EVAPORATION. 


175 


ing  the  closed  part,  A  B,  slightly  downward,  and  pouring  the  ink  in  at  C,  held 
in  a  slanting  position.  When  the  bottle  is  placed  in  the  upright  position,  the 
surface  of  the  ink  in  the  bottle  will  remain  above  the  surface  of  the  ink  in  C, 
because  the  atmospheric  pressure  acting  in  C  will  balance  the  weight  of  the 
ink  in  A  B,  together  with  the  pressure  of  the  air  confined  in  A  B.  The  evap- 
oration from  the  surface  in  A  B  having  saturated  the  space  above  it  will  cease, 
and  the  only  evaporation  which  will  have  a  tendency  to  thicken  the  ink  will  be 
that  which  takes  place  at  the  surface  in  C  ;  but  this  surface  being  very  small, 
the  evaporation  will  be  inconsiderable.  In  such  an  ink-bottle  ink  may  remain 
several  months  without  thickening. 

The  reciprocal  processes  of  evaporation  and  condensation  are  the  means 
whereby  the  whole  surface  of  that  part  of  the  globe  which  constitutes  land  is 
supplied  with  the  fresh  moisture  and  water  necessary  to  sustain  the  organiza- 
tion and  to  maintain  the  functions  of  the  animal  and  vegetable  world.  Thence 
sap  and  juice  are  supplied  to  vegetables,  and  fluids  to  animals  ;  rivers  and 
lakes  are  fed,  and  carry  back  to  the  ocean  their  "waters,  after  supplying  the 
uses  of  the  living  world. 

The  extensive  surface  of  the  ocean  undergoes  a  never-ceasing  process  of 
evaporation,  and  dismisses  into  the  atmosphere  a  quantity  of  pure  water  pro- 
portionate to  its  extent  of  surface  and  the  temperature  of  the  air  above  it,  and 
to  the  state  of  that  air  with  respect  to  saturation.  This  vapor  is  carried  with 
currents  of  air  through  every  part  of  the  atmosphere  which  surrounds  the 
globe. 

When  by  various  meteorological  causes  the  temperature  of  the  air  is  re- 
duced, it  will  frequently  happen  that  it  will  come  below  that  limit  at  which  the 
suspended  vapor  is  in  a  state  of  saturation.  A  deposition  or  condensation  will 
therefore  take  place,  and  rain  or  aqueous  clouds  will  be  formed.  If  the  con- 
densed vapor  collect  in  spherical  drops,  it  will  be  precipitated,  and  fall  on  the 
surface  of  the  earth  in  the  form  of  rain  ;  but,  from  some  unknown  cause,  it  fre- 
quently happens  that,  instead  of  collecting  in  drops,  the  condensed  vapor  is 
formed  into  hollow  bubbles,  enclosing  within  them  a  fluid  lighter,  bulk  for  bulk, 
than  the  atmosphere.  These  bubbles  are  also  found  to  have  a  repulsive  influ- 
ence on  each  other,  like  that  of  bodies  similarly  electrified.  They  float,  there- 
fore, in  the  atmosphere,  their  mutual  repulsion  preventing  them  coalescing  so 
as  to  form  drops.  In  this  state,  having  by  the  laws  of  optics  a  certain  degree 
of  opacity,  they  become  distinctly  visible  and  form  clouds. 

The  vapor  suspended  in  the  air  during  a  hot  summer's  day  is  so  elevated  in 
its  temperature  as  to  be  below  the  point  of  saturation,  and  therefore,  though  the 
actual 'quantity  suspended  be  very  considerable,  yet,  while  the  air  is  capable 
of  sustaining  more,  no  condensation  can  take  place ;  but  in  the  evening,  after 
the  sun  has  departed,  the  source  of  heat  being  withdrawn,  the  temperature  of 
the  air  undergoes  a  great  depression,  and  the  quantity  of  vapor  suspended  in 
the  atmosphere,  now  at  a  lower  temperature,  first  attains  and  subsequently 
passes  the  point  of  saturation. 

A  deposition  of  moisture  then  takes  place  by  the  condensation  of  the  redun- 
dant vapor  of  the  atmosphere,  and  the  small  particles  of  moisture  which  fall  on 
the  surface,  coalescing  by  their  natural  cohesion,  form  clear,  pellucid  drops  on 
the  surface  of  the  ground,  and  are  known  by  the  name  of  dew. 

The  clouds  in  which  the  condensed  vesicles  of  vapor  are  collected,  are  affect- 
ed by  an  attraction  which  draws  them  toward  the  mountains  and  highest  points  , 
of  the  surface  of  the  earth.  Collected  there,  they  undergo  a  change,  by  which  ' 
they  form  into  drops,  and  are  deposited  in  the  form  of  rain  ;  and  hence,  by  their  \ 
natural  gravitation,  they  find  their  way  through  the  pores  and  interstices  of  the  < 
earth,  and  in  channels  along  its  surface,  forming,  in  the  one  case,  wells  and  / 


176 


EVAPORATION. 


springs  in  various  parts  of  the  earth,  where  they  find  a  natural  exit,  or  where 
an  artificial  exit  is  given  to  them,  and,  in  the  other  case,  obeying  the  form  of 
the  surface  of  the  country  through  which  they  are  carried,  they  wind  in  narrow 
channels,  first  deepening  and  widening  as  they  proceed,  and  are  fed  by  tributary 
streams  until  they  form  into  great  rivers,  or  spread  into  lakes,  and  at  length 
discharge  their  waters  into  the  sea. 

The  process  of  evaporation  is  not  confined  to  the  sea,  but  takes  place  from 
the  surface  of  the  soil,  and  from  all  vegetable  and  animal  productions.  The 
showers  which  fall  in  summer,  first  scattered  in  a  thin  sheet  of  moisture  over 
the  surface  of  the  country,  speedily  return  to  the  form  of  vapor,  and  carry  with 
them,  in  the  latent  form,  a  quantity  of  heat,  which  they  take  from  every  object 
in  contact  with  them,  thus  moderating  the  temperature  of  the  earth,  and  re- 
freshing the  animal  and  vegetable  creation. 

A  remarkable  example  of  evaporation  on  a  large  scale  is  supplied  by  that 
great  inland  sea,  the  Mediterranean.  That  natural  reservoir  of  water  receives 
an  extraordinary  number  of  large  rivers,  among  which  may  be  mentioned  the 
Nile,  the  Danube,  the  Dnieper,  the  Rhone,  the  Ebro,  the  Don,  and  many  oth- 
ers. It  has  no  communication  with  the  ocean,  except  by  the  straits  of  Gibral- 
ter,  and  there,  instead  of  an  outward  current,  there  is  a  rapid  and  never-ceas- 
ing inward  flow  of  water.  We  are,  therefore,  compelled  to  conclude  that  the 
evaporation  from  the  surface  of  this  sea  carries  off  the  enormous  quantity  of 
water  constantly  supplied  from  these  sources.  This  may,  in  some  degree,  be 
accounted  for  by  the  fact,  that  the  Mediterranean  is  surrounded  by  vast  tracts 
of  land  on  every  side  except  the  west.  The  wind,  whether  it  blow  from  the 
south,  the  north,  or  from  the  east,  has  passed  over  a  considerable  extent  of 
land,  and  is  generally  in  a  state,  with  respect  to  vapor,  considerably  below  sat- 
uration. These  dry  currents  of  wind,  coming  in  contact  with  the  surface  of 
the  Mediterranean,  draw  off  water  with  avidity,  and,  passing  off,  are  succeeded 
by  fresh  portions  of  air,  which  repeat  the  same  process. 


CONDUCTION    OF   HEAT. 


Conducting  Powers  of  Bodies. — Liquids  Non-Conductors. — Eflfect  of  Feathers  and  Wool  on  Ani- 
mals.— Clothing. — Familiar  Examples. 


COIDUGTION   OF   HEAT. 


If  two  solid  bodies,  having  different  temperatures,  be  placed  in  close  con- 
tact, it  will  be  observed  that  the  hotter  body  will  gradually  fall  in  temperature, 
and  the  colder  gradually  rise,  until  the  temperatures  become  equal.  This  pro- 
cess is  not,  like  radiation,  sudden,  but  very  gradual ;  the  colder  body  receives 
increased  temperature  slowly,  and  the  hotter  loses  it  at  the  same  rate.  Differ- 
ent bodies,  however,  exhibit  a  different  facility  in  this  gradual  transmission  of 
heat  by  contact.  In  some  it  passes  more  rapidly  from  the  hotter  to  the  colder, 
and  in  others  the  equalization  of  temperature  is  not  produced  until  after  the 
lapse  of  a  considerable  time. 

This  quality  in  bodies,  by  which  heat  passes  from  one  to  the  other  through 
their  dimensions,  is  called  their  conducting  power,  and  the  heat  thus  transmitted 
is  said  to  be  conducted  by  the  body.  One  body  is  said  to  be  a  better  conductor 
than  another,  when  the  equalization  of  temperature  is  effected  more  speedily  ; 
and  when  the  equalization  is  accomplished  more  slowly,  the  body  is  said  to  be 
a  bad  conductor. 

To  make  this  process  more  intelligible,  let  us  suppose  A,  fig.  1,  a  small 
square  block  of  red-hot  iron,  and  let  B  C  be  a  bar  of  iron,  the  section  of  which 

Fig.  1. 


1 

r! 

t 

ti 

t'l 

t'" 

c 

Ify- 

^ 

is  square.  Let  the  extremity,  B,  be  placed  close  against  the  block  A,  and  let 
a  screen,  S,  pierced  by  A  B,  be  placed  so  as  to  intercept  the  effect  of  radiation 
from  A.     Let  thermometers,  t  I',  &c.,  be  inserted  at  different  points  of  the  bar 


r 


ISO 


CONDUCTION  OF  HEAT. 


B  C,  in  small  cavities  provided  for  the  purpose  and  filled  with  mercury.  This 
mercury  will  take  the  temperature  of  the  bar,  and  will  communicate  it  to  each 
thermometer  successively.  Before  the  bar  is  placed  in  contact  with  the  red- 
hot  block  A,  the  thermometers  will  all  indicate  the  same  temperature.  At  the 
first  moment,  when  the  bar  is  placed  in  contact  with  A,  none  of  the  thermome- 
ters will  be  aff'ected  by  it ;  but,  after  the  lapse  of  a  short  time,  the  first  ther- 
mometer, t,  will  be  observed  to  rise  slowly ;  after  another  interval,  the  ther- 
mometer l'  will  begin  to  be  affected  ;  and  the  other  thermometers,  after  like 
intervals,  will  be  successively  aff'ected  in  the  same  way ;  but  the  thermometer 
t,  by  continuing  to  rise,  will  indicate  a  higher  temperature  than  t' ,  and  t'.  a 
higher  temperature  than  t" ,  and  so  on.  After  the  lapse  of  a  considerable  time, 
the  temperatures  of  all  the  thermometers  will  be  the  same ;  and  if  the  block  A 
be  observed,  it  will  be  found  to  have  the  common  temperature  indicated  by  all 
the  thermometers. 

It  appears,  from  this  experiment,  that  the  propagation  of  heat  in  this  manner 
through  the  dimensions  of  the  bar  is  very  slow,  and  it  would  seem  to  take  place 
from  particle  to  particle  of  the  matter  composing  the  bar.  The  first  particle  in 
contact  with  the  source  of  heat  acquires  a  certain  temperature  ;  this  being 
greater  than  the  contiguous  particles,  an  interchange  takes  place  between  the 
two,  on  a  principle  exactly  similar  to  the  interchange  of  heat  by  radiation.  In 
fact,  two  contiguous  particles  in  this  case  may  be  regarded,  under  the  same 
circumstances,  as  two  bodies  having  diff'erent  temperatures  placed  in  the  foci 
of  the  two  reflectors,//,  fig.  2.     In  that  case,  the  hotter  body  radiates  heat 


Fig.  2. 


on  the  colder,  and  the  colder  on  the  hotter,  in  unequal  quantities,  until  their 
temperatures  are  equalized.  Every  two  successive  particles  in  the  bar  B  C, 
fig.  1,  beginning  from  the  source  of  heat,  appear  to  act  on  each  other  in  the 
same  way. 

Let  a  number  of  bars  of  diff'erent  substances  of  equal  dimensions,  be  suc- 
cessively exposed  in  this  manner,  to  the  same  source  of  heat,  and  let  thermom- 
eters be  applied  to  similar  points  in  them,  it  will  be  found  that  thermometers 
in  the  same  situation  on  diff'erent  bars,  will,  after  the  lapse  of  the  same  time 
from  the  commencement  of  the  contact,  be  diff'erently  aff'ected.  In  those  bars 
which  are  good  conductors  the  thermometer  will  be  more  elevatedthan  in  those 
which  are  bad  conductors  ;  and,  in  general,  the  conducting  power  of  the  diff'e- 
rent bars  may  be  estimated  by  the  effect  produced  on  thermometers  at  a  given 
distance  from  the  source  of  heat,  after  the  lapse  of  a  given  time.  In  experi- 
ments of  this  nature  it  is,  howe\=!r,  necessary  to  guard  against  the  effects  of 
radiation  ;  because,  if  two  different  bars  radiate  differently,  it  is  possible  that 
the  indications  of  the  thermometer  may  be  so  interfered  with,  by  their  different 
powers  of  radiation,  that  their  conducting  power  cannot,  with  certainty,  be  in- 
ferred.    In  a  course  of  experiments  instituted  on  this  subject  by  Despretz,  he 


CONDUCTION  OF  HEAT.  l&l 


employed  bars  of  the  same  size,  covered  with  a  coating  of  varnish.  Heat  v/as 
applied  by  a  lamp  at  one  end,  and  its  progress  along  the  bar  indicated  by  a 
thermometer  at  the  other  ;  the  lamp  was  applied  until  its  utmost  effect  on  ihe 
thermometer  was  ascertained ;  and  the  greatest  heat  to  which  the  thermometer 
could  thus  be  raised  by  the  effect  of  the  lamp,  was  taken  as  the  measure  of  ihe 
conducting  power  of  the  bar.  The  following  table  exhibits  the  results  ot 
Despretz's  experiments  on  different  substances  : — 

Conducting  power. 

Gold, 100- 

Platinum, 98-1 

Silver, 97-3 

Copper, 89-82 

Iron, 37-41 

Zinc, 36-37 

Tin 30-38 

Lead 17-96 

Marble 2-34 

Porcelain 1-22 

Brick  earth 1-13 

From  this  table  it  is  obvious  that  the  metals  are,  by  far,  the  best  conductors 
of  heat,  and  that  the  conducting  power  of  earthy  substances  is  prodigiously 
inferior. 

Similar  experiments  were  made  on  different  species  of  wood,  by  MM.  A. 
Delarive,  and  A.  DecandoUe.  From  these  experiments  it  appears  that,  gene- 
rally, the  more  dense  woods  are  those  which  conduct  heat  best.  This  rule, 
however,  is  not  invariable,  for  the  conducting  power  of  nut-wood  was  found  to 
be  considerably  greater  than  that  of  oak.  It  was,  also,  found,  that  heat  was 
better  conducted  in  the  direction  of  the  fibres  than  across  them. 

In  bodies  of  the  same  kind,  the  rate  at  which  heat  is  conducted,  from  the 
hotter  to  the  colder,  depends  on  the  extent  of  the  surface  of  contact,  and  is 
proportional  to  that  surface.  Thus  if  two  spheres  or  balls  of  metal,  at  different 
temperatures,  be  placed  in  contact,  they  will  touch  only  in  a  single  point, 
and  the  transmission  of  heat  will  be  extremely  slow  ;  but  if  two  cubes  of  the 
same  metal  be  placed  face  to  face,  their  surface  of  contact  will  be  considerable, 
and  the  transition  of  heat  will  be  proportionally  rapid. 

Bodies  of  a  porous,  soft,  or  spongy  texture,  and  especially  those  of  a  fibrous 
nature,  such  as  wool,  feathers,  fur,  &c.,  are  the  worst  conductors  of  heat. 
Such  a  body  may  be  placed  in  contact  with  another  body  of  a  much  higher  or 
much  lower  temperature  than  itself,  without  exhibiting  any  change  of  temper- 
ature, for  a  long  period  of  time. 

From  what  has  been   above   explained,  it  appears  that,   besides  a  tendency 
to  equilibrium  of  temperature,  which  arises  from  the  interchange  of  heat  by 
radiation,  bodies  have  a  like  tendency  to  calorific   equilibrium  by  the  transmis- 
sion of  heat  by  contact.     After  the  lapse  of  a  sufficient  time,  every  two  bodies 
in  contact  distribute  between  them  the  heat  they  contain  in  such  portions  as  to 
I  render  their  temperature  equal.     The  manner  in  which  this  effect  is,  generally, 
1  produced  in  liquids  and  gases  differs,  however,   materially   from  the  nature  of 
'  the  process  in  solids.     The  constituent  particles  of  solid  bodies  being  incapa- 
1  ble  of  changing  their  material  position  and  arrangement,  the  heat  can  only  pass 
[  through  them,  from  particle  to  particle,  by  a  slow  process  ;  but  when  the  parti- 
)  cles  forming  any  stratum  of  a  liquid  are  heated,  their  mass,  expanding,  becomes 
'  lighter,  bulk  for  bulk,  than  the  stratum  immediately  above  it,  and  ascends,  al- 
)  lowing  the  superior  strata  to  descend.     Thus  a  source  of  heat  applied  to  the 
'>  bottom  of  a  vessel  containing  a  liquid,  immediately  causes   the   liquid  near  the 
i  bottom  to  form  an  upward  current,  while  the  superior  liquid  forms  a  downward 


CONDUCTION  OF  HEAT. 


one  ;  and  a  constant  series  of  currents  upward  and  downward  is  thus  estab  ( 
lished.  The  portion  of  the  liquid  which  receives  heat  below,  is  thus  continu  ' 
ally  mixed  through  the  other  parts,  and  the  heat  is  diffused  by  the  motion  of  , 
the  particles  among  each  other  ;  the  same  effect  takes  place  in  gases.  If  a  ' 
lower  stratum  be  heated,  it  acquires  a  tendency  to  ascend  to  the  higher,  and  the  ! 
colder  strata  descend. 

If,  however,  heat  be  applied  to  the  highest  stratum  of  the  liquid,  this  effect 
caniiot  ensue  ;  and  it  is  found  that,  in  this  case,  the  particles  maintaining  their 
mutual  arrangement,  the  transmission  of  heat  takes  place  in  the  same  manner 
as  if  the  liquid  were  solid.  In  fact,  the  heat  is,  in  this  case,  conducted  through 
the  liquid.  Liquids,  in  this  manner,  are  observed  to  have  extremely  low  con- 
ducting powers  ;  so  low  that,  for  a  long  period,  they  were  supposed  to  be  alto- 
gether incapable  of  conducting  heat.  They  have  been  ascertained  by  experi- 
ment, however,  not  to  be  altogether  destitute  of  the  power  of  conduction. 

Let  a  small  quantity  of  spirits  of  wine  be  poured  on  the  surface  of  water, 
at  the  temperature  of  32°,  and  let  a  thermometer  be  immersed  in  the  water  at 
a  small  depth  below  the  common  surface  of  the  water  and  spirits  ;  let  the  spir- 
its be  now  inflamed  and  caused  to  burn  on  the  surface  of  the  water.  After  the 
lapse  of  a  considerable  time  the  thermometer  will  show  a  very  slight  indication 
of  increased  temperature,  by  the  downward  transmission  of  heat  from  the  burn- 
ing spirits. 

This,  and  other  experiments  of  a  like  nature,  are  extremely  difficult  of  man- 
agement, and  very  uncertain  in  their  results.  It  often  happens  that  the  eleva- 
tion of  the  thermometer  is  caused  by  currents  of  the  liquid  produced  by  heat 
conducted  downward  by  the  sides  of  the  vessels  containing  the  liquid.  Al- 
though the  liquid  itself  may  fail  to  conduct  the  heat  downward,  yet  the  vessel 
containing  it,  having  a  better  conducting  power,  will  transmit  the  heat  to  in- 
ferior strata  of  the  liquid,  and  currents  may  thus,  to  a  certain  extent,  be  estab- 
lished. An  ingenious  method  of  evading  this  difficulty  was  suggested  by  Mr. 
Murray,  who  conducted  the  experiment  in  vessels  composed  of  ice.  The  heat 
received  by  the  sides  of  the  vessel  was,  in  this  case,  expended  in  the  liquefac- 
tion of  the  ice,  and  had  no  tendency,  therefore,  to  disturb  the  result  of  the  in- 
vestigation. 

The  process  of  cooling,  which  a  hot  body  undergoes  when  suspended  in  air, 
is  chiefly  owing  to  the  radiation  of  heat  from  its  surface  :  but  another  cause 
of  the  diminution  of  heat  conspires  with  this.  The  particles  of  air  in  contact 
with  the  surface  of  the  body,  receive  heat  from  it,  and  thus  becoming  specifi- 
cally lighter  by  their  dilatation,  ascend,  and  give  place  to  others,  on  which  a 
like  effect  is  produced.  Thus  heat  is  imparted,  constantly,  to  fresh  portions 
of  the  air,  and  carried  off  by  them.  If  a  hot  body  be  suspended  in  a  liquid, 
the  process,  as  to  its  cooling,  is  altogether  produced  by  this  means,  for  in  that 
case  no  radiation  takes  place. 

The  covering  of  wool  and  feathers,  which  nature  has  provided  for  the  infe- 
rior classes  of  animals,  has  a  property  of  conducting  heat  very  imperfectly, 
and  hence,  it  has  the  effect  of  keeping  the  body  cool  in  hot  weather,  and  warm 
in  cold  weather.  The  heat  which  is  produced  by  powers  provided  in  the  ani- 
mal economy,  within  the  body,  has  a  tendency,  when  in  a  cold  atmosphere,  to 
escape  faster  than  it  is  generated,  the  covering  being  a  non-conductor,  intercepts 
it,  and  keeps  it  confined. 

Man  is  endowed  with  faculties  which  enable  him  to  fabricate,  for  himself, 
covering  similar  to  that  with  which  nature  has  provided  other  animals.  Clothes 
are  generally  composed  of  some  light,  non-conducting  substances,  which  pro- 
tect the  body  from  the  inclement  heat  or  cold  of  the  external  air.  In  summer, 
clothing  keeps  the  body  cool,  and  in   winter,   warm.     Woollen  substances  are 


CONDUCTION  OP  HEAT. 


183 


worse  conductors  than  those   composed  of  cotton  or  linen.     A  flannel  shirt  ' 
more   effectually  intercepts  heat  than   a  cotton  or  a  linen  one  ;  and  whether 
in  warm  or  in  cold  climates,  attains  the  end  of  clothing  more  effectually. 

If  we  would  preserve  ice  from  melting,  the  most  effectual  means  would  be 
to  wrap  it  in  blankets,  which  would  retard,  for  a  long  time,  the  approach  of 
heat  to  it  from  any  external  source. 

Glass  and  porcelain  are  slow  conductors  of  heat,  and  hence  maybe  explain- 
ed the  fact,  that  vessels  formed  of  this  material  are,  frequently,  broken  by  sud- 
denly introducing  boiling  water  into  them.  If  a  small  quantity  of  boiling  water 
be  poured  into  a  thick  glass  tumbler,  the  bottom,  with  which  the  water  first 
comes  into  contact,  is  suddenly  heated,  and  it  expands  ;  but  the  heat,  passing 
very  slowly  through  it,  fails  to  affect  the  upper  part  of  the  vessel,  which,  there- 
fore, undergoes  a  corresponding  expansion  :  the  lower  part  enlarging,  while 
the  upper  part  remains  unaltered,  a  crack  is  produced,  which  detaches  the  bot- 
tom of  the  tumbler  from  the  upper  part  of  it. 

In  the  construction  of  an  ice-house,  the  walls,  roof,  and  floor,  should  be  sur- 
rounded with  some  substance  which  conducts  heat  imperfectly.     A  lining  of 

,  straw-matting,  or  of  wooUen-blinkets,  will  answer  this  purpose.     Air  being  a 

'  bad  conductor  of  heat,  the   building  is,  sometimes,  constructed  with  double 

I  walls,  having  a  space  between  them.  The  ice  is  thus  surrounded  by  a  wall 
of  air,  as  it  were,  which  is,  in  a  great  degree,  impenetrable  by  heat,  provided 
no  source  of  radiation  be  present.  Furnaces,  intended  to  heat  apartments, 
should  be  surrounded  with  non-conducting  substances,  to  prevent  the  waste  of 
heat. 

When  wine-coolers  are  formed  of  a  double  casing,  the  space  between  may 
be  filled  with  some  non-conducting  substance,  such  as  powdered  charcoal,  or 
wool,  or  it  may  be  left  merely  filled  with  air. 

The  practical  application  of  non-conduction  is  illustrated  in  the  construction 
and  management  of  the  boilers  and  steam-pipes  of  steam-machinery. 

In  places  where  fuel  is  expensive  and  consumed  in  great  quantity,  every 
possible  expedient  that  can  conduce  to  its  economy  is  resorted  to.  In  Corn- 
wall, where  very  powerful  engines  are  worked  for  the  drainage  of  the  mines 
and  the  preparation  of  the  ore,  and  to  which  fuel  has  to  be  carried  from  a  con- 
siderable distance,  the  boilers  are  surrounded  by  a  hollow  casing,  stuffed  with 
saw-dust.  This  is  found  to  be  a  nearly  perfect  non-conductor  of  heat.  All  the 
pipes  which  conduct  steam  to  the  cylinders  are  similarly  coated.  The  conse- 
quence of  this  is,  that  the  boiler-houses,  notwithstanding  the  large  furnaces 
continually  burning  in  them,  are  extremely  cool  rooms,  and  in  summer  are 
much  cooler  than  the  external  atmosphere.  The  steam  cylinders  are  also, 
sometimes  cased  in  wood. 

In  the  machinery  used  in  the  British  steamships,  it  has  been  the  practice 
to  invest  the  boiler  with  a  coating  of  patent  felt  and  to  cover  the  great  steam- 
pipe  in  the  same  manner.  This  non-conducting  coating  prevents  the  con- 
stant waste  of  steam  by  the  condensation  produced  by  radiation. 

I       Charcoal  in  powder  is  a  good  non-conductor  of  heat,  and  is  sometimes  used 

i  to  protect  ice  from  fusion. 

I       Fresh  provisions  are  sometimes  exported  to  distant  places  enveloped  in  ice. 

i  In  this  case  it  would  be  advantageous  to  envelope  the  ice  itself  in  a  casing  of 

]  saw-dust. 

I       In    concluding  these  discourses  on  heat,    it   may  be  proper  to  enumerate 

[  the    most   ordinary  sources   of  this  principle.     They  may  be   stated  as  fol- 

I  lows  : — 

\       1 .  Solar  Light. — The  sources  from  which  heat  might,  by  possibility,  be  ra- 

•  dialed  toward  the  earth  from  distant  regions  of  the  universe  are,  1st,  the  sun ; 


184 


CONDUCTION  OF  HEAT. 


the 


lanets,  and  satellites,  and  the  moon;  and  3d,  the  fixed  stars.  But  it 
has  been  shown  that  the  moon  does  not  supply  heat  enough  to  affect  the 
most  delicate  differential  thermometer,  even  when  condensed  by  a  burning- 
glass.  It  follows,  then,  a  fortiori,  that  the  planets  and  satellites  can  produce 
no  sensible  effect.  As  to  the  stars,  it  has  been  proved  that  their  heating  pow- 
er must  be  less  than  that  of  the  sun  in  the  same  proportion  as  their  apparent 
magnitude  is  less,  and  as  no  telescope  has  ever  exhibited  them  with  any 
sensible  magnitude,  however  high  the  power,  we  may  safely  infer  that  their 
heating  power  is  unappreciable. 

2.  Electricity  is  a  source  of  heat  in  whatever  manner  it  may  be  evolved. 

3.  The  condensation  of  gases,  solidification  of  liquids,  and  percussion  or 
compression  of  solids. 

4.  Chemical  combination  and  decomposition. 

5.  The  functions  of  animal  life. 


\ 


RELATIOI  OP  HEAT  AID  LIGHT. 


Probable  Identity  of  Heat  and  Ligbt. — Incandescence. — Probable  Temperature  of. — Gases  cannot 
be  made  Incandescent. — The  Absorption  and  Reflection  of  Heat  depend  on  Color. — Burning 
Glass — Heat  of  Sun's  Rays. — Heat  of  artificial  Light. — Moonlight. — Phosphorescence. 


RELATION  OF  HEAT  AND  LIGHT. 


187 


RELATION  OF  HEAT  AID  LIGHT, 


The  whole  body  of  natural  phenomena  in  which  the  effects  of  heat  and  light 
are  concerned,  demonstrate  an  intimate  physical  connexion  between  these 
agents.  Sunlight  is  warm,  the  light  of  red  coals  is  warm,  and  the  more  bril- 
liant light  of  flame  excites  still  more  intense  heat.  If  every  degree  of  light 
were  productive  of  heat,  and,  reciprocally,  every  degree  of  heat  productive  of 
light,  we  should  not  hesitate  to  infer  that  heat  and  light  are  two  distinct  effects 
of  the  same  physical  principle ;  and  such  an  inference  would  be  corroborated 
if  it  appeared  that  the  energy  of  the  luminous  and  calorific  effects  were  pro- 
portionate to  each  other,  the  most  brilliant  light  always  producing  the  most  in- 
tense heat,  and  the  most  fierce  temperature  always  accompanied  by  the  strong- 
est illuminating  power. 

Some  of  the  more  obvious  phenomena  countenance  these  views.  All  the 
ordinary  sources  of  light,  are  also  sources  of  heat ;  and  by  whatever  artificial 
means  natural  light  is  condensed,  so  as  to  increase  its  splendor,  the  heat  which 
it  produces  is  at  the  same  time  rendered  more  intense.  The  direct  rays  of  the 
sun,  playing  on  the  bulb  of  a  thermometer,  will  elevate  its  temperature  to  a  cer- 
tain extent ;  but  if  a  certain  number  of  these  rays  be  concentrated  on  the  same 
bulb  by  a  concave  reflector,  or  burning  lens,  then  the  elevation  of  temperature 
will  be  much  more  sudden  and  extensive.  These,  however,  are  only  the  first 
and  more  prominent  effects  which  obtrude  themselves  on  our  observation.  It 
requires  little  attention  to  the  phenomena  of  nature,  much  less  to  those  which 
are  exhibited  by  the  processes  of  science  and  art,  to  discover  that  the  heat 
which  accompanies  light  is  not  always  proportionate  to  the  splendor  of  the 
light  ;  and  further,  that  heat  of  considerable  intensity,  both  as  regards  the  ther- 
mometric  effects,  and  the  sensation  it  produces,  may  be  either  absolutely  ac- 
companied by  light,  or,  at  least,  if  it  have  light,  the  intensity  of  that  light  is  so 
small  as  to  be  below  the  limit  of  the  sensibility  of  the  eve. 

The  fact  of  the  existence  of  heat  unaccompanied  by  any  sensible  degree  of 


RELATION  OF  HEAT  AND  LIGHT. 


light,  and  of  light  unaccompanied  by  any  sensible  degree  of  heat,  on  the  one 
hand,  and  of  an  extensive  and  complicated  group  of  properties,  in  which  light 
and  heat  agree  in  their  physical  characters,  on  the  other,  have  given  rise  to  two 
distinct  hypotheses  respecting  the  nature  of  these  principles.  By  the  one  they 
are  regarded  as  distinct  physical  agents,  which  enjoy  some  common  properties, 
while  on  the  other  they  are  assumed  to  be  the  same  principle  manifesting  itself 
in  difTerent  ways,  according  to  the  property  which,  under  different  circumstan- 
ces, acts  with  the  greatest  degree  of  energy.  Our  object  at  present  shall 
be  confined  to  the  statement  of  the  principal  effects  upon  which  one  or  the 
other  theory  must  be  founded,  and  which  any  theory  must  explain  before  its 
validity  can  be  admitted. 

If  heat  be  communicated  to  solid  bodies  which  are  difficult  of  fusion,  it  is 
observed  that  after  having  absorbed  a  certain  quantity,  they  begin  to  become 
luminous.  If  the  process  be  conducted  in  a  dark  chamber,  the  body  will 
gradually  begin  to  be  visible  by  emitting  a  dull  red  light.  This  luminous 
quality  gradually  increases  as  the  body  absorbs  heat,  and  at  length  it  emits 
sufficient  light  to  render  the  surrounding  objects  visible  ;  and  the  color  of  the 
light  changes  from  an  obscure,  dusky  red,  gradually  to  the  color  of  bright  red. 
The  body  is  then  said,  in  common  language,  to  be  red-hot.  If  the  communica- 
tion of  heat  be  still  continued,  the  color  of  the  light  will  change  to  an  orange, 
and  subsequently  will  become  yellow.  If  the  application  of  heat  be  still  fur- 
ther continued,  it  will  at  length  emit  a  clear  white  light,  the  color  of  sunlight; 
the  body  is  then  said  to  be  white-hot. 

The  state  in  which  a  heated  body  naturally  incapable  of  emitting  light  be- 
comes luminous,  is  called  a  state  of  incandescence.  The  term  ignition  is  some- 
times applied  to  this  state  ;  but  the  former  term  is  preferable,  since  ignition  is 
sometimes  used  to  express  the  commencement  of  inflammation  or  combustion, 
which  is  a  process  of  a  totally  different  nature. 

The  temperature  at  which  a  body  becomes  incandescent  is  extremely  diffi- 
cult to  be  ascertained  with  exactness,  being  beyond  the  reach  of  the  mercurial 
thermometer.  The  uncertainty  of  the  indications  of  pyrometers,  and  other 
means  by  which  fierce  temperatures  are  measured,  has  been  before  noticed. 
There  are,  however,  some  circumstances  which  render  it  probable  that  bodies 
in  general,  which  have  been  rendered  incandescent  by  increase  of  temperature, 
have  attained  that  state  at  nearly  the  same  temperature.  Mr.  Wedgwood  placed 
some  gilding  on  a  piece  of  porcelain,  and  exposed  both  to  the  heat  of  an  in- 
tense furnace,  until  the  porcelain  became  red-hot:  no  difference  could  be  ob- 
served in  the  time  of  the  porcelain  and  the  gilding  upon  it  becoming  luminous, 
yet  these  substances  are  of  so  very  different  a  nature  that  it;  might  be  expected 
that  a  difference  in  their  incandescence  would  be  observable. 

The  point  of  fusion  seems  to  have  no  relation  whatever  to  the  point  of  in- 
candescence. While  yet  solid,  some  bodies  attain  a  cleg-r  white  heat  without 
fusion.  Others  again,  such  as  silver  and  lead,  fuse  before  they  become  lumin- 
ous. If  the  boiling  point  of  a  body  be  below  its  point  of  incandescence,  it  can- 
not attain  the  latter  state  unless  its  vaporization  be  resisted  by  pressure.  It  is 
supposed  that  liquids  submitted  to  a  pressure  which  will  resist  their  vaporiza- 
tion, are  capable  of  attaining  a  state  of  incandescence.  Thus,  in  some  experi- 
ments of  Perkins,  water  is  said  to  have  been  rendered  red  hot  without  being 
permitted  to  expand  into  vapor. 

The  determination  of  the  temperature  at  which  bodies  become  incandescent 
has  occupied  the  attention  of  several  distinguished  philosophers.  Newton 
fixed  it  at  the  temperature  of  635°  ;  but  there  is  no  doubt  that  this  is  consider- 
ably below  the  true  temperature.  Newton  possessed  very  imperfect  means 
of  determining  the  temperature,  and  measured  it  by  observing  the  rate  at  which 


RELATIOSf  OF  HEAT  AND  LIGHT. 


189 


red-hot  iron  cooled,  calculating  the  heat  lost  by  the  time  of  cooling.  Mercury 
boils  at  the  temperature  of  662°  ;  and  yet  it  is  certain  that  it  emits  no  sensible 
light,  since  it  is  perfectly  invisible  in  a  dark  room.  Mr.  Daniel,  from  experi- 
ments made  with  his  pyrometer,  fixed  the  temperature  of  incandescence  at 
980°  ;  but  this,  again,  is  proved  to  be  higher  than  the  true  temperature  of  in- 
candescence, since  antimony,  at  its  fusing  point,  is  visible  in  the  dark,  and  yet 
this  metal  melts  at  810°.  Sir  Humphrey  Davy  fixed  the  temperature  of  in- 
candescence at  812°. 

The  uncertainty  attending  the  temperature  at  virhich  incandescence  commen- 
ces cannot  be  surprising  when  we  consider  that  besides  the  difficulty  of  accu- 
rately measuring  high  temperatures,  there  are  no  other  means  of  determining 
the  fact  of  incipient  incandescence  than  the  evidence  of  the  sight.  Now  there 
are  many  reasons  for  concluding  that  sight  is  a  very  imperfect  measure  of  illu- 
mination. Objects  illuminated  in  different  degrees,  exhibited  to  the  same  in- 
dividual, will  give  him  very  imperfect  notions  of  their  actual  comparative 
brightness.  Let  two  pieces  of  white  paper  be  differently  illuminated  by  com- 
mon candles  :  let  one  be  exposed  to  the  light  of  a  single  candle,  and  the  other 
to  the  light  of  ten  candles,  and  let  them  be  viewed  by  any  number  of  individu- 
als ;  it  will  be  found  that  no  two  will  agree  in  their  estimates  of  the  relative 
degree  of  ilhimination.  If,  then,  the  eye  be  so  imperfect  a  judge  of  the  degree 
of  illumination,  it  is  extremely  probable  that  when  the  illumination  becomes  so 
faint  as  to  be  barely  perceptible,  it  will  begin  to  be  perceived  by  different  per- 
sons when  it  arrives  at  different  degrees  of  intensity.  It  is  extremely  proba- 
ble, if  not  certain,  that  the  same  object  placed  in  a  dark  room  will  be  pro- 
nounced to  be  luminous  by  one  person,  and  not  so  by  another;  and  it  is  abso- 
lutely certain  that  an  object  may  be  luminous  to  the  eyes  of  certain  animals 
when  it  is  perfectly  invisible  to  the  human  eye.  Sight,  therefore,  is  by  no 
means  a  certain  test  of  the  presence  of  light,  and,  consequently,  is  an  extremely 
inadequate  means  of  determining  the  commencement  of  incandescence.  If, 
however,  incandescence  be  defined  to  be  the  commencement  of  that  state  in 
which,  whether  light  be  actually  emitted  or  not,  sufficient  light  is  emitted  sensibly 
to  affect  the  human  eye,  then  the  temperature  of  incipient  incandescence  must 
be  taken  as  the  average  or  mean  of  the  results  given  by  different  observers.  In 
this  sense  we  shall  not,  perhaps,  be  very  wide  of  the  truth  if  it  be  fixed  at  a 
temperature  of  between  700°  and  800°.  To  attempt  to  fix  the  temperature 
more  accurately  would  be  inconsistent  with  the  results  of  experience,  and  the 
imperfect  nature  of  our  means  of  estimating  them. 

Analogy  would  lead  us  to  conclude  that  all  bodies  in  the  solid  and  liquid 
state  are  susceptible  of  incandescence.  Since  analogy,  likewise,  countenan- 
ces the  supposition  that  all  bodies  are  susceptible  of  existing  in  these  states,  it 
is  likewise  probable  that  all  bodies  whatever  are  susceptible  of  incandescence. 
Practically,  however,  the  attainment  of  the  state  of  incandescence  is  rendered 
impossible  in  a  vast  number  of  bodies,  from  various  causes.  In  some  cases, 
long  before  the  requisite  increase  of  temperature  can  be  attained  the  forces 
which  hold  the  constituent  parts  of  bodies  together  are  destroyed  by  the  antag- 
onist forces  introduced  by  the  heat  itself;  so  that  the  body  is  decomposed  or 
resolved  into  its  constituent  parts.  In  other  cases  combustion  takes  place, 
by  which  the  body  to  which  heat  is  communicated,  or  some  parts  of  it,  com- 
bine with  other  elements,  and  form  new  compounds.  These  circumstances 
destroy  the  identity  of  the  body,  and  cause  a  total  change  in  its  nature  and  con- 
stitution, long  before  incandescence  can  be  looked  for. 

It  is  generally  held  that  air  and  the  gases  form  an  exception  to  this  general 
effect.  No  heat  ever  yet  attained  has  rendered  a  body  in  the  gaseous  form  red- 
hot  ;  and  yet  such  bodies  have  been  certainly  raised  to  a  temperature  sufficient 


190 


RELATION  OF  HEAT  AND  LIGHT. 


to  render  solids  luminous.  If,  therefore,  they  be  susceptible  of  incandescence, 
their  point  of  incandescence  must  be  far  above  the  point  of  incandescence  of 
bodies  in  the  solid  or  liquid  form.  Mr.  Wedgwood  constructed  a  spiral  tube  of 
porcelain,  which  was  carried  through  a  crucible  surrounded  with  sand.  To 
one  end  of  it  was  attached  a  pair  of  bellows,  and  the  air  thus  driven  through 
it  was  received  from  the  other  extremity  into  a  globular  vessel,  furnished  with 
a  valve  by  which  air  was  allowed  to  escape,  but  none  to  enter.  In  the  side  of 
this  globular  vessel  was  an  opening,  in  which  was  inserted  a  piece  of  glass, 
through  which  the  interior  could  be  viewed.  The  sand  in  the  crucible  being 
then  rendered  red-hot,  air  was  blown  through  the  earthern  tube,  and  made  to 
pass  into  the  glass  vessel  at  the  other  end  of  the  tube.  When  viewed  through 
the  glass  in  the  side  of  the  vessel,  it  was  observed  not  to  be  luminous ;  but 
a  piece  of  gold  wire  introduced  into  that  part  of  the  vessel  near  the  mouth 
of  the  spiral  tube,  was  immediately  rendered  red-hot  by  the  blast  of  hot  air 
which  issued  from  it.  The  air,  therefore,  had  a  temperature  at  least  equal  to 
the  temperature  of  the  incandescence  of  gold. 

Such  ejiperiments  render  it  manifest  that  gases  are  incapable  of  attaining 
incandescence  at  the  same  temperature  as  that  at  which  solids  become  luminous  ; 
but  it  appears  to  me  that  we  cannot  hence  infer  that  the  matter  of  the  gas  is 
not  susceptible  of  incandescence,  even  at  the  temperature  at  which  other  bodies 
pass  into  that  state  ;  for  if  a  gas  were  liquified,  and  confined  by  pressure  so  as 
to  prevent  it  from  dilating  again  into  the  form  of  gas,  it  is  probable  that  in  that 
state  a  quantity  of  heat  would  render  it  incandescent  which  would  be  altogether 
incapable  of  producing  the  same  effect  on  it  in  the  form  of  gas. 

Established  facts  and  analogy  founded  on  them,  therefore,  lead  to  the  con- 
clusion, that  if  a  sufficient  quantity  of  heat  be  supplied  to  any  body,  that  body 
will  at  length  become  luminous  ;  and,  therefore,  that  light  is  invariably  a  con- 
sequence of  heat,  when  that  heat  attains  a  certain  degree  of  intensity  ;  the 
quantity  of  heat  necessary  for  the  production  of  light  differing  according  to  the 
nature  of  the  body  which  contains  that  heat,  those  having  a  less  specific  heat 
requiring  a  less  supply  of  heat  to  render  them  luminous. 

Let  us  now  inquire  how  far  the  presence  of  heat  is  a  necessary  consequence 
of  the  presence  of  light. 

It  has  been  proved  that  the  least  refrangible  rays  of  solar  light  are  those  which 
possess  the  quality  of  heat  in  the  highest  degree  ;  the  most  refrangible  luminous 
rays,  though  still  indicating  the  presence  of  the  calorific  principle,  exhibit  that 
in  a  very  slight  degree  ;  while  the  invisible  chemical  rays,  still  more  refrangi- 
ble than  these,  produce  no  susceptible  effect  on  the  thermometer.  We  are, 
therefore,  led  to  infer,  that,  in  solar  light,  the  heating  qualities  of  the  rays  in- 
crease as  their  refrangibility  diminishes. 

When  light  falls  on  an  opaque  body,  it  is  either  wholly  or  partially  absorbed. 
If  it  be  generally  absorbed,  that  portion  which  is  not  absorbed  is  reflected,  or 
driven  back  into  the  space  from  which  the  light  came.  Now  it  is  clear  that, 
so  far  as  light  is  the  means  of  communicating  heat  to  any  opaque  body  under 
these  circumstances,  this  heat  must  proceed  altogether  from  the  light  which  is 
absorbed. 

It  has  been  explained,  that  the  solar  light  is  composed  of  lights  of  several 
different  colors.  When  this  light  falls  on  an  opaque  body,  it  happens  that  lights 
of  certain  colors  are  absorbed  by  the  surface  of  the  body,  and  the  remainder  of 
the  solar  light  is  reflected.  On  this  fact  depend  all  the  phenomena  of  the  col- 
ors of  natural  bodies.  When  a  body  appears  to  be  of  a  red  color,  it  reflects 
from  its  surface  that  portion  of  the  sun's  light  which  is  red,  and  it  absorbs  all 
the  other  colors.  Again,  if  a  body  appear  green,  it  absorbs  all  the  sun's  light 
which  strikes  upon  it  except  the  green  light,  and  that  alone  is  reflected,  and  so 


191 


on ;  similar  reasoning  being  applied  to  all  other  shades  of  color.  If  a  body 
appears  perfectly  black,  it  absorbs  all  the  sun's  light,  and  reflects  none  ;  if  it 
be  perfectly  white,  it  reflects  all  the  sun's  light  and  absorbs  none  ;  but  perfect 
colors,  whether  black  or  white,  or  of  whatever  other  tint  they  may  be,  do  not 
exist  in  nature.  No  body  exhibits  an  absolute  black  or  an  absolute  white,  how- 
ever near  these  limits  they  may  approach. 

If  an  opaque  body  of  any  color  be  exposed  to  the  direct  rays  of  the  sun,  it 
will  be  observed  to  rise  in  its  temperature,  or  become  warm.  If  it  be  of  a 
black  color,  it  will  exhibit  a  rapid  and  considerable  increase  of  temperature. 
Next  to  black,  a  body  of  a  blue  color  will  absorb  most  heat ;  next  follow  green, 
yellow,  and  red,  and  white  least  of  all. 

That  black  should  absord  most  heat,  and  white  least,  follows  immediately 
from  the  fact  that  a  body  of  a  black  color  absorbs  nearly  all  the  solar  rays,  and 
with  them  their  heat ;  while  a  body  of  a  white  color  reflects  nearly  all  the  rays, 
and  with  them  reflects  their  heat.  Of  all  the  constituent  parts  of  solar  light, 
that  which  possesses  the  least  heating  power  is  the  blue  light.  A  body,  therefore, 
which  reflects  this  only,  must  absorb  all  the  most  powerful  heating  rays ;  and  hence 
we  see  why  an  opaque  object  of  a  blue  color  receives  the  most  heat,  next  to  black. 
The  green  light  has  a  certain  heating  power,  less  than  the  red  or  yellow,  but 
more  than  the  blue.  A  body,  therefore,  which  reflects  the  green  light,  absorb- 
ing the  others,  reflects  more  heat  than  a  blue  or  black  object,  but  less  than 
objects  of  those  colors  which  occupy  the  lower  part  of  the  prismatic  spectrum. 
Such  a  body,  therefore,  receives  less  heat  from  the  solar  light  than  those  of  a 
darker  shade,  and  more  than  those  of  a  lighter.  The  application  of  the  same 
reasoning  will  explain  why  bodies  of  a  yellow  or  red  color  absorb  still  less 
heat. 

If  several  pieces  of  cloth,  of  the  same  size  and  quality,  but  of  different  col- 
ors— black,  blue,  green,  yellow,  and  white — be  thrown  on  the  surface  of  snow 
in  clear  daylight,  but  especially  in  sunshine,  it  will  be  found  that  the  black 
cloth  will  quickly  melt  the  snow  beneath  it  and  sink  downward.  The  blue 
will  do  the  same,  but  less  rapidly  ;  the  green  still  less  so ;  the  yellow  slightly, 
and  the  white  not  at  all.  These  effects  illustrate  the  principle  just  explained. 
We  see,  also,  that  the  warmth  or  coolness  of  clothing  depends  as  well  on  its 
color  as  its  quality.  A  white  dress,  or  one  of  a  light  color,  will  always  be 
cooler  than  one  of  the  same  quality  of  a  dark  color,  and  especially  so  in  clear 
weather,  when  there  is  much  sunshine.  A  white,  or  light  color,  reflects  heat 
copiously,  and  absorbs  little ;  while  a  black  and  dark  color  absorbs  copiously 
and  reflects  little.  From  this  we  see  that  experience  has  supplied  the  place  of 
science  in  directing  the  choice  of  clothing.  The  use  of  light  colors  always 
prevails  in  summer,  and  that  of  dark  colors  in  winter. 

Of  transparent  objects,  some,  such  as  air  and  the  gases,  are  almost  perfectly 
so,  transmitting  nearly  all  the  light  to  which  they  are  exposed.  Such  bodies 
are,  consequently,  invisible  ;  since  the  light  which  passes  through  them,  and 
which  alone  can  affect  the  sight,  suffers  no  effect  different  from  that  which  it 
would  undergo  if  they  were  not  present,  and  if  the  space  through  which  it 
passed  were  an  absolute  vacuum.  Such  bodies,  since  they  arrest  no  portion  of 
the  light  in  its  progress,  receive  no  heat  from  it.  The  same  is  true  of  some 
liquids,  as  pure  water  ;  and  of  some  solids,  though  in  a  less  degree,  as  plate 
glass.  The  rays  of  solar  light,  passing  through  a  pane  of  plate  glass,  produce 
little  effect  on  its  temperature  ;  but  some  little  effect  is  produced,  since  no  glass, 
however  pure,  is  perfectly  transparent ;  but  even  were  it  admitted  that  glass 
and  other  transparent  bodies  were  absolutely  transparent  to  all  the  luminous 
rays  of  solar  light,  it  might  happen  that  they  would  absorb  those  invisible  cal- 
orific rays  which  are  proved  to  exist  in  it,  and  to  be  less  refrangible  than  any 


192 


RELATION  OF  HEAT  AND  LIGHT. 


luminous  rays.     However,  in  general,  so  far  as  the  transmission  of  sunlight  is  ( 
concerned,  bodies  which  are  absolutely  transparent,  or  nearly  so,  are  found  to  * 
arrest  an  extremely  small  portion  of  the  calorific   principle  of  the  sun's  light.  ! 
This  effect,  therefore,  is  generally  consistent  with  the  supposition  that  the  cal-  ' 
orific  principle  is  a  quality  of  the  solar  rays.     But  numerous  bodies  are  imper-  ! 
fectly  transparent,  or  transparent  only  to  lights  of  a  particular  color  ;  and  in 
this  respect  transparent  objects  bear  an  analogy  to  opaque  ones.     The  color  of 
a  transparent  object  when  we  look  through  it  depends  on  the  color  of  the  light 
which  it  transmits.     Thus  stained  glass  exhibits  various  colors  according  to  its 
quality  vv^hen  viewed  from  the  interior  of  a  window  in  which  it  is  set.     A  piece 
of  blue  glass  admits  a  blue  light  to  pass  through  it,  but  intercepts  other  colors. 
Red  glass,  in  like  manner,  allows  a  red  light  to  penetrate  it,  but  stops  the  pas- 
sage of  lights  of  other  colors.     The  lights  which  are  intercepted  by  partially 
transparent  objects,  are  partly  absorbed  by  them  and  partly  reflected.     The  por- 
tion which  is  reflected  is  of  that  color  which  the  object  appears  when  viewed, 
no  source  of  light  being  behind  it ;  and  the  remainder  is  absorbed.     Let  us 
suppose  that  the  light  which  penetrates  a  piece  of  stained  glass  were  mixed 
with  the  light  which  is  reflected,  the  mixture  would  not  give  the  complete  solar 
light  which  strikes  upon  it ;  the  part  which  it  absorbs  would  still  be  wanting ; 
if  that  were   added,  the   mixture   of  the   three  would  form  white  solar  light. 
Hence  we  see  the  reason  why  a  window  of  stained  glass  exhibits  one  set  of 
colors  when  viewed  from  the  interior,  and  a  different  set  of  colors  when  viewed 
from  the  exterior.     When  viewed  from  the  interior  the  color  which  it  transmits 
is  seen  ;  when  viewed  from  the  exterior,  only  the  color  which  it  reflects  is  ob- 
served. 

To  determine  the  effects  of  the  sun's  light  in  heating  a  transparent  object,  it 
is  necessary  first  to  ascertain  the  color  of  the  light  transmitted  through  it,  and 
next  the  color  of  the  light  reflected  by  it.  These  two  colors  being  subtracted 
from  the  combination  of  color  exhibited  in  the  prismatic  spectrum,  the  remain- 
der will  be  the  color  of  the  light  absorbed. 

A  partially  transparent  object,  therefore,  will  always  absorb  most  heat  when 
the  colors  which  its  transmits  and  reflects  are  those  which  occupy  the  upper 
portion  of  the  prismatic  spectrum  ;  for,  in  that  case,  the  lights  which  it  absorbs 
are  those  which  occupy  the  lower  portion  of  the  spectrum,  and  are  the  most 
powerful  in  their  calorific  effects. 

Hence  we  see  the  reason  why  the  colored  glasses  used  by  Sir  William  Her- 
schel  to  mitigate  the  sun's  light  in  his  telescopes,  were  so  frequently  cracked  by 
the  heat  they  absorbed.  The  splendor  of  the  light  in  a  large  telescope,  ren- 
dered it  necessary  to  use  glasses  of  a  very  dark  color,  and  consequently  such 
as  absorbed  the  most  calorific  colors. 

The  calorific  power  of  the  sun's  rays  may  be  exhibited  in  a  very  conspicu- 
ous manner,  by  concentrating  a  large  number  of  them  into  a  small  space,  by 
means  of  a  burning-glass.  Such  an  instrument  is  usually  formed  either  of  a 
large  concave  reflector,  by  which  the  rays,  falling  on  an  extensive  surface, 
are  reflected  in  lines  which  all  tend  toward  one  point,  or  by  a  large  con- 
vex lens  of  glass,  which,  when  the  rays  pass  through  it,  bend  them,  or  refract 
them,  in  directions  converging  all  to  the  same  pomt.  In  either  case,  the  effect 
of  the  rays  is  increased  in  the  proportion  which  the  magnitude  of  the  point 
into  which  they  are  collected  bears  to  the  magnitude  of  the  reflector  or  the 
lens.  From  experiments  performed  in  this  way  by  Count  Rumford,  it  appears, 
however,  that  no  change  in  the  heating  power  of  individual  rays  is  produced 
by  this  means,  and  that  the  increased  energy  of  their  calorific  action  arises 
altogether  from  a  great  number  of  them  being  concentrated  in  a  small  space. 

The  heating  power  of  the  sun's  rays,  when  collected  by  a  burning-glass,  far 


RELATION  OP  HEAT  AND  LIGHT. 


193 


exceeds  the  heat  of  a  powerful  furnace.  A  piece  of  gold  placed  in  the  focus 
of  such  a  glass,  has  not  only  been  melted,  but  has  actually  been  converted  into 
vapor,  by  Lavoisier.  This  fact  was  proved  by  a  piece  of  silver  placed  at  some 
height  above  tlje  gold,  having  been  gilded  by  the  condensation  of  the  vapor  of 
the  gold  on  its  surface. 

Artificial  lights  are  generally  accompanied  by  heat  in  various  degrees,  and, 
generally,  the  more  intensely  brilliant  the  light,  the  more  powerful  will  be  the 
calorific  effects.  It  would  appear,  however,  from  some  remarkable  differences 
which  are  observed  in  the  transmission  of  artificial  hght  through  transparent 
bodies,  that  the  invisible  calorific  rays  exist  in  such  light  in  a  much  greater 
proportion  than  in  solar  light.  If  a  screen  of  plate-glass  be  placed  before  a 
coal  fire,  although  scarcely  any  light  will  be  intercepted,  nearly  all  the  heat 
will  be  immediately  stopped.  This  has  been  generally  adduced  as  a  proof 
that  light  and  heat  are  distinct  principles,  since  the  glass,  in  this  case,  is  said 
to  separate  them.  The  effect,  however,  admits  of  explanation  with  equal  fa- 
cility, on  the  supposition  that  heat  is  a  quality  of  light,  and  that  the  luminous 
property  may  have  so  weak  a  force  in  some  rays,  as  to  be  incapable  of  affect- 
ing the  sight.  The  light  from  the  fire,  in  the  case  just  mentioned,  is  generally 
of  a  red  color,  like  that  of  the  rays  at  the  lowest  point  of  the  luminous  spec- 
trum;  it  is  probable,  therefore,  that  it  may  contain  also  the  more  calorific  in- 
visible rays  which  are,  in  that  neighborhood,  in  the  spectrum.  If  this  be  ad- 
mitted, the  light  emitted  by  a  fire  will  consist  of  a  much  larger  proportion  of 
the  invisible  calorific  rays  than  is  found  in  sunlight.  The  proportion,  there- 
fore, which  the  visible  rays  transmitted  by  the  glass  bears  to  the  invisible  rays 
which  may  not  be  transmuted,  will  be  much  less  than  in  sunlight,  and  conse- 
quently the  rays  transmitted  by  the  glass  will  possess  comparatively  a  much 
less  heating  power. 

One  of  the  most  remarkable  exceptions  to  the  general  fact,  that  the  presence 
of  light  necessarily  infers  the  presence  of  heat,  is  the  fact,  that  moonlight,  in 
whatever  degree  it  can  be  concentrated  by  the  most  powerful  burning-glasses, 
has  never  yet  been  found  to  affect  the  most  sensible  thermometer.  De-la-Hire 
collected  the  rays  of  the  full  moon,  when  on  the  meridian,  by  a  burning-glass 
of  about  three  feet  in  diameter,  in  the  focus  of  which  he  placed  a  delicate  air 
thermometer.  The  density  of  the  lunar  rays  was  in  this  case  increased  in  the 
proportion  of  about  300  to  1,  and  yet  not  the  slightest  effect  was  produced. 
This  anomaly  is,  however,  easily  accounted  for.  Admitting  that  the  moon  ab- 
sorbs no  part  of  the  invisible  calorific  rays  of  the  solar  light,  it  will  follow  that 
the  heating  power  of  moonlight  cannot  be  in  a  greater  proportion  to  that  of 
sunlight  than  the  relative  brilliancy  of  the  two  lights.  Now,  to  determine  the 
comparative  splendor  of  moonlight  and  sunlight,  let  the  moon,  when  seen  in  the 
firmament  during  the  day,  be  compared  with  a  white  cloud  near  it ;  its  bright- 
ness, and  that  of  the  cloud,  will  appear  very  nearly  the  same.  Assuming  that 
they  are  exactly  the  same,  it  will  follow  that  in  the  day,  when  the  whole  fir- 
mament is  covered  with  white  fleecy  clouds,  the  brilliancy  of  the  light  would 
be  the  same  as  if  the  whole  firmament  were  covered  with  an  illuminated  sur- 
face similar  to  that  of  the  moon.  The  light,  therefore,  of  a  cloudy  day  of  this 
kind,  will  be  as  much  more  brilliant  than  the  light  of  the  moon,  as  the  magni- 
tude of  the  whole  firmament  is  greater  than  that  portion  of  it  occupied  by  the 
full  moon.  This  proportion  is  nearly  that  of  300,000  to  1  ;  and  hence  the 
light  of  a  cloudy  day  is  300,000  times  brighter  than  moonlight :  consequently, 
the  intensity  of  the  moon's  rays  is  certainly  not  greater  than  3oo\7^o  P^'"^  ^^  the 
intensity  of  sunlight.  In  the  experiment  of  De-la-Hire,  just  explained,  where 
the  moon's  rays  were  concentrated  in  the  proportion  of  300  to  1,  the  effect  of 
the  concentrated  light  in  the  focus  of  a  burning-glass   would  not  amount   to 

VOL..  1 1.— 13 


194 


RELATION  OF  HEAT  AND  LIGHT. 


more  than  the  one  thousandth  part  of  the  effect  of  the  direct  unconcentrated 
light  of  the  sun.  Now  it  was  found  that,  under  favorable  circumstances,  the 
sunlight,  acting  on  the  bulb  of  a  thermometer,  caused  it  to  rise  about  230°  ; 
it  follows,  therefore,  that  the  effect  of  the  concentrated  light  of  the  moon,  in 
the  experiment  just  mentioned,  could  not  exceed  the  fifth  part  of  a  degree  ; 
but  even  this  is  greater  than  its  true  effects,  because  the  light  of  the  moon  has 
been  here  compared  with  the  light  of  a  cloudy  day,  which  is  less  intense  than 
the  direct  rays  of  the  sun.  From  this  and  other  reasons,  it  is  probable  that, 
admitting  the  moon's  rays  to  possess  the  calorific  power,  they  could  not,  in 
the  experiment  of  De-la-Hire,  affect  the  thermometer  to  an  extent  even  of  the 
twentieth  of  a  degree. 

There  are  certain  bodies  which,  at  a  comparatively  low  temperature,  possess 
the  property  of  emitting  light,  presenting  an  appearance  of  a  lambent  flame,  the 
color  being  different  in  different  bodies,  and  apparently  depending  on  the  color 
of  the  body  itself ;  this  process  is  called  phosphorescence.  The  minerals  which 
possess  this  property  in  the  highest  degree,  are  fluorspar  and  phosphate  of 
lime'.  Some  bodies  exhibit  this  effect  at  the  commencement  of  spontaneous 
combustion.  Certain  kinds  of  meat  and  fish,  when  putrefaction  begins,  are 
luminous  in  the  dark.  If  four  drachms  of  the  substance  of  whiting,  herring,  or 
mackerel,  be  put  into  a  phial  containing  two  ounces  of  sea-water,  or  of  pure 
water  holding  in  solution  half  a  drachm  of  common  salt,  the  phial,  when  ex- 
posed in  a  dark  place,  after  the  lapse  of  three  days,  exhibits  a  Inminous  ring 
on  the  surface  of  the  liquid.  The  whole  liquid,  when  agitated,  becomes  lu- 
minous, and  continues  so  for  some  time.  "When  these  liquids  are  frozen,  the 
phosphorescence  disappears,  but  it  reappears  when  they  are  again  thawed.  A 
moderate  increase  of  temperature  causes  an  increase  in  the  luminous  appear- 
ance, but  a  boiling  heat  extinguishes  it.  The  light  thus  produced  has  no  sen- 
sible effect  on  the  thermometer. 


ACTION  AND  REACTION. 


197 


ACTION  AND  REACTION. 


The  effects  of  inertia  or  inactivity  are  such  as  may  be  manifested  by  a  sin- 
gle insulated  body,  without  reference  to,  or  connexion  with,  any  other  body 
whatever  ;  they  might  all  be  recognised,  if  there  were  but  one  body  existing 
in  the  universe.  There  are,  however,  other  important  results  of  this  universal 
property  of  matter,  to  the  development  of  which  two  bodies  at  least  are  neces- 
sary. If  a  mass  of  matter,  moving  in  any  direction,  encounter  another  equal 
mass  which  is  quiescent,  the  two  masses  will  move  together  after  the  impact ; 
but  it  will  be  observed,  that  their  speed  after  the  impact  will  be  only  half  that 
of  the  former  mass.  Thus  the  body  which  was  moving  before  the  impact  loses 
half  its  velocity,  and  that  which  was  quiescent  receives  exactly  the  same  amount 
of  motion  ;  the  one,  therefore,  receives  just  so  much  motion  as  the  other  loses, 
and  therefore  the  actual  quantity  of  motion  after  the  impact  is  the  same  as  be- 
fore it. 

Again,  let  A  and  B  be  two  masses,  B  being  twice  that  of  A.  If,  as  before, 
A  strikes  B  with  a  certain  velocity,  B  being  previously  quiescent,  it  will  be 
found  that  the  velocity  of  the  combined  masses  of  A  and  B  after  the  impact 
will  be  just  one  third  of  the  velocity  of  A  before  it.  Thus,  after  the  impact  A 
loses  two  thirds  of  its  velocity,  and  B  consisting  of  two  masses,  each  equal 
to  A,  each  of  these  receives  one  third  of  A's  motion,  so  that  the  whole 
motion  received  by  B  is  two  thirds  of  the  motion  of  A  before  impact. 
By  the  impact,  therefore,  as  much  motion  exactly  is  received  by  B  as  is  lost 

A  similar  result  will  be  obtained,  whatever  proportion  may  subsist  between 
the  masses  A  and  B.  Suppose  B  to  be  ten  times  A,  then  the  whole  motion  of 
A  must,  after  the  impact,  be  distributed  among  the  parts  of  the  united  masses 
A  and  B  ;  but  these  united  masses  are  in  this  case  eleven  times  the  mass  of  A. 
Now,  as  they  all  move  with  a  common  motion,  it  follows  that  A's  former  mo- 
tion must  be  equally  distributed  among  them,  so  that  each  part  shall  have  an 
eleventh  part  of  it ;  therefore  the  velocity  after  impact  will  be  the  eleventh 


198 


ACTION  AND  REACTION. 


part  of  the  velocity  of  A  before  it.     Thus  A  loses  by  the  impact  ten  eleventh 
parts  of  its  motion,  vphich  are  precisely  what  B  receives. 

Again,  if  the  masses  of  A  and  B  be  5  and  7,  then  the  united  mass  after  im- 
pact will  be  12.  The  motion  of  A  before  impact  will  be  equally  distributed 
between  these  12  parts,  so  that  each  part  will  have  a  12th  of  it ;  but  5  of  these 
parts  belong  to  the  mass  A,  and  7  to  B.  Hence,  B  will  receive  -^r^,  while  A 
retains  -f-^. 

In  general,  therefore,  when,  a  mass,  A,  in  motion  impinges  on  a  mass,  B,  at 
rest,  to  find  the  motion  of  the  united  mass  after  impact,  divide  the  whole  motion 
of  A  into  as  many  equal  parts  as  there  are  equal  component  masses  in  A  and 
B  together,  and  then  B  will  receive  by  the  impact  as  many  parts  of  this  motion 
as  it  has  equal  component  masses. 

This  is  an  immediate  consequence  of  the  property  of  inertia.  If  we  were 
to  suppose  that,  by  their  mutual  impact,  A  were  to  give  to  B  either  more  or 
less  motion  than  that  which  it,  A,  loses,  it  would  necessarily  follow  that  either 
A  or  B  must  have  a  power  of  producing  or  of  resisting  motion,  which  would  be 
inconsistent  with  the  quality  of  inertia  already  defined.  For,  if  A  give  to  B 
more  motion  than  it  loses,  all  the  overplus  or  excess  must  be  excited  in  B  by 
the  action  of  A  ;  and  therefore  A  is  not  inactive,  but  is  capable  of  exciting  mo- 
tion which  it  does  not  possess.  On  the  other  hand,  B  cannot  receive  from  A 
less  motion  than  A  loses,  because  then  B  must  be  admitted  to  have  the  power  \ 
by  its  resistance  of  destroying  all  the  deficiency ;  a  power  essentially  active,  \ 
and  inconsistent  with  the  quality  of  inertia.  ) 

If  we  contemplate  the  efl^ects  of  impact,  which  we  have  now  described,  as 
facts  ascertained  by  experiment  (which  they  may  be),  we  may  take  them  as 
.  further  verification  of  the  universality  of  the  quality  of  inertia.  But,  on  the 
other  hand,  we  may  view  them  as  phenomena  which  may  certainly  be  pre- 
dicted from  the  previous  knowledge  of  that  quality ;  and  this  is  one  of  many 
instances  of  the  advantage  which  science  possesses  over  knowledge  merely 
practical.  Having  obtained  by  observation  or  experience  a  certain  number  of 
simple  facts,  and  thence  deduced  the  general  qualities  of  bodies,  we  are  ena- 
bled, by  demonstrative  reasoning,  to  discover  ulher  facts  which  have  never 
fallen  under  our  observation,  or,  if  so,  may  have  never  excited  attention.  In 
this  way  philosophers  have  discovered  certain  small  motions  and  slight  chan- 
ges which  have  taken  place  among  the  heavenly  bodies,  and  have  directed 
the  attention  of  astronomical  observers  to  them,  instructing  them  with  the 
greatest  precision  as  to  the  exact  moment  of  time,  and  the  point  of  the  firma- 
ment to  which  they  should  direct  the  telescope,  in  order  to  witness  the  pre- 
dicted event. 

Since,  by  the  quality  of  inertia,  a  body  can  neither  generate  nor  destroy  mo- 
tion, it  follows  that  when  two  bodies  act  upon  each  other,  in  any  way  what- 
ever, the  total  quantity  of  motion  in  a  given  direction,  after  the  action  takes 
place,  must  be  the  same  as  before  it,  for  otherwise  some  motion  would  be  pro- 
duced by  the  action  of  the  bodies,  which  would  contradict  the  principle  that 
they  are  inert.  The  word  "  action"  is  here  applied,  perhaps  improperly,  but 
according  to  the  usage  of  mechanical  writers,  to  express  a  certain  phenomenon 
or  effect.  It  is,  therefore,  not  to  be  understood  as  implying  any  active  princi- 
ple in  the  bodies  to  which  it  is  attributed. 

In  the  cases  of  collision  of  which  we  have  spoken,  one  of  the  masses,  B,  was 
supposed  to  be  quiescent  before  the  impact.  We  shall  now  suppose  it  to  be 
moving  in  the  same  direction  as  A,  that  is,  toward  C,  but  with  a  less  velocity, 
so  that  A  shall  overtake  it,  and  impinge  upon  it.  After  the  impact,  the  two 
masses  will  move  toward  C  with  a  common  velocity,  the  amount  of  which  we 
now  propose  to  determine. 


ACTION  AND  REACTION. 


199 


If  the  masses  A  and  B  be  equal,  then  their  motions  or  velocities  added  to- 
gether must  be  the  motion  of  the  united  mass  after  impact,  since  no  motion  can 
either  be  created  or  destroyed  by  that  event.  But  as  A  and  B  move  with  a 
common  motion,  this  sum  must  be  equally  distributed  between  them,  and 
therefore  each  will  move  with  a  velocity  equal  to  half  the  sum  of  their  ve- 
locities before  the  impact.  Thus,  if  A  have  the  velocity  7,  and  B  have  5,  the 
velocity  of  the  united  mass  after  impact  is  6,  being  the  half  of  12,  the  sum  of  7 
and  5. 

If  A  and  B  be  not  equal,  suppose  them  divided  into  equal  component  parts, 
and  let  A  consist  of  8,  and  B  of  6,  equal  masses  :  let  the  velocity  of  A  be  17, 
so  that,  the  motion  of  each  of  the  8  parts  being  17,  the  motion  of  the  whole  will 
be  136.  In  the  same  manner,  let  the  velocity  of  B  be  10,  the  motion  of  each 
part  being  10,  the  whole  motion  of  the  6  parts  will  be  60.  The  sum  of  the 
two  motions,  therefore,  toward  C  is  196  ;  and  since  none  of  this  can  be 
lost  by  the  impact,  nor  any  motion  added  to  it,  this  must  also  be  the  whole 
motion  of  the  united  masses  after  impact.  Being  equally  distributed  among 
the  14  component  parts  of  which  these  united  masses  consist,  each  part 
will  have  a  fourteenth  of  the  whole  motion.  Hence,  196  being  divided 
by  14,  we  obtain  the  quotient  14,  which  is  the  velocity  with  which  the  whole 
moves. 

In  general,  therefore,  when  two  masses,  moving  in  the  same  direction,  im- 
pinge one  upon  the  other,  and,  after  impact,  move  together,  their  common  ve- 
locity may  be  determined  by  the  following  rule :"  Express  the  masses  and 
velocities  by  numbers  in  the  usual  way,  and  multiply  the  numbers  expressing 
the  masses  by  the  numbers  which  express  the  velocities  ;  the  two  products 
thus  obtained  being  added  together,  and  their  sum  divided  by  the  sum  of  the 
numbers  expressing  the  masses,  the  quotient  will  be  the  number  expressing  the 
required  velocity." 

From  the  preceding  details,  it  appears  that  motion  is  not  adequately  estimated 
by  speed  or  velocity.  For  example,  a  certain  mass,  A,  moving  at  a  determinate 
rate,  has  a  certain  quantity  of  motion.  If  another  equal  mass,  B,  be  added  to 
A,  and  a  similar  velocity  be  given  to  it,  as  much  more  motion  will  evidently  be 
called  into  existence.  In  other  words,  the  two  equal  masses  A  and  B  tmited 
have  twice  as  much  motion  as  the  single  mass  A  had  when  moving  alone,  and 
with  the  same  speed.  The  same  reasoning  will  show  that  /Aree  equal  masses 
will,  with  the  same  speed,  have  three  times  the  motion  of  any  one  of  them.  In 
general,  therefore,  the  velocity  being  the  same,  the  quantity  of  motion  will  al- 
ways be  increased  or  diminished  in  the  same  proportion  as  the  mass  moved  is 
increased  or  diminished. 

On  the  other  hand,  the  quantity  of  motion  does  not  depend  on  the  mass  only, 
but  also  on  the  speed.  If  a  certain  determinate  mass  move  with  a  certain 
determinate  speed,  another  equal  mass  which  moves  with  twice  the  speed, 
that  is,  which  moves  over  twice  the  space  in  the  same  time,  will  have 
twice  the  quantity  of  motion.  In  this  manner,  the  mass  being  the  same, 
the  quantity  of  motion  will  increase  or  diminish  in  the  same  proportion  as  the 
velocity. 

The  true  estimate,  then,  of  the  quantity  of  motion  is  found  by  multiplyino- 
together  the  numbers  which  express  the  mass  and  the  velocity.  Thus,  in  the 
example  which  has  been  last  given  of  the  impact  of  masses,  the  quantities  of 
motion  before  and  after  impact  appear  to  be  as  follow : — 


ACTION  AND  REACTION. 


Before  impact. 
Mass  of  A           8 
Velocity  of  A   17 

t^uantity  of  motion  of  A  8  X  H*  or  136 

Mass  of  B  6 

Velocity  of  B    10 

Quantity  of  motion  of  B  6  X  10     or    60 


After  impact. 
Mass  of  A                8 
Common  velocity  14  

Quantity  of  motion  of  A       8  X  14   or 

Mass  of  B  6 

Common  velocity  14  

Quantity  of  motion  of  B       6  X  14    or 


112 


By  this  calculation  it  appears  that  in  the  impact  A  has  lost  a  quantity  of  mo- 
tion expressed  by  24,  and  that  B  has  received  exactly  that  amount.  The  effect, 
therefore,  of  the  impact  is  a  transfer  of  motion  from  A  to  B  ;  but  no  new^  mo- 
tion is  produced  in  the  direction  A  C  which  did  not  exist  before.  This  is  ob- 
viously consistent  v/'iih  the  property  of  inertia,  and,  indeed,  an  inevitable  re- 
sult of  it. 

This  phenomenon  is  an  example  of  a  law^  deduced  from  the  property  of  iner- 
tia, and  generally  expressed  thus  :  "  Action  and  reaction  are  equal,  and  in  con- 
trary directions."  The  student  must,  how^ever,  be  cautious  not  to  receive  these 
terms  in  their  ordinary  acceptation.  After  the  full  explanation  of  inertia,  in  the 
lecture  on  matter  and  its  physical  properties,  it  is,  perhaps,  scarcely  necessary 
here  to  repeat  that,  in  the  phenomena  manifested  by  the  motion  of  two  bodies, 
there  can  be  neither  "  action"  nor  "  reaction,"  properly  so  called.  The  bodies 
are  absolutely  incapable  either  of  action  or  resistance.  The  sense  in  which 
these  words  must  be  received,  as  used  in  the  law,  is  merely  an  expression  of 
the  transfer  of  a  certain  quantity  of  motion  from  one  body  to  another,  which  is 
called  ah  action  in  the  body  which  loses  the  motion,  and  a  reaction  in  the  body 
which  receives  it.  The  accession  of  motion  to  the  latter  is  said  to  proceed 
from  the  action  of  the  former  ;  and  the  Joss  of  the  same  motion  in  the  former  is 
ascribed  to  the  reaction  of  the  latter.  The  whole  phraseology  is,  however,  most 
objectionable  and  unphilosophical,  and  is  calculated  to  create  wrong  notions. 

The  bodies  impinging  were,  in  the  last  case,  supposed  to  move  in  the  same 
direction.  We  shall  now  consider  the  case  in  which  they  move  in  opposite 
directions. 

First,  let  the  masses  A  and  B  be  supposed  to  be  equal,  and  moving  in  oppo- 
site directions  with  the  same  velocity.     Let  C,  fig.  1,  be  the  point  at  which 

Fig.  1. 

A                                    C  B 

Q ^_ M 


they  meet.  The  equal  motions  in  opposite  directions  will,  in  this  case,  destroy 
each  other,  and  both  masses  will  be  reduced  to  a  state  of  rest.  Thus  the  mass 
A  loses  all  its  motion  in  the  direction  A  C,  which  it  may  be  supposed  to  trans- 
fer to  B  at  the  moment  of  impact.  But  B,  having  previously  had  an  equal 
quantity  of  motion  in  the  direction  B  C,  will  now  have  two  equal  motions  im- 
pressed upon  it,  in  directions  immediately  opposite  ;  and,  these  motions  neu- 
tralizing each  other,  the  mass  becomes  quiescent.  In  this  case,  therefore,  as 
in  all  the  former  examples,  each  body  transfers  to  the  other  all  the  (notion  which 
it  loses,  consistently  with  the  principle  of  "  action  and  reaction." 

The  masses  A  and  B  being  still  supposed  equal,  let  them  move  toward  C 
with  different  velocities.  Let  A  move  with  the  velocity  10,  and  B  with  the 
velocity  6.  Of  the  10  parts  of  motion  with  which  A  is  endued,  6  being  trans- 
ferred to  B,  will  destroy  the  equal  velocity  6,  which  B  has  in  the  direction  B 
C.    The  bodies  will  then  move  together  in  the  direction  C  B,  the  four  remain- 


*  The  sign  X  vvben  placed  between  two  numbers  means  that  they  are  to  be  multiplied  together. 


ACTION  AND  REACTION. 


201 


ing  parts  of  A's  motion  being  equally  distributed  between  Ihem.  Each  body- 
will,  therefore,  have  two  parts  of  A's  original  motion,  and  2  therefore  will  be 
their  common  velocity  after  impact.  In  this  case,  A  loses  8  of  the  10  parts  of 
its  motion  in  the  direction  A  C.  On  the  other  hand,  B  loses  the  entire  of  its  6 
parts  of  motion  in  the  direction  B  C,  and  receives  2  parts  in  the  direction  A  C. 
This  is  equivalent  to  receiving  8  parts  of  A's  motion  in  the  direction  A  C.  Thus, 
according  to  the  law  of  "  action  and  reaction,"  B  receives  exactly  what  A  loses. 

Finally,  suppose  that  both  the  masses  and  velocities  of  A  and  B  are  unequal. 
Let  the  mass  of  A  be  8,  and  its  velocity  9  ;  and  let  the  mass  of  B  be  6,  and  its 
velocity  5.  The  quantity  of  motion  of  A  will  be  72,  and  that  of  B,  in  the  oppo- 
site direction,  will  be  30.  Of  the  72  parts  of  motion  which  A  has  in  the  direc- 
tion A  C,  30,  being  transferred  to  B,  will  destroy  all  its  30  parts  of  motion  in 
the  direction  B  C,  and  the  two  masses  will  move  in  the  direction  C  B,  with 
the  remaining  42  parts  of  motion,  which  will  be  equally  distributed  among  their 
14  component  masses.  Each  component  part  will,  therefore,  receive  three 
parts  of  motion  ;  and  accordingly  3  will  be  the  common  velocity  of  the  united 
mass  after  impact. 

When  two  masses,  moving  in  opposite  directions,  impinge  and  move  together, 
their  common  velocity  after  impact  may  be  found  by  the  following  rule  :  "  Mul- 
tiply the  numbers  expressing  the  masses  by  those  which  express  the  velocities 
respectively,  and  subtract  the  lesser  product  from  the  greater ;  divide  the  re- 
mainder by  the  sum  of  the  numbers  expressing  the  masses,  and  the  quotient 
will  be  the  common  velocity ;  the  direction  will  be  that  of  the  mass  which  has 
the  greater  quantity  of  motion." 

It  may  be  shown,  without  difficulty,  that  the  example  which  we  have  just 
given  obeys  the  law  of  "  action  and  reaction." 

After  impact. 


Before  impact 
Mass  of  A  8 

Velocity  of  A     9 


Quantity  motion  in  direction  A  C  8X9  or  72 

Mass  of  B  6 

Velocity  of  B     5 


Quantity  motion  in  direction  B  C   6X5  or  30 


Mass  of  A  8 

Common  velocity     3 
Quantity  motion  in  direction  A  C    8X3  or  24 

Mass  of  B  6 

Common  velocity     3 
Quantity  motion  in  direction  AC    6X3  or  18 


Hence  it  appears  that  the  quantity  of  motion  in  the  direction  A  C,  of  which  A 
has  been  deprived  by  the  impact,  is  48,  the  difference  between  72  and  24.  On 
the  other  hand,  B  loses  by  the  impact  the  quantity  30  in  the  direction  B  C, 
which  is  equivalent  to  receiving  30  in  the  direction  A  C.  But  it  also  acquires 
a  quantity  18  in  the  direction  A  C,  which,  added  to  the  former  30,  gives  a  total 
of  48  received  by  B  in  the  direction  A  C.  Thus  the  same  quantity  of  motion 
which  A  loses  in  the  direction  A  C,  is  received  by  B  in  the  same  direction. 
The  law  of  "  action  and  reaction"  is,  therefore,  fulfilled. 

The  examples  of  the  equality  of  action  and  reaction  in  the  collision  of  bodies 
may  be  exhibited  experimentally  by  a  very  simple  apparatus.  Let  A  and  B, 
fig.  2,,  be  two  balls  of  soft  clay,  or  any  other  substance  which  is  inelastic,  or 
nearly  so,  and  let  these  be  suspended  from  C  by  equal  strings,  so  that  they  may 
be  in  contact ;  and  let  a  graduated  arch,  of  which  the  centre  is  C,  be  placed  so 
that  the  balls  may  oscillate  over  it.  One  of  the  balls  being  moved  from  its 
place  of  rest  along  the  arch,  and  allowed  to  descend  upon  the  other  through  a 
certain  number  of  degrees,  will  strike  the  other  with  a  velocity  corresponding 
to  that  number  of  degrees,  and  both  balls  will  then  move  together  with  a  velo- 
city which  nmy  be  estimated  by  the  number  of  degrees  of  the  arch  through 
which  they  rise. 

In  all  these  cases  in  which  we  have  explained  the  law  of  "  action  and  reac- 


202 


ACTION  AND  REACTION. 


tion,"  the  transfer  of  motion  from  one  body  to  the  other  has  been  made  by  im- 
pact or  collision.  This  phenomenon  has  been  selected  only  because  it  is  the 
most  ordinary  way  in  which  bodies  are  seen  to  affect  each  other.  The  law  is, 
however,  universal,  and  will  be  fulfilled  in  whatever  manner  the  bodies  may 

Fig.  2. 

c    ■         •  • 


AB 


affect  each  other.  Thus  A  may  be  connected  with  B  by  a  flexible  string,  which, 
at  the  commencement  of  A's  motion,  is  slack.  Until  the  string  becomes  stretched, 
that  is,  until  A's  distance  from  B  becomes  equal  to  the  length  of  the  string,  A 
will  continue  to  have  all  the  motion  first  impressed  upon  it.  But  when  the 
string  is  stretched,  a  part  of  that  motion  is  transferred  to  B,  which  is  then  drawn 
after  A  ;  and  whatever  motion  B  in  this  way  receives,  A  must  lose.  All 
that  has  been  observed  of  the  effect  of  motion  transferred  by  impact  will  be 
equally  applicable  in  this  case. 

Again,  if  B,  fig.  3,  be  a  magnet,  moving  in  the  direction  B  C  with  a  certain 
quantity  of  motion,  and,  while  it  is  so  moving,  a  mass  of  iron  be  placed  at  rest 


A 

ft.- 


Fig.  3. 
B 


at  A,  the  attraction  of  the  magnet  will  draw  the  iron  after  it  toward  C,  and  will 
thus  communicate  to  the  iron  a  certain  quantity  of  motion  in  the  direction  of  C. 
All  the  motion  thus  communicated  to  the  iron  A  must  be  lost  by  the  magnet  B. 

If  the  magnet  and  the  iron  were  both  placed  quiescent  at  B  and  A,  the  at- 
traction of  the  magnet  would  cause  the  iron  to  move  from  A  toward  B  ;  but  the 
magnet,  in  this  case,  not  having  any  motion,  cannot  be  literally  said  to  transfer 
a  motion  to  the  iron.  At  the  moment,  however,  when  the  iron  begins  to  move 
from  A  toward  B,  the  magnet  will  be  observed  to  begin  also  to  move  from  B 
toward  A;  and  if  the  velocities  of  the  two  bodies  be  expressed  by  numbers, 
and  respectively  multiplied  by  the  numbers  expressing  their  masses,  the  quan- 
tities of  motion  thus  obtained  will  be  found  to  be  exactly  equal.  We  have  al- 
ready explained  why  a  quantity  of  motion  received  in  the  direction  B  A  is 
equivalent  to  the  same  quantity  lost  in  the  direction  A  B.  Hence  it  appears 
that  the  magnet,  in  receiving  as  much  motion  in  the  direction  B  A  as  it  gives 
in  the  direction  A  B,  suffers  an  effect  which  is  equivalent  to  losing  as  much 
motion  directed  toward  C  as  it  has  communicated  to  the  iron  in  the  same  direction. 

In  the  same  manner,  if  the  body  B  had  any  property  in  virtue  of  which  it 
might  repel  A,  it  would  itself  be  repelled  with  the  same  quantity  of  motion.    In 


ACTION  AND  EEACTION. 


203 


a  word,  whatever  be  the  manner  in  which  the  bodies  may  affect  each  other, 
whether  by  collision,  traction,  attraction,  or  repulsion,  or  by  whatever  other 
name  the  phenomenon  may  be  designated,  still  it  is  an  inevitable  consequence, 
that  any  motion,  in  a  given  direction,  which  one  of  the  bodies  may  receive, 
must  be  accompanied  by  a  loss  of  motion  in  the  same  direction,  and  to  the  same 
amount,  by  the  other  body,  or  the  acquisition  of  as  much  motion  in  the  contrary 
direction  ;  or,  finally,  by  a  loss  in  the  same  direction,  and  an  acquisition  of 
motion  in  the  contrary  direction,  the  combined  amount  of  which  is  equal  to  the 
motion  received  by  the  former. 

From  the  principle,  that  the  force  of  a  body  in  motion  depends  on  the  mass 
and  the  velocity,  it  follows  that  any  body,  however  small,  may  be  made  to  move 
with  the  same  force  as  any  other  body,  however  great,  by  giving  to  the  smaller 
body  a  velocity  which  bears  to  that  of  the  greater  the  same  proportion  as  the 
mass  of  the  greater  bears  to  the  mass  of  the  smaller.  Thus  a  feather,  ten  thou- 
sand of  which  would  have  the  same  weight  as  a  cannon-ball,  would  move  with 
the  same  force  if  it  had  ten  thousand  times  the  velocity ;  and,  in  such  a  case, 
these  two  bodies,  encountering  in  opposite  directions,  would  mutually  destroy 
each  other's  motion. 

The  consequences  of  the  property  of  inertia,  which  have  been  explained  in 
the  present  and  previous  lecture,  have  been  given  by  Newton  in  his  Prin- 
cipia,  and,  after  him,  in  most  English  treatises  on  mechanics,  under  the  form 
of  three  propositions,  which  are  called  the  "  laws  of  motion."  They  are  as 
follow : —  ■  - 

I. 

"Every  body  must  persevere  in  its  state  of  rest,  or  of  uniform  motion  in  a  straight  line, 
unless  it  be  compelled  to  change  that  state  by  forces  impressed  upon  it." 

II. 
"  Every  change  of  motion  must  be  proportional  to  the  impressed  force,  and  must  be  in 
the  direction  of  that  straight  line  in  which  the  force  is  impressed." 

III. 
"  Action  must  always  be  equal,  and  contrary  to  reaction ;  or  the  actions  of  two  bodies 
upon  each  other  must  be  equal,  and  directed  toward  contrary  sides." 


When  inertia  and  ybrce  are  defined,  the  first  law  becomes  an  identical  propo- 
sition. The  second  law  cannot  be  rendered  perfectly  intelligible  until  the  stu- 
dent has  read  the  discourse  on  the  composition  and  resolution  of  forces  ;  for,  in 
fact,  it  is  intended  as  an  expression  of  the  whole  body  of  results  in  that  dis- 
course. The  third  law  has  been  explained  in  the  present  lectur.e,  as  far  as  it 
can  be  rendered  intelligible  in  the  present  stage  of  our  progress. 

We  have  noticed  these  formularies  more  from  a  respect  for  the  authorities  by 
which  they  have  been  adopted,  than  from  any  persuasion  of  their  utility.  Their 
full  import  cannot  be  comprehended  until  nearly  the  whole  of  elementary  me- 
chanics has  been  acquired,  and  then  all  such  summaries  become  useless. 

The  consequences  deduced  from  the  consideration  of  the  quality  of  inertia  in 
this  lecture,  will  account  for  many  eff'ects  which  fall  under  our  notice  daily,  and 
with  which  we  have  become  so  familiar  that  they  have  almost  ceased  to  excite 
curiosity.  One  of  the  facts  of  which  we  have  most  frequent  practical  illustra- 
tion is,  that  the  quantity  of  motion,  or  moving  force,  as  it  is  sometimes  called, 
is  estimated  by  the  velocity  of  the  motion  and  the  weight  or  mass  of  the  thing 
moved  conjointly. 

If  the  same  force  impel  two  balls,  one  of  one  pound  weight,  and  the  other  of 
two  pounds,  it  follows,  since  the  balls  can  neither  g^ive  force  to  themselves  nor 


204 


ACTION  AND  REACTION. 


resist  that  which  is  impressed  upon  them,  that  they  will  move  with  the  same 
force.  But  the  lighter  ball  will  move  with  twice  the  speed  of  the  heavier.  The 
impressed  force  which  is  manifested  by  giving  velocity  to  a  double  mass  in  the 
one,  is  engaged  in  giving  a  double  velocity  to  the  other. 

If  a  cannon-ball  were  forty  times  the  weight  of  a  musket-ball,  but  the  musket- 
ball  moved  with  forty  times  the  velocity  of  the  cannon-ball,  both  would  strike 
any  obstacle  with  the  same  force  and  would  overcome  the  same  resistance  ;  for 
the  one  would  acquire  from  its  velocity  as  much  force  as  the  other  derives  from 
its  weight. 

A  very  small  velocity  may  be  accompanied  by  enormous  force,  if  the  mass 
which  is  moved  with  that  velocity  be  proportionally  great.  A  large  ship  float- 
ing near  the  pier-wall  may  approach  it  with  so  small  a  velocity  as  to  be  scarcely 
perceptible,  and  yet  the  force  will  be  so  great  as  to  crush  a  small  boat. 

A  grain  of  shot,  flung  from  the  hand  and  striking  the  person,  will  occasion 
no  pain,  and,  indeed,  will  scarcely  be  felt,  while  a  block  of  stone  having  the 
same  velocity  would  occasion  death. 

If  a  body  in  motion  strike  a  body  at  rest,  the  striking  body  must  sustain  as 
great  a  shock  from  the  collision  as  if  it  had  been  at  rest  and  struck  by  the  oth- 
er body  with  the  same  force  ;  for  the  loss  of  force  which  it  sustains  in  the  one 
direction  is  an  efl^ect  of  the  same  kind  as  if,  being  at  rest,  it  had  received  as 
much  force  in  the  opposite  direction.  If  a  man,  walking  rapidly,  or  running, 
encounters  another  standing  still,  he  suffers  as  much  from  the  collision  as  the 
man  against  whom  he  strikes. 

If  a  leaden  bullet  be  discharged  against  a  plank  of  hard  wood,  it  will  be  found 
that  the  round  shape  of  the  ball  is  destroyed,  and  that  it  has  itself  suffered  a 
force  by  the  impact,  which  is  equivalent  to  the  eflfect  which  it  produces  upon 
the  plank. 

When  two  bodies  moving  in  opposite  directions  meet,  each  body  sustains  as 
great  a  shock  as  if,  being  at  rest,  it  had  been  struck  by  the  other  body  with  the 
united  forces  of  the  two.  Thus,  if  two  equal  balls,  moving  at  the  rate  of  ten 
feet  in  a  second,  meet,  each  will  be  struck  with  the  same  force  as  if,  being  at 
rest,  the  other  had  moved  against  it  at  the  rate  of  twenty  feet  in  a  second.  In 
this  case,  one  part  of  the  shock  sustained  arises  from  the  loss  of  force  in  one 
direction,  and  another  from  the  reception  of  force  in  the  opposite  direction. 

For  this  reason,  two  persons  walking  in  opposite  directions  receive  from 
their  encounter  a  more  Adolent  shock  than  might  be  expected.  If  they  be  of 
nearly  equal  weight,  and  one  be  walking  at  the  rate  of  three,  and  the  other  four 
miles  an  hour,  each  sustains  the  same  shock  as  if  he  had  been  at  rest,  and  struck 
by  the  other  running  at  the  rate  of  seven  miles  an  hour. 

This  principle  accounts  for  the  destructive  effects  arising  from  ships  running 
foul  of  each  o'ther  at  sea.  If  two  ships  of  500  tons  burden  encounter  each  other, 
sailing  at  ten  knots  an  hour,  each  sustains  the  shock  which,  being  at  rest,  it 
would  receive  from  a  vessel  of  1,000  tons  burden  sailing  ten  knots  an  hour. 

It  is  a  mistake  to  suppose  that,  when  a  large  and  small  body  encounter,  the 
small  body  suffers  a  greater  shock  than  the  large  one.  The  shock  which  they 
sustain  must  be  the  same ;  but  the  large  body  may  be  better  able  to  bear  it. 

When  the  fist  of  a  pugilist  strikes  the  body  of  his  antagonist,  it  sustains  as 
great  a  shock  as  it  gives ;  but  the  part  being  more  fitted  to  endure  the  blow,  the 
injury  and  pain  are  inflicted  on  his  opponent.  This  is  not  the  case,  however, 
when  fist  meets  fist.  Then  the  parts  in  collision  are  equally  sensitive  and  vul- 
nerable, and  the  effect  is  aggravated  by  both  having  approached  each  other  with 
great  force.  The  eflfect  of  the  blow  is  the  same  as  if  one  fist,  being  held  at  rest, 
were  struck  by  the  other  with  the  combined  force  of  both. 


COMPOSITION  &  BESOLUTIOI  OP  FORCE. 


Motion  and  Pressure. — Force. — Attraction. — Parallelogram  of  Forces. — Resultant. — Components. — 
Composition  of  Force. — Resolution  of  Force. — Illustrative  Experiments. — Composition  of  Pres- 
sures.— Theorems  regulating  Pressures  also  regulate  Motion. — Examples. — Resolution  of  Mo- 
tion.— Forces  in  Equilibrium. — Composition  of  Motion  and  Pressure. — Illustrations. — Boat  in  a 
CuiTent. — Motions  of  Fishes. — Flight  of  Birds. — Sails  of  a  Vessel. — Tacking. — Equestrian  Feats. — 
Absolute  and  rel stive  Motion. 


COMPOSITION  AND  RESOLUTION  OP  FORCE. 


207 


COMPOSITION  &  RESOLUTIOI  OF  FORCE. 


Motion  and  pressure  are  terms  too  familiar  to  need  explanation.  It  may  be 
observed,  generally,  that  definitions  in  the  first  rudiments  of  a  science  are  sel- 
dom, if  ever,  comprehended.  The  force  of  words  is  learned  by  their  applica- 
tion ;  and  it  is  not  until  a  definition  becomes  useless,  that  w^e  are  taught  the 
meaning  of  the  terms  in  which  it  is  expressed.  Moreover,  we  are  perhaps  jus- 
tified in  saying  that,  in  the  mathematical  sciences,  the  fundamental  notions  are 
of  so  uncompounded  a  character,  that  definitions,  when  developed  and  enlarged 
upon,  often  draw  us  into  metaphysical  subtleties  and  distinctions,  which,  what- 
ever be  their  merit  or  importance,  would  be  here  altogether  misplaced.  We 
shall,  therefore,  at  once  take  it  for  granted,  that  the  words  motion  and  pressure 
express  phenomena  or  effects  which  are  the  subjects  of  constant  experience 
and  hourly  observation ;  and  if  the  scientific  use  of  these  words  be  more  pre- 
cise than  their  general  and  popular  application,  that  precision  will  soon  be 
learned  by  their  frequent  use  in  the  present  treatise. 

Force  is  the  name  given  in  mechanics  to  whatever  produces  motion  or  pres- 
sure. This  word  is  also  often  used  to  express  the  motion  or  pressure  itself; 
and  when  the  cause  of  the  motion  or  pressure  is  not  known,  this  is  the  only 
correct  use  of  the  word.  Thus,  when  a  piece  of  iron  moves  toward  a  magnet, 
it  is  usual  to  say  that  the  cause  of  the  motion  is  "  the  attraction  of  the  magnet ;" 
but  in  efiect  we  are  ignorant  of  the  cause  of  this  phenomenon,  and  the  name 
attraction  would  be  better  applied  to  the  effect,  of  which  we  have  experience. 
In  like  manner  the  attraction  and  repulsion  of  electrified  bodies  should  be  un- 
derstood, not  as  names  for  unknown  causes,  but  as  words  expressing  observed 
appearances  or  effects. 

When  a  certain  phraseology  has,  however,  gotten  into  general  use,  it  is  nei- 
ther easy  nor  convenient  to  supersede  it.  We  shall,  therefore,  be  compelled, 
in  speaking  of  motion  or  pressure,  to  use  the  language  of  causation  ;  but  must 
advise  the  student  that  it  is  effects,  and  not  causes,  which  will  be  expressed. 

If  two  forces  act  upon  the  same  point  of  a  body  in  different  directions,  a  sin- 


208 


COMPOSITION  AND  RESOLUTION  OF  FORCE. 


gle  force  may  be  assigned,  which,  acting  on  that  point,  will  produce  the  same 
result  as  the  united  effects  of  the  other  two. 

Let  P,  fig.  1,  be  the  point  on  which  the  two  forces  act,  and  let  their  direc- 
tions be  P  A  and  P  B.     From  the  point  P,  upon  the  line  P  A,  take  a  length 


Fig.  1. 


P  a,  consisting  of  as  many  inches  as  there  are  ounces  in  the  force  P  A ;  and, 
in  like  manner,  take  P  b,  in  the  direction  P  B,  consisting  of  as  many  inches 
as  there  are  ounces  in  the  force  P  B.  Through  a  draw  a  line  parallel  to  P  B, 
and  through  b  draw  a  line  parallel  to  P  A,  and  suppose  these  lines  meet  at  c. 
Then  draw  PC.  A  single  force,  acting  in  the  direction  P  C,  and  consisting 
of  as  many  ounces  as  the  line  P  c  consists  of  inches,  will  produce  upon  the 
point  P  the  same  effect  as  the  two  forces  P  A  and  P  B  produce  acting  to- 
gether. 

The  figure  V  a  cb  is  called,  in  geometry,  a  parallelogram ;  the  lines  V  a,V  h, 
are  called  its  sides,  and  the  line  P  c  is  called  its  diagonal.  Thus  the  method 
of  finding  an  equivalent  for  two  forces,  which  we  have  just  explained,  is  gen- 
erally called  ".the  parallelogram  of  forces,"  and  is  usually  expressed  thus  :  "  If 
two  forces  be  represented  in  quantity  and  direction  by  the  sides  of  a  parallelo- 
gram, an  equivalent  force  will  be  represented  in  quantity  and  direction  by  its 
diagonal." 

A  single  force,  which  is  thus  mechanically  equivalent  to  two  or  more  other 
forces,  is  called  their  resultant,  and  relatively  to  it  they  are  called  its  compo- 
nents. In  any  mechanical  investigation,  when  the  result  is  used  for  the  com- 
ponents, which  it  always  may  be,  the  process  is  called  "  the  composition  of 
force."  It  is,  however,  frequently  expedient  to  substitute  for  a  single  force  two 
or  more  forces,  to  which  it  is  mechanically  equivalent,  or  of  which  it  is  the  re- 
sultant.   This  process  is  called  "  the  resolution  of  force." 

To  verify  experimentally  the  theorem  of  the  parallelogram  of  forces  is  not 
diflScult.  Let  two  small  wheels,  M  N,  fig.  2,  with  grooves  in  their  edges  to 
receive  a  thread,  be  attached  to  an  upright  board,  or  to  a  wall.  Let  a  thread  be 
passed  over  them,  having  weights,  A  and  B,  hooked  upon  loops  at  its  extrem- 
ities. From  any  part,  P,  of  the  thread  between  the  wheels  let  a  weight,  C,  be 
suspended ;  it  will  draw  the  thread  downward,  so  as  to  form  an  angle,  M  P  N, 
and  the  apparatus  will  settle  itself  at  rest  in  some  determinate  position.  In  this 
state  it  is  evident  that,  since  the  weight  C,  acting  in  the  direction  P  C,  balan- 
ces the  weights  A  and  B,  acting  in  the  directions  P  M  and  P  N,  these  two 
forces  must  be  mechanically  equivalent  to  a  force  equal  to  the  weight  C,  and 
acting  directly  upward  from  P.  The  weight  C  is  therefore  the  quantity  of  the 
resultant  of  the  forces  P  M  and  P  N  ;  and  the  direction  of  the  resultant  is  that 
of  a  line  drawn  directly  upward  from  P. 

To  ascertain  how  far  this  is  consistent  with  the  theorem  of  "  the  parallelo- 


COMPOSITION  AND  RESOLUTION  OF  FORCE. 


gram  of  forces,"  let  a  line,  P  0,  be  drawn  upon  the  upright  board  to  which  the 
wheels  are  attached,  from  the  point  P  upward,  in  the  direction  of  the  thread 
C  P.     Also,  let  lines  be  drawn  upon  the  board  immediately  under  the  threads 


Fig.  2. 


P  M  and  P  N.  From  the  point  P,  on  the  line  P  0,  take  as  many  inches  as 
there  are  ounces  in  the  weight  C.  Let  the  part  of  P  0  thus  measured  be  P  c, 
and  from  c  draw  c  a  parallel  to  P  N,  and  c  b  parallel  to  P  M.  If  the  sides  P  a 
and  P  5  of  the  parallelogram  thus  formed  be  measured,  it  will  be  found  that  P  a 
will  consist  of  as  many  inches  as  there  are  ounces  in  the  weight  A,  and  P  Z»  of 
as  many  inches  as  there  are  ounces  in  the  weight  B. 

In  this  illustration,  ounces  and  inches  have  been  used  as  the  subdivisions  of 
weight  and  length.  It  is  scarcely  necessary  to  state,  that  any  other  measures 
of  these  quantities  would  serve  as  well,  only  observing  that  the  same  denomi- 
nations must  be  preserved  in  all  parts  of  the  same  investigation. 

Among  the  philosophical  apparatus  of  the  University  of  London,  is  a  very 
simple  and  convenient  instrument  which  I  have  constructed  for  the  experimen- 
tal illustration  of  this  important  theorem.  The  wheels  M  N  are  attached  to  the 
tops  of  two  tall  stands,  the  heights  of  which  may  be  varied  at  pleasure  by  an 
adjusting  screw.     A  jointed  parallelogram,  A  B  C  D,  fig.  3,  is  formed,  whose 


sides  are  divided  into  inches,  and  the  joints  at  A  and  B  are  moveable,  so  as  to 
vary  the  lengths  of  the  sides  at  pleasure.  The  joint  C  is  fixed  at  the  extremity 
of  a  ruler,  also  divided  into  inches,  while  the  opposite  joint  A  is  attached  to  a 
brass  loop,  which  surrounds  the  diagonal  ruler  loosely,  so  as  to  slide  freely 
along  it.  An  adjusting  screw  is  provided  in  this  loop,  so  as  to  clamp  it  in  any 
required  position. 
VOT>.  1I._i4 


210 


COMPOSITION  AND  RESOLUTION  OF  FORCE. 


In  making  the  experiment,  the  sides  A  B  and  A  D,  C  B  and  C  D,  are  ad- 
justed by  the  joints  B  and  A  to  the  same  number  of  inches  respectively  as  there 
are  ounces  in  the  weights  A  and  B,  fig.  2.  Then  the  diagonal  A  C  is  adjusted 
by  the  loop  and  screw  at  A,  to  as  many  inches  as  there  are  ounces  in  the 
weight  C.  This  done,  the  point  A  is  placed  behind  P,  fig.  2,  and  the  paral- 
lelogram is  held  upright,  so  that  the  diagonal  A  C  shall  be  in  the  direction  of 
the  vertical  thread  P  C.  The  sides  A  B  and  AD  will  then  be  found  to  take 
the  direction  of  the  threads  P  M  and  P  N.  By  changing  the  weights  and  the 
lengths  of  the  diagonal  and  sides  of  the  parallelogram,  the  experiment  may  be 
easily  varied  at  pleasure. 

In  the  examples  of  the  composition  of  forces  which  we  have  here  given,  the 
effects  of  the  forces  are  the.  production  of  pressures ;  or,  to  speak  more  cor- 
rectly, the  theorem  which  we  have  illustrated  is  "  the  composition  of  pres- 
sures." For  the  point  P  is  supposed  to  be  at  rest,  and  to  be  drawn  or  pressed 
in  the  directions  P  M  and  P  N.  In  the  definition  which  has  been  given  of  the 
word ybrce,  it  is  declared  to  include  motions  as  well  as  pressures.  In  fact,  if 
motion  be  resisted,  the  effect  is  converted  into  pressure.  The  same  cause, 
acting  upon  a  body,  will  either  produce  motion  or  pressure,  according  as  the 
body  is  free  or  restrained.  If  the  body  be  free,  motion  ensues  ;  if  restrained, 
pressure,  or  both  these  effects  together.  It  is,  therefore,  consistent  with  anal- 
ogy to  expect  that  the  same  theorems  which  regulate  pressures  will  also 
be  applicable  to  motions,  and  we  find  accordingly  a  most  exact  corres- 
pondence. 

If  a  body  have  a  motion  in  the  direction  A  B,  and  at  the  point  P  it  receive 
another  motion,  such  as  would  carry  it  in  the  direction  P  C,  fig.  4,  were  it  pre- 


Fig.  4. 


viously  quiescent  at  P,  it  is  required  to  determine  the  direction  which  the 
body  will  take,  and  the  speed  with  which  it  will  move,  under  these  circum- 
stances. 

Let  the  velocity  with  which  the  body  is  moving  from  A  to  B  be  such,  that  it 
would  move  through  a  certain  space,  suppose  P  N,  in  one  second  of  time,  and 
let  the  velocity  of  the  motion  impressed  upon  it  at  P  be  such,  that,  if  it  had  no 
previous  motion,  it  would  move  from  P  to  M  in  one  second.  From  the  point 
M  draw  a  line  parallel  to  P  B,  and  from  N  draw  a  line  parallel  to  P  C,  and 
suppose  these  lines  to  meet  at  some  point,  as  0.  Then  draw  the  line  P  O.  In 
consequence  of  the  two  motions,  which  are  at  the  same  time  impressed  upon  the 
body  at  P,  it  will  move  in  the  straight  line  from  P  to  0. 

Thus  the  two  motions,  which  are  expressed  in  quantity  and  direction  by  the 
sides  of  a  parallelogram,  will,  when  given  to  the  same  body,  produce  a  single 
motion,  expressed  in  quantity  and  direction  by  its  diagonal :  a  theorem  which 
is  to  motions  exactly  what  the  former  was  to  pressures. 

There  are  various  methods  of  illustrating  experimentally  the  composition  of 


COMPOSITION  AND  RESOLUTION  OF  FORCE. 


motion.  An  ivory  ball,  being  placed  upon  a  perfectly  level,  square  table,  at 
one  of  the  corners,  and  receiving  two  equal  impulses,  in  the  directions  of  the 
sides  of  the  table,  will  move  along  the  diagonal.  Apparatus  for  this  experiment 
differ  from  each  other  only  in  the  way  of  communicating  the  impulses  to  the 
ball. 

As  two  motions  simultaneously  communicated  to  a  body  are  equivalent  to  a 
single  motion  in  an  intermediate  direction,  so  also  a  single  motion  may  be  me- 
chanically replaced  by  two  motions  in  directions  expressed  by  the  sides  of  any 
parallelogram,  whose  diagonal  represents  the  single  motion.  This  process  is 
"  the  resolution  of  motion,"  and  gives  considerable  clearness  and  facility  to 
many  mechanical  investigations. 

It  is  frequently  necessary  to  express  the  portion  of  a  given  force,  which  acts 
in  some  given  direction  different  from  the  immediate  direction  of  the  force  it- 
self.    Thus,  if  a  force  act  from  A,  fig.  5,  in  the  direction  A  C,  we  may  require 


Fig.  5. 


M. 


B 


to  estimate  what  part  of  that  force  acts  in  the  direction  A  B.  If  the  force  be  a 
pressure,  take  as  many  inches,  A  P,  from  A,  on  the  line  A  C,  as  there  are 
ounces  in  the  force,  and  from  P  draw  P  M  perpendicular  to  A  B  ;  then  the 
part  of  the  force  which  acts  along  A  B  will  be  as  many  ounces  as  there  are 
inches  in  A  M.  The  force  A  B  is  mechanically  equivalent  to  two  forces,  ex- 
pressed by  the  sides  A  M  and  A  N  of  the  parallelogram ;  but  A  N,  being  per- 
pendicular to  A  B,  can  have  no  effect  on  a  body  at  A,  in  the  direction  of  A  B, 
and  therefore  the  effective  part  of  the  force  A  P,  in  the  direction  A  B,  is  ex- 
pressed by  AM. 

Any  number  of  forces  acting  on  the  same  point  of  a  body  may  be  replaced 
by  a  single  force  which  is  mechanically  equivalent  to  them,  and  which  is, 
therefore,  their  resultant.  This  composition  may  be  effected  by  the  successive 
application  of  the  parallelogram  of  forces.  Let  the  several  forces  be  called  A, 
B,  C,  D,  E,  &c.  Draw  the  parallelogram  whose  sides  express  the  forces  A 
and  B,  and  let  its  diagonal  be  A'.  The  force  expressed  by  A'  will  be  equiva- 
lent to  A  and  B.  Then  draw  the  parallelogram  whose  sides  express  the  forces 
A'  and  C,  and  let  its  diagonal  be  B'.  This  diagonal  will  express  a  force  me- 
chanically equivalent  to  A'  and  C.  But  A'  is  mechanically  equivalent  to  A  and 
B,  and  therefore  B'  is  mechanically  equivalent  to  A,  B,  and  C.  Next  construct 
a  parallelogram  whose  sides  express  the  forces  B'  and  D,  and  let  its  diagonal 
be  C.  The  force  expressed  by  C  will  be  mechanically  equivalent  to  the  forces 
B'  and  D  ;  but  the  force  B'  is  equivalent  to  A,  B,  C,  and  therefore  C  is  equiv- 
alent to  A,  B,  C,  and  D.  By  continuing  this  process,  it  is  evident  that  a  sin- 
gle force  may  be  found  which  will  be  equivalent  to,  and  may  be  always  substi- 
tuted for,  any  number  of  forces  which  act  upon  the  same  point. 

If  the  forces  which*  act  upon  the  point  neutralize  each  other,  so  that  no  mo- 
tion can  ensue,  they  are  said  to  be  in  equilibrium. 

Examples  of  the  composition  of  motion  and  pressure  are  continually  present- 
ing themselves.  They  occur  in  almost  every  instance  of  motion  or  force  which 
falls  under  our  observation.  The  difficulty  is  to  find  an  example  which,  strictly 
speaking,  is  a  simple  motion. 


212 


COMPOSITION  AND  RESOLUTION  OF  FORCE. 


When  a  boat  is  rowed  across  a  river,  in  which  there  is  a  current,  it  will  not 
move  in  the  direction  in  which  it  is  impelled  by  the  oars.  Neither  will  it  take 
the  direction  of  the  stream,  but  will  proceed  exactly  in  that  intermediate  direc- 
tion which  is  determined  by  the  composition  of  force. 

Let  A,  fig.  6,  be  the  place  of  the  boat  at  starting ;  and  suppose  that  the  oars 
are  so  worked  as  to  impel  the  boat  toward  B  with  a  force  which  would  carry 
it  to  B  in  one  hour,  if  there  were  no  current  in  the  river.     But,  on  the  other 


Fig.  6. 

B 

B               D 

\ 

'A 

\ 

\        / 

'' 

L _!ii 

hand,  suppose  the  rapidity  of  the  current  is  such  that,  without  any  exertion  of 
the  rowers,  the  boat  would  float  down  the  stream  in  one  hour  to  C.  From  C 
draw  C  D  parallel  to  A  B,  and  draw  the  straight  line  A  D  diagonally.  The 
combined  effect  of  the  oars  and  the  current  will  be,  that  the  boat  will  be  car- 
ried along  A  D,  and  will  arrive  at  the  opposite  bank  in  one  hour,  at  the 
point  D. 

If  the  object  be,  therefore,  to  reach  the  point  B,  starting  from  A,  the  rowers 
must  calculate,  as  nearly  as  possible,  the  velocity  of  the  current.  They  must 
imagine  a  certain  point,  E,  at  such  a  distance  above  B  that  the  boat  would  be 
floated  by  the  stream  from  E  to  B  in  the  time  taken  in  crossing  the  river  in  the 
direction  A  E,  if  there  were  no  current.  If  they  row  toward  the  point  E,  the 
boat  will  arrive  at  the  point  B,  moving  in  the  line  A  B. 

In  this  case  the  boat  is  impelled  by  two  forces,  that  of  the  oars  in  the  direc- 
tion A  E,  and  that  of  the  current  in  the  direction  A  C.  The  result  will  be, 
according  to  the  parallelogram  of  forces,  a  motion  in  the  diagonal  A  B. 

The  wind  and  tide  acting  upon  a  vessel  is  a  case  of  a  similar  kind.  Sup- 
pose that  the  wind  is  made  to  impel  the  vessel  in  the  direction  of  the  keel, 

Fig.  7. 


B/ 


while  the  tide  may  be  acting  in  any  direction  oblique  to  that  of  the  keel.  The 
course  of  the  vessel  is  determined  exactly  in  the  same  manner  as  that  of  the 
boat  in  the  last  example. 


'COMPOSITION  AND  RESOLUTION  OF  FORCE. 


213 


The  action  of  the  oars  themselves,  in  impelling  the  boat,  is  an  example  of 
the  composition  of  force.  Let  A,  fig.  7,  be  the  head,  and  B  the  stern  of  the 
boat.  The  boatman  presents  his  face  toward  B,  and  places  the  oars  so  that 
their  blades  press  against  the  water  in  the  directions  C  E,  D  F.  The  resist- 
ance of  the  water  produces  forces  on  the  side  of  the  boat,  in  the  directions  G 
L  and  H  L,  which,  by  the  composition  of  force,  are  equivalent  to  the  diagonal 
force  K  L,  in  the  direction  of  the  keel. 

Similar  observations  will  apply  to  almost  every  body  impelled  by  instruments 
projecting  from  its  sides  and  acting  against  a  fluid.  The  motions  of  fishes,  the 
act  of  swimming,  the  flight  of  birds,  are  all  instances  of  the  same  kind. 

The  action  of  wind  upon  the  sails  of  a  vessel,  and  the  force  thereby  trans- 
mitted to  the  keel,  modified  by  the  rudder,  is  a  problem  which  is  solved  by  the 
principles  of  the  composition  and  resolution  of  force  ;  but  it  is  of  too  compli- 
cated and  difiicult  a  nature  to  be  introduced  with  all  its  necessary  conditions 
and  limitations  in  this  place.  The  question  may,  however,  be  simplified,  if  we 
consider  the  canvass  of  the  sails  to  be  stretched  so  completely  as  to  form  a 
plane  surface.     Let  A  B,  fig.  8,  be  the  position  of  the  sail,  and  let  the  wind 


Fig.  8. 


blow  in  the  direction  C  D.  If  the  line  C  D  be  taken  to  express  the  force  of 
the  wind,  let  D  E  C  F  be  a  parallelogram,  of  which  it  is  the  diagonal.  The 
force  C  D  is  equivalent  to  two  forces,  one  in  the  direction  F  D  of  the  plane  of 
the  canvass,  and  the  other  E  D  perpendicular  to  the  sail.  The  efi'ect,  there- 
fore, is  the  same  as  if  there  were  two  winds,  one  blowing  in  the  direction  of 
F  D  or  B  A,  that  is,  against  the  edge  of  the  sail,  and  the  other,  E  D,  blowing 
full  against  its  face.  It  is  evident  that  the  former  will  produce  no  eflfect  what- 
ever upon  the  sail,  and  that  the  latter  will  urge  the  vessel  in  the  direction 
D  G. 

Let  us  now  consider  this  force  D  G  as  acting  in  the  diagonal  of  the  parallel- 
ogram D  H  G  I.  It  will  be  equivalent  to  two  forces,  D  H  and  D  I,  acting 
along  the  sides.  One  of  these  forces,  D  H,  is  in  the  direction  of  the  keel,  and 
the  other,  D  I,  at  right  angles  to  the  length  of  the  vessel,  so  as  to  urge  it  side- 
wise.  The  form  of  the  vessel  is  evidently  such  as  to  offer  a  great  resistance 
to  the  latter  force,  and  very  little  to  the  former.  It  consequently  proceeds  with 
considerable  velocity  in  the  direction  D  H  of  its  keel,  and  makes  way  very 
slowly  in  the  sideward  direction  D  I.     The  latter  effect  is  called  leeway. 

From  this  explanation,  it  will  be  easily  understood  how  a  wind  which  is 


COMPOSITION  AND  RESOLUTION  OF  FORCE. 


nearly  opposed  to  the  course  of  a  vessel  may,  nevertheless,  be  made  to  impel 
it  by  the  effect  of  sails.  The  angle  B  D  V,  formed  by  the  sail  and  the  direc- 
tion of  the  keel,  may  be  very  oblique,  as  may  also  be  the  angle  C  D  B,  formed 
by  the  direction  of  the  wind  and  that  of  the  sail.  Therefore  the  angle  C  D  V, 
made  up  of  these  two,  and  which  is  that  formed  by  the  direction  of  the  wind 
and  that  of  the  keel,  may  be  very  oblique.    In  fig.  9,  the  wind  is  nearly  contrary 

Fig.  9. 


to  the  direction  of  the  keel,  and  yet  there  is  an  impelling  force  expressed 
by  the  line  D  H,  the  line  C  D  expressing,  as  before,  the  whole  force  of  the 
wind. 

In  this  example  there  are  two  successive  decompositions  of  force.  First, 
the  original  force  of  the  wind  C  D  is  resolved  into  two,  E  D  and  F  D ;  and 
next  the  element  E  D,  or  its  equal  D  G,  is  resolved  into  D  I  and  D  H ;  so 


that  the  original  force  is  resolved  into  three,  viz.,  F  D,  D  I,  D  H,  which,  taken 
together,  are  mechanically  equivalent  to  it.    The  part  F  D  is  entirely  ineffect- 


COMPOSITION  AND  RESOLUTION  OF  FORCE. 


215 


ual ;  it  glides  off  on  the  surface  of  the  canvass  without  producing  any  effect 
upon  the  vessel.     The  part  D  I  produces  leeway,  and  the  part  D  H  impels. 

If  the  wind,  however,  be  directly  contrary  to  the  course  which  it  is  required 
that  the  vessel  should  take,  there  is  no  position  which  can  be  given  to  the  sails 
which  will  impel  the  vessel.  In  this  case,  the  required  course  itself  is  resolv- 
ed into  two,  in  which  the  vessel  sails  alternately,  a  process  which  is  called 
tacking.  Thus,  suppose  the  vessel  is  required  to  move  from  A  to  E,  fig.  10, 
the  wind  setting  from  E  to  A.  The  motion  A  B  being  resolved  into  two,  by 
being  assumed  as  the  diagonal  of  a  parallelogram,  the  sides  A  a,  a  B,  of  the 
parallelogram  are  successively  sailed  over,  and  the  vessel  by  this  means  ar- 
rives at  B,  instead  of  moving  along  the  diagonal  A  B.  In  the  same  manner 
she  moves  along  B  Z>,  6  C,  C  c,  c  D,  D  t?,  (i  E,  and  arrives  at  E.  She  thus 
sails  continually  at  a  sufficient  angle  with  the  wind  to  obtain  an  impelling  force, 
yet  at  a  sufficiently  small  angle  to  make  way  in  her  proposed  course. 

The  consideration  of  the  effect  of  the  rudder,  which  we  have  omitted  in  the 
preceding  illustration,  affords  another  instance  of  the  resolution  of  force.  We 
shall  not,  however,  pursue  this  example  further. 

A  body  falling  from  the  top  of  the  mast,  when  the  vessel  is  in  full  sail,  is  an 
example  of  the  composition  of  motion.  It  might  be  expected  that,  during  the 
descent  of  the  body,  the  vessel,  having  sailed  forward,  would  leave  it  behind, 
and  that,  therefore,  it  would  fall  in  the  water  behind  the  stern,  or  at  least  on 
the  deck,  considerably  behind  the  mast.  On  the  other  hand,  it  is  found  to  fall 
at  the  foot  of  the  mast,  exactly  as  it  would  if  the  vessel  were  not  in  motion. 
To  account  for  this,  let  A  B,  fig.  11,  be  the  position  of  the  mast  when  the  body 


at  the  top  is  disengaged.  The  mast  is  moving  onward  with  the  vessel  in  the 
direction  A  C,  so  that  in  the  time  which  the  body  would  take  to  fall  to  the 
deck  the  top  of  the  mast  would  move  from  A  to  C.  But  the  body,  being  on  the 
mast  at  the  moment  it  is  disengaged,  has  this  motion  A  C  in  common  with  the 
mast,  and,  therefore,  in  its  descent  it  is  affected  by  two  motions,  viz.,  that  of 
the  vessel  expressed  by  A  C,  and  its  descending  motion  expressed  by  A  B. 
Hence,  by  the  composition  of  motion,  it  will  be  found  at  the  opposite  angle,  D, 
of  the  parallelogram,  at  the  end  of  the  fall.  During  the  fall,  however,  the  mast 
has  moved  with  the  vessel,  and  has  advanced  to  C  D,  so  that  the  body  falls  at 
the  foot  of  the  mast. 

An  instance  of  the  composition  of  motion,  which  is  worthy  of  some  attention, 
as  it  affords  a  proof  of  the  diurnal  motion  of  the  earth,  is  derived  from  observ- 
ing the  descent  of  a  body  from  a  very  high  tower.  To  render  the  explanation 
of  this  more  simple,  we  shall  suppose  the  tower  to  be  on  the  equator  of  the 
earth.  Let  E  P  Q,  fig.  12,  be  a  section  of  the  earth  through  the  equator,  and 
let  P  T  be  the  tower.     Let  us  suppose  that  the  earth  moves  on  its  axis  in  the 


216 


COMPOSITION  AND  RESOLUTION  OF  FORCE. 


direction  E  P  Q.  The  foot  P  of  the  tower  will,  therefore,  in  one  day,  move 
over  the  circle  E  P  Q,  while  the  top  T  moves  over  the  greater  circle  T  T'  R. 
Hence  it  is  evident  that  the  top  of  the  tower  moves  with  greater  speed  than 
the  foot,  and  therefore  in  the  same  time  moves  through  a  greater  space.  Now 
suppose  a  body  placed  at  the  top ;  it  participates  in  the  motion  which  the  top 
of  the  tower  has  in  common  with  the  earth.  If  it  be  disengaged,  it  also  re- 
ceives the  descending  motion  T  P.  Let  us  suppose  that  the  body  would  take 
five  seconds  to  fall  from  T  to  P,  and  that  in  the  same  time  the  top  T  is  moved 
by  the  rotation  of  the  earth  from  T  to  T',  the  foot  being  moved  from  P  to  P'. 
The  falling  body  is  therefore  endued  with  two  motions,  one  expressed  by  T  T', 
and  the  other  by  T  P.  The  combined  effect  of  these  will  be  found  in  the  usual 
way  by  the  parallelogram.  Take  T  p,  equal  to  T  T',  the  body  will  move  from 
T  to  jo  in  the  time  of  the  fall,  and  will  meet  the  ground  at  p.  But  since  T  T' 
is  greater  than  P  P',  it  follows  that  p  must  be  at  a  distance  from  P'  equal  to 
the  excess  of  T  T'  above  P  P'.  Hence  the  body  will  not  fall  exactly  at  the 
foot  of  the  tower,  but  at  a  certain  distance  from  it,  in  the  direction  of  the  earth's 
motion,  that  is,  eastward.  This  is  found,  by  experiment,  to  be  actually  the 
case ;  and  the  distance  from  the  foot  of  the  tower,  at  which  the  body  is  ob- 
served to  fall,  agrees  with  that  which  is  computed  from  the  motion  of  the  earth, 
to  as  great  a  degree  of  exactness  as  could  be  expected  from  the  nature  of  the 
experiment. 

The  properties  of  compounded  motions  cause  some  of  the  equestrian  feats 
exhibited  at  public  spectacles  to  be  performed  by  a  kind  of  exertion  very  dif- 
ferent from  that  the  spectators  generally  attribute  to  the  performer.  For  ex- 
ample, the  horseman,  standing  on  the  saddle,  leaps  over  a  garter  extended  over 
the  horse  at  right  angles  to  his  motion  ;  the  horse  passing  under  the  garter,  the 
rider  lights  upon  the  saddle  at  the  opposite  side.  The  exertion  of  the  per- 
former, in  this  case,  is  not  that  which  he  would  use  were  he  to  leap  from  the 
ground  over  a  garter  at  the  same  height.  In  the  latter  case,  he  would  make 
an  exertion  to  rise,  and  at  the  same  time  to  project  his  body  forward.  In  the 
case,  however,  of  the  horseman,  he  merely  makes  that  exertion  which  is  ne- 
cessary to  rise  directly  upward  to  a  sufficient  height  to  clear  the  garter.  The 
motion  which  he  has  in  common  with  the  horse,  compounded  with  the  eleva- 
tion acquired  by  his  muscular  power,  accomplishes  the  leap. 

To  explain  this  more  fully,  let  ABC,  fig.  13,  be  the  direction  in  which  the 


horse  moves,  A  being  the  point  at  which  the  rider  quits  the  saddle,  and  C  the 
point  at  which  he  returns  to  it.  Let  D  be  the  highest  point  which  is  to  be 
cleared  in  the  leap.  At  A  the  rider  makes  a  leap  toward  the  point  E,  and  this 
must  be  done  at  such  a  distance  from  B,  that  he  would  rise  from  B  to  E  in  the 
time  in  which  the  horse  moves  from  A  to  B.  On  departing  from  A,  the  rider 
has,  therefore,  two  motions,  represented  by  the  lines  A  E  and  A  B,  by  which 
he  will  move  from  the  point  A  to  the  opposite  angle,  D,  of  the  parallelogram. 
At  D,  the  exertion  of  the  leap  being  overcome  by  the  weight  of  his  body,  he 
begins  to  return  downward,  and  would  fall  from  D  to  B  in  the  time  in  which 
the  horse  moves  from  B  to  C.     But  at  D  he  still  retains  the  motion  which  he 


COMPOSITION  AND  RESOLUTION  OF  FORCE. 


217 


had  in  common  with  the  horse,  and  therefore,  in  leaving  the  point  D,  he  has 
two  motions,  expressed  by  the  lines  D  F  and  D  B.  The  compounded  effects 
of  these  motions  carry  him  from  D  to  C.  Strictly  speaking,  his  motion  from  A 
to  D,  and  from  D  to  C,  is  not  in  straight  lines,  but  in  a  curve.  It  is  not  neces- 
sary here,  however,  to  attend  to  this  circumstance. 

If  a  billiard-ball  strike  the  cushion  of  the  table  obliquely,  it  will  be  reflected 
from  it  in  a  certain  direction,  forming  an  angle  with  the  direction  in  which  it 
struck  it.  This  affords  an  example  of  the  resolution  and  composition  of  mo- 
tion. We  shall  first  consider  the  effect  which  would  ensue  if  the  ball  struck 
the  cushion  perpendicularly. 

Let  A  B,  fig.  14,  be  the  cushion,  and  C  D  the  direction  in  which  the  ball 


moves  toward  it.  If  the  ball  and  the  cushion  were  perfectly  inelastic,  the  re- 
sistance of  the  cushion  would  destroy  the  motion  of  the  ball,  and  it  would  be 
reduced  to  a  state  of  rest  at  D.  If,  on  the  other  hand,  the  ball  were  perfectly 
elastic,  it  would  be  reflected  from  the  cushion,  and  would  receive  as  much  mo- 
tion from  D  to  C,  after  the  impact,  as  it  had  from  C  to  D  before  it.  Perfect 
elasticity,  however,  is  a  quality  which  is  never  found  in  these  bodies.  They 
are  always  elastic,  but  imperfectly  so.  Consequently  the  ball,  after  the  impact, 
will  be  reflected  from  D  toward  C,  but  with  a  less  motion  than  that  with  which 
it  approached  from  C  to  D. 

Now  let  us  suppose  that  the  ball,  instead  of  moving  from  C  to  D,  moves 
from  E  to  D.  The  force  with  which  it  strikes  D,  being  expressed  by  D  E', 
equal  to  E  D,  may  be  resolved  into  two,  D  F  and  D  C  The  resistance  of 
the  cushion  destroys  D  C,  and  the  elasticity  produces  a  contrary  force  in  the 
direction  D  C,  but  less  than  D  C  or  D  C,  because  that  elasticity  is  imperfect. 
The  line  D  C  expressing  the  force  in  the  direction  C  D,  let  D  G  (less  than 
D  C)  express  the  reflective  force  in  the  direction  D  C.  The  other  element, 
D  F,  into  which  the  force  D  E'  is  resolved  by  the  impact,  is  not  destroyed  or 
modified  by  the  cushion,  and  therefore,  on  leaving  the  cushion  at  D,  the  ball  is 
influenced  by  two  forces,  D  F  (which  is  equal  to  C  E)  and  D  G.  Consequently 
it  will  move  in  the  diagonal  D  H. 

The  angle  E  D  C  is,  in  this  case,  called  the  "  angle  of  incidence,"  and  C  D 
H  is  called  the  "  angle  of  reflection."  It  is  evident,  from  what  has  just  been 
inferred,  that,  the  ball  being  imperfectly  elastic,  the  angle  of  incidence  must 
always  be  less  than  the  angle  of  reflection,  and,  with  the  same  obliquity  of 
incidence,  the  more  imperfect  the  elasticity  is,  the  less  will  be  the  angle  of  re- 
flection. 

In  the  impact  of  a  perfectly  elastic  body,  the  angle  of  reflection  would  be 
equal  to  the  angle  of  incidence.    For  then  the  line  D  G,  expressing  the  reflec- 


218 


COMPOSITION  AND  RESOLUTION  OF  FORCE. 


tive  force,  would  be  taken  equal  to  C  D,  and  the  angle  C  D  H  would  be  equal 
to  C  D  E.  This  is  found  by  experiment  to  be  the  case  when  light  is  reflected 
from  a  polished  surface  of  glass  or  metal. 

Motion  is  sometimes  distinguished  into  absolute  and  relative.  What  "  rela- 
tive motion"  means  is  easily  explained.  If  a  man  walk  upon  the  deck  of  a  ship 
from  stem  to  stern,  he  has  a  relative  motion  which  is  measured  by  the  space 
upon  the  deck  over  which  he  walks  in  a  given  rime.  But  while  he  is  thus 
walking  from  stem  to  stern,  the  ship  and  its  contents,  including  himself,  are 
impelled  through  the  deep  in  the  opposite  .direction.  If  it  so  happen  that  the 
motion  of  the  man  from  stem  to  stern  be  exactly  equal  to  the  motion  of  the  ship 
in  the  contrary  way,  the  man  will  be,  relatively  to  the  surface  of  the  sea  and 
that  of  the  earth,  at  rest.  Thus,  relatively  to  the  ship,  he  is  in  motion,  while, 
relatively  to  the  surface  of  the  earth,  he  is  at  rest.  But  still  this  is  not  abso- 
lute rest.  The  surface  itself  is  moving  by  the  diurnal  rotation  of  the  earth 
upon  its  axis,  as  well  as  by  the  annual  motion  in  its  orbit  round  the  sun. 
These  motions,  and  others  to  which  the  earth  is  subject,  must  be  all  com- 
pounded by  the  theorem  of  the  parallelogram  of  forces,  before  we  can  obtain 
the  absolute  state  of  the  body  with  respect  to  motion  or  rest. 


CENTRE   OF   GRATITY. 


Terrestrial  Attraction  the  combined  Action  of  parallel  Forces. — Single  equivalent  Force. — Exam- 
ples.— Method  of  iinding  the  Centre  of  Gravity. — Line  of  Direction. — Globe. — Oblate  Spheroid. — 
Prolate  Spheroid. — Cube. — Straight  "Wand. — Flat  Plate. — Triangular  Plate. — Centre  of  Gravity 
not  always  within  the  Body. — A  Ring. — Experiments. — Stable,  instable,  and  neutral  Equilibrium. — 
Motion  and  Position  of  the  Arms  and  Feet. — Effect  of  the  Knee-joint. — Positions  of  a  Dancer. — 
Porter  under  a  Load. — Motion  of  a  Q,uadruped. — Rope  Dancing. — Centre  of  Gravity  of  tv^o  Bod- 
ies separated  from  each  other. — Mathematical  and  experimental  Examples. — The  Conservation  of 
the  Motion  of  the  Centre  of  Gravity. — Solar  System. — Centre  of  Gravity  sometimes  called  Centre 
of  Inertia. 


CENTRE  OF  GRAVITY. 


221 


CEKTRE   OF  GRAYITY, 


By  the  earth's  attraction,  all  the  particles  which  compose  the  mass  of  a  body 
are  solicited  by  equal  forces  in  parallel  directions  downward.  If  these  com- 
ponent particles  were  placed  in  mere  juxtaposition,  without  any  mechanical 
connexion,  the  force  impressed  on  any  one  of  them  could  in  nowise  affect  the 
others,  and  the  mass  would  in  such  a  case  be  contemplated  as  an  aggregation 
of  small  particles  of  matter,  each  urged  by  an  independent  force.  But  the 
bodies  which  are  the  subjects  of  investigation  in  mechanical  science  are  not 
found  in  this  state.  Solid  bodies  are  coherent  masses,  the  particles  of  which 
are  firmly  bound  together,  so  that  any  force  which  affects  one,  being  modified 
according  to  circumstances,  will  be  transmitted  through  the  whole  body.  Li- 
quids accommodate  themselves  to  the  shape  of  the  surfaces  on  which  they  rest, 
and  forces  affecting  any  one  part  are  transmitted  to  others,  in  a  manner  de- 
pending on  the  peculiar  properties  of  this  class  of  bodies. 

As  all  bodies,  which  are  subjects  of  mechanical  inquiry,  on  the  surface  of 
the  earth,  must  be  continually  influenced  by  terrestrial  gravity,  it  is  desirable 
to  obtain  some  easy  and  summary  method  of  estimating  the  effect  of  this  force. 
To  consider  it,  as  is  unavoidable  in  the  first  instance,  the  combined  action  of 
an  infinite  number  of  equal  and  parallel  forces  soliciting  the  elementary  mole- 
cules downward,  would  be  attended  with  manifest  inconvenience.  An  infinite 
number  of  forces,  and  an  infinite  subdivision  of  the  mass,  would  form  parts  of 
every  mechanical  problem. 

To  overcome  this  difficulty,  and  to  obtain  all  the  ease  and  simplicity  which 
can  be  desired  in  elementary  investigations,  it  is  only  necessary  to  determine 
some  force,  whose  single  effect  shall  be  equivalent  to  the  combined  effects  of 
the  gravitation  of  all  the  molecules  of  the  body.  If  this  can  be  accomplished, 
that  single  force  might  be  introduced  into  all  problems  to  represent  the  whole 
effect  of  the  earth's  attraction,  and  no  regard  need  be  had  to  any  particles  of 
the  body,  except  that  on  which  this  force  acts.  < 

To  discover  such  a  force,  if  it  exist,  we  shall  first  inquire  what  properties  ] 


222 


CENTRE  OF  GRAVITY. 


Fig.  1. 


must  necessarily  characterize  it.  Let  A  B,  fig.  1,  be  a  solid  body  placed  near 
the  surface  of  the  earth.  Its  particles  are  all  solicited  downward,  in  the  di- 
rections represented  by  the  arrows.  Now,  if  there  be  any  single  force  equiva- 
lent to  these  combined  effects,  two  properties  may  be  at  once  assigned  to  it : 
1,  it  must  be  presented  downward,  in  the  common  direction  of  those  forces  to 
which  it  is  mechanically  equivalent ;  and,  2,  it  must  be  equal  in  intensity  to 
their  sum,  or,  what  is  the  same,  to  the  force  with  which  the  whole  mass  would 
descend.  We  shall  then  suppose  it  to  have  this  intensity,  and  to  have  the  di- 
rection of  the  arrow  D  E.  Now,  if  the  single  force,  in  the  direction  D  E,  be 
equivalent  to  all  the  separate  attractions  which  affect  the  particles,  we  may 
suppose  all  these  attractions  removed,  and  the  body  A  B  influenced  only  by  a 
single  attraction,  acting  in  the  direction  D  E.  This  being  admitted,  it  follows 
that  if  the  body  be  placed  on  a  prop,  immediately  under  the  direction  of  the 
line  D  E,  or  be  suspended  from  a  fixed  point  immediately  above  its  direction, 
it  will  remain  motionless.  For  the  whole  attracting  force  in  the  direction  D  E 
will,  in  the  one  case,  press  the  body  on  the  prop,  and,  in  the  other  case,  will 
give  tension  to  the  cord,  rod,  or  whatever  other  means  of  suspension  be  used. 

But  suppose  the  body  were  suspended  from  some  point  P,  not  in  the  direc- 
tion of  the  line  D  E.  Let  P  C  be  the  direction  of  the  thread  by  which  the 
body  is  suspended.  Its  whole  weight,  according  to  the  supposition  which  we 
have  adopted,  must  then  act  in  the  direction  C  E.  Taking  C  F  to  represent 
the  weight,  it  may  be  considered  as  mechanically  equivalent  to  two  forces  (74), 
C  I  and  C  H.  Of  these,  C  H,  acting  directly  from  the  point  P,  merely  pro- 
duces pressure  upon  it,  and  gives  tension  to  the  cord  PC;  but  C  I,  acting  at 
right  angles  to  C  P,  produces  motion  round  P  as  a  centre,  and  in  the  direction 
C  I,  toward  a  vertical  line  P  G,  drawn  through  the  point  P.  If  the  body  A  B 
had  been  on  the  other  side  of  the  line  P  G,  it  would  have  moved,  in  like  man- 
ner, toward  it,  and  therefore  in  the  direction  contrary  to  its  present  motion. 

Hence  we  must  infer,  that,  when  the  body  is  suspended  from  a  fixed  point, 
it  cannot  remain  at  rest,  if  that  fixed  point  be  not  pkced  in  the  direction  of  tke 
line  D  E  ;  and,  on  the  other  hand,  that  if  the  fixed  point  be  in  the  direction  of 
that  line,  it  cannot  move.     A  practical  test  is  thus  suggested,  by  which  the 


CENTRE  OF  GRAVITY. 


223 


line  D  E  maybe  at  once  discovered.  Let  a  thread  be  attached  to  any  point  of 
the  body,  and  let  it  be  suspended  by  this  thread  from  a  hook  or  other  fixed 
point.  The  direction  of  the  thread,  when  the  body  becomes  quiescent,  will  be 
that  of  a  single  force  equivalent  to  the  gravitation  of  all  the  component  parts  of 
the  mass. 

An  inquiry  is  here  suggested  :  Does  the  direction  of>  the  equivalent  force, 
thus  determined,  depend  on  the  position  of  the  body  with  respect  to  the  surface 
of  the  earth,  and  how  is  the  direction  of  the  equivalent  force  affected  by  a 
change  in  that  position  ?  This  question  may  be  at  once  solved  if  the  body  be 
suspended  by  different  points,  and  the  directions  which  the  suspending  thread 
takes  in  each  cSse  relatively  to  the  figure  and  dimensions  of  the  body  exam- 
ined. 

The  body  being  suspended  in  this  manner  from  any  point,  let  a  small  hole 
be  bored  through  it,  in  the  exact  direction  of  the  thread,  so  that  if  the  thread 
were  continued  below  the  point  where  it  is  attached  to  the  body,  it  would  pass 
through  this  hole.  The  body  being  successively  suspended  by  several  differ- 
ent points  on  its  surface,  let  as  many  small  holes  be  bored  through  it  in  the 
same  manner.  If  the  body  be  then  cut  through,  so  as  to  discover  the  direc- 
tions which  the  several  holes  have  taken,  they  will  be  all  found  to  cross  each 
other  at  one  point  within  the  body ;  or  the  same  fact  may  be  discovered  thus  : 
a  thin  wire,  which  nearly  fills  the  holes,  being  passed  through  any  one  of 
them,  it  will  be  found  to  intercept  the  passage  of  a  similar  wire  through  any 
other. 

This  singular  fact  teaches  us — what,  indeed,  can  be  proved  by  mathematical 
reasoning  without  experiment — that  there  is  one  point  in  every  body  through 
which  the  single  force,  which  is  equivalent  to  the  gravitation  of  all  its  parti- 
cles, must  pass  in  whatever  position  the  body  be  placed.  This  point  is  called 
the  centre  of  gravity. 

In  whatever  situation  a  body  may  be  placed,  the  centre  of  gravity  will  have 
a  tendency  to  descend  in  the  direction  of  a  line  perpendicular  to  the  horizon, 
and  which  is  called  the  line  of  direction  of  the  weight.  If  the  body  be  alto- 
gether free  and  unrestricted  by  any  resistance  or  impediment,  the  centre  of 
gravity  will  actually  descend  in  this  direction,  and  all  the  other  points  of  the 
body  will  move  with  the  same  velocity  in  parallel  directions,  so  that,  during 
its  fall,  the  position  of  the  parts  of  the  body  with  respect  to  the  ground  will  be 
unaltered.  But  if  the  body,  as  is  most  usual,  be  subject  to  some  resistance 
or  restraint,  it  will  either  remain  unmoved,  its  weight  being  expended  in 
exciting  pressure  on  the  restraining  points  or  surfaces,  or  it  will  move  in 
a  direction  and  with  a  velocity  depending  on  the  circumstances  which  re- 
strain it. 

In  order  to  determine  these  effects — to  predict  the   pressure   produced  by 
the  weight  if  the  body  be  quiescent,  or  the  mixed  effects  of  motion  and  pres- 
sure if  it  be  not  so — it  is  necessary  in  all  cases  to  be  able  to  assign  the  place 
of  the  centre  of  gravity.     When  the  magnitude  and  figure,  of  the  body,  and  the 
\  density  of  the  matter  which  occupies  its  dimensions,  are  known,  the  place  of 
'  the  centre  of  gravity  can  be  determined  with  the  greatest  precision  by  mathe- 
\  matical  calculation.     The  process  by  which  this  is  accomplished,  however,  is 
'  not  of  a  sufficiently  elementary  nature  to  be  properly  introduced  into  this  trea- 
I  tise.     To  render  it  intelligible  would  require  the  aid  of  some  of  the  most  ad- 

>  vanced  analytical  principles  ;  and  even  to  express  the  position  of  the  point  in 
I  question,  except  in  very  particular  instances,  would  be  impossible,  without  the 

>  aid  of  peculiar  symbols. 

(  There  are  certain  particular  forms  of  body  in  which,  when  they  are  uni- 
)  formly  dense,  the  place  of  the  centre  of  gravity  can  be  easily  assigned,  and 


224 


CENTRE  OF  GRAVITY. 


proved  by  reasoning  which  is  generally  intelligible  ;  but  in  all  cases  whatever 
this  point  may  be  easily  determined  by  experiment. 

If  a  body  uniformly  dense  have  such  a  shape  that  a  point  may  be  found,  on 
either  side  of  which,  in  all  directions  around  it,  the  materials  of  the  body  are 
similarly  distributed,  that  point  will  obviously  be  the  centre  of  gravity.  For 
if  it  be  supported,  the  gravitation  of  the  particles  on  one  side  drawing  them 
downward,  is  resisted  by  an  effect  of  exactly  the  same  kind  and  of  equal  amount 
on  the  opposite  side,  and  so  the  body  remains  balanced  on  the  point. 

The  most  remarkable  body  of  this  kind  is  a  globe,  the  centre  of  which  is 
evidently  its  centre  of  gravity. 

A  figure,  such  as  fig.  2,  called  an  oblate  spheriod,  has  its  centre  of  gravity 

Fig.  2. 


at  its  centre,  C.     Such  is  the  figure  of  the  earth.     The  same  may  be  observed 
of  the  elliptical  solid,  fig.  3,  which  is  called  a  prolate  spheroid. 

Fig.  3. 


A  cube,  and  some  other  regular  solids,  bounded  by  plane  surfaces,  have  a 
point  within  them,  such  as  above  described,  and  which  is  therefore  their  centre 
of  gravity.     Such  are  figs.  4.  and  5. 


Fig.  4. 


Fig.  5. 


CENTRE  OF  GRAVITY. 


A  Straight  wand,  of  uniform  thickness,  has  its  centre  of  gravity  at  the  centre 
of  its  length  ;  and  a  cylindrical  body  has  its  centre  of  gravity  in  its  centre,  at  the 
middle  of  its  length  or  axis.     Such  is  the  point  C,  fig.  6. 


Fig.  6. 


Fig.  7. 


c 


A  flat  plate  of  any  uniform  substance,  and  which  has  in  every  part  an  equal 
thickness,  has  its  centre  of  gravity  at  the  middle  of  its  thickness,  and  under  a 
point  of  its  surface,  which  is  to  be  determined  by  its  shape.  If  it  be  circular 
or  elliptical,  this  point  is  its  centre.  If  it  have  any  regular  form,  bounded  by 
straight  edges,  it  is  that  point  which  is  equally  distant  from  its  several  angles, 
as  C  in  fig.  7. 

There  are  some  cases  in  which,  although  the  place  of  the  centre  of  gravity 
is  not  so  obvious  as  in  the  examples  just  given,  still  it  may  be  discovered  with- 
out any  mathematical  process,  which  is  not  easily  understood.  Suppose  ABC, 
fig.  8,  to  be  a  flat  triangular  plate  of  uniform  thickness  and  density.     Let  it  be 

Fig.  8. 


imagined  to  be  divided  into  narrow  bars,  by  lines  parallel  to  the  side  A  C,  as 
represented  in  the  figure.  Draw  B  D  from  the  angle  B  to  the  middle  point  D 
of  the  side  AC.  It  is  not  difficult  to  perceive  that  B  D  will  divide  equally  all 
the  bars  into  which  the  triangle  is  conceived  to  be  divided.  Now,  if  the  flat 
triangular  plate  A  B  C  be  placed  in  a  horizontal  position  on  a  straight  edge 
coinciding  with  the  line  B  D,  it  will  be  balanced ;  for  the  bars  parallel  to  A  C 
will  be  severally  balanced  by  the  edge  immediately  under  their  middle  point, 
since  that  middle  point  is  the  centre  of  gravity  of  each  bar.  Since,  then,  the 
triangle  is  balanced  on  the  edge,  the  centre  of  gravity  must  be  somewhere  im- 
mediately over  it,  and  must  therefore  be  within  the  plate,  at  some  point  under 
the  line  B  D. 

The  same  reasoning  will  prove  that  the  centre  of  gravity  of  the  plate  is  un- 
der the  line  A  E,  drawn  from  the  angle  A  to  the  middle  point  E  of  the  side 
B  C.  To  perceive  this,  it  is  only  necessary  to  consider  the  triangle  divided 
into  bars  parallel  to  B  C,  and  thence  to  show  that  it  will  be  balanced  on  an 

VOL,.  II 15 


226 


CENTRE  OF  GRAVITY. 


edge  placed  under  A  E.  Since,  then,  the  centre  of  gravity  of  the  plate  is 
under  the  line  B  D,  and  also  under  A  E,  it  must  be  under  the  point  G,  at 
which  these  lines  cross  each  other  ;  and  it  is  accordingly  at  a  depth  beneath  G, 
equal  to  half  the  thickness  of  the  plate. 

This  may  be  experimentally  verified  by  taking  a  piece  of  tin  or  card,  and 
cutting  it  into  a  triangular  form.  The  point  G  being  found  by  drawing  B  D 
and  A  E,  which  divide  two  sides  equally,  it  will  be  balanced  if  placed  upon 
the  point  of  a  pin  at  G. 

The  centre  of  gravity  of  a  triangle  being  thus  determined,  we  shall  be  able 
to  find  the  position  of  the  centre  of  gravity  of  any  plate  of  uniform  thickness 
and  density  which  is  bounded  by  straight  edges. 

The  centre  of  gravity  is  not  always  included  within  the  volume  of  the  body, 
that  is,  it  is  not  enclosed  by  its  surfaces.  Numerous  examples  of  this  can  be 
produced.  If  a  piece  of  wire  be  bent  into  any  form,  the  centre  of  gravity  will 
rarely  be  in  the  wire.  Suppose  it  be  brought  to  the  form  of  a  ring.  In  that 
case,  the  centre  of  gravity  of  the  wire  will  be  the  centre  of  the  circle,  a  point 
not  forming  any  part  of  the  wire  itself :  nevertheless  this  point  may  be  proved 
to  have  the  characteristic  property  of  the  centre  of  gravity  ;  for  if  the  ring  be 
suspended  by  any  point,  the  centre  of  the  ring  must  always  settle  itself  under 
the  point  of  suspension.  If  this  centre  could  be  supposed  to  be  connected 
with  the  ring  by  very  fine  threads,  whose  weight  would  be  insignificant,  and 
which  might  be  united  by  a  knot  or  otherwise  at  the  centre,  the  ring  would  be 
balanced  upon  a  point  placed  under  the  knot. 

In  like  manner,  if  the  wire  be  formed  into  an  ellipse,  or  any  other  curve 
similarly  arranged  round  a  centre  point,  that  point  will  be  its  centre  of  gravity. 

To  find  the  centre  of  gravity  experimentally,  the  method  explained  in  fig.  1 
may  be  used.  In  this  case  two  points  of  suspension  will  be  sufficient  to  de- 
termine it  ;  for  the  directions  of  the  suspending  cord,  being  continued  through 
the  body,  will  cross  each  other  at  the  centre  of  gravity.  These  directions  may 
also  be  found  by  placing  the  body  on  a  sharp  point,  and  adjusting  it  so  as  to  be 
balanced  upon  it.  In  this  case,  a  line  drawn  through  the  body  directly  upward 
from  the  point  will  pass  through  the  centre  of  gravity,  and  therefore  two  such 
lines  must  cross  at  that  point. 

If  the  body  have  two  flat  parallel  surfaces,  like  sheet  metal,  stiff*  paper,  card, 
board,  &c.,  the  centre  of  gravity  may  be  found  by  balancing  the  body  in  two 
positions  on  a  horizontal  straight  edge.  The  point  where  the  lines  marked  by 
the  edge  cross  each  other  will  be  immediately  under  the  centre  of  gravity. 
This  may  be  verified  by  showing  that  the  body  will  be  balanced  on  a  point 
thus  placed,  or  that,  if  it  be  suspended,  the  point  thus  determined  will  always 
come  under  the  point  of  suspension. 

The  position  of  the  centre  of  gravity  of  such  bodies  may  also  be  found  by 
placing  the  body  on  a  horizontal  table  having  a  straight  edge.  The  body  being 
moved  beyond  the  edge  until  it  is  in  that  position  in  which  the  slightest  distur- 
bance will  cause  it  to  fall,  the  centre  of  gravity  will  then  be  immediately  over 
the  edge.  This  being  done  in  two  positions,  the  centre  of  gravity  will  be  de- 
termined as  before. 

It  has  been  already  stated  that  when  the  body  is  perfectly  free,  the  centre 
of  gravity  must  necessarily  move  downward,  in  a  direction  perpendicular  to  a 
horizontal  plane.  When  the  body  is  not  free,  the  circumstances  which  re- 
strain it  geuerally  permit  the  centre  of  gravity  to  move  in  certain  directions, 
but  obstruct  its  motion  in  others.  Thus,  if  a  body  be  suspended  from  a  fixed 
point  by  a  flexible  cord,  the  centre  of  gravity  is  free  to  move  in  every  direc- 
tion except  those  which  would  carry  it  farther  from  the  point  of  suspension 
than  the  length  of  the  cord.     Hence  if  we  conceive  a  globe  or  sphere  to  sur- 


CENTRE  OF  GRAVITY. 


227 


round  the  point  of  suspension  on  every  side  to  a  distance  equal  to 'that  of  the 
centre  of  gravity  from  the  point  of  suspension,  w^hen  the  cord  is  fully  stretched, 
the  centre  of  gravity  will  be  at  liberty  to  move  in  every  direction  within  this 
sphere. 

There  are  an  infinite  variety  of  circumstances  under  which  the  motion  of  a 
body  may  be  restrained,  and  in  which  a  most  important  and  useful  class  of 
mechanical  problems  originate.  Before  we  notice  others,  we  shall,  however, 
examine  that  which  has  just  been  described  more  particularly. 


Let  P,  fig.  9,  be  the  point  of  suspension,  and  C  the  centre  of  gravity,  and 
suppose  the  body  to  be  so  placed  that  C  shall  be  within  the  sphere  already 
described.  The  cord  will  therefore  be  slackened,  and  in  this  state  the  body 
will  be  free.  The  centre  of  gravity  will  therefore  descend  in  the  perpendicu- 
lar direction  until  the  cord  becomes  fully  extended  ;  the  tension  will  then  pre- 
vent its  further  motion  in  the  perpendicular  direction.  The  downward  force 
must  now  be  considered  as  the  diagonal  of  a  parallelogram,  and  equivalent  to 
two  forces  C  D  and  C  E,  in  the  directions  of  the  sides,  as  already  explained 
in  fig.  1.  The  force  C  D  will  bring  the  centre  of  gravity  into  the  direction 
P  F,  perpendicularly  under  the  point  of  suspension.  Since  the  force  of  grav- 
ity acts  continually  on  C  in  its  approach  to  P  F,  it  will  move  toward  that  line 
with  accelerated  speed,  and  when  it  has  arrived  there,  it  will  have  acquired  a 
force  to  which  no  obstruction  is  immediately  opposed,  and  consequently  by  its 
inertia  it  retains  this  force,  and  moves  beyond  P  F  on  the  other  side.  But 
when  the  point  C  gets  into  the  line  P  F,  it  is  in  the  lowest  possible  position  ; 
for  it  is  at  the  lowest  point  of  the  sphere  which  limits  its  motion.  When  it 
passes  to  the  other  side  of  P  F,  it  must  therefore  begin  to  ascend,  and  the 
force  of  gravity,  which  in  the  former  case  accelerated  its  descent,  will  now, 
for  the  same  reason,  and  with  equal  energy,  oppose  its  ascent.  This  will  be 
easily  understood.  Let  C  be  any  point  which  it  may  have  attained  in  ascend- 
ing :  C  G',  the  force  of  gravity,  is  now  equivalent  to  C  D'  and  C  E'.  The 
latter,  as  before,  produces  tension  ;  but  the  former,  C  D',  is  in  a  direction  im- 
mediately opposed  to  the  motion,  and  therefore  retards  it.  This  retardation 
will  continue  until  all  the  motion  acquired  by  the  body  in  its  desceftt  from  the 
first  position  has  been  destroyed,  and  then  it  will  begin  to  return  to  P  F,  and 
so  it  will  continue  to  vibrate  I'rom  the  one  side  to  the  other  until  the  friction  on 
the  point  P,  and  the  resistance  of  the  air,  gradually  deprive  it  of  its  motion,  and 
bring  it  to  a  state  of  rest  in  the  direction  P  F. 


228 


CENTRE  OF  GRAVITY. 


But  for -the  effects  of  friction  and  atmospheric  resistance,  the  body  would 
continue  for  ever  to  oscillate  equally  from  side  to  side  of  the  line  P  F. 

The  phenomenon  just  developed  is  only  an  example  of  an  extensive  class. 
Whenever  the  circumstances  which  restrain  the  body  are  of  such  a  nature  that 
the  centre  of  gravity  is  prevented  from  descending  below  a  certain  level,  but 
not,  on  the  other  hand,  restrained  from  rising  above  it,  the  body  will  remain  at 
rest  if  the  centre  of  gravity  be  placed  at  the  lowest  limit  of  its  level ;  any  dis- 
turbance will  cause  it  to  oscillate  around  this  state,  and  it  cannot  return  to  a 
state  of  rest  until  friction  or  some  other  cause  have  deprived  it  of  the  motion 
communicated  by  the  disturbing  force. 

Under  the  circumstances  which  we  have  just  described,  the  body  could  not 
maintain  itself  in  a  state  of  rest  in  any  position  except  that  in  which  the  centre 
of  gravity  is,  at  the  lowest  point  of  the  space  in  which  it  is  free  to  move.  This, 
however,  is  not  always  the  case.  Suppose  it  were  suspended  by  an  inflex- 
ible rod  instead  of  a  flexible  string  :  the  centre  of  gravity  would  then  not  only 
be  prevented  from  receding  from  the  point  of  suspension,  but  also  from  ap- 
proaching it ;  in  fact,  it  would  be  always  kept  at  the  same  distance  from  it. 
Thus,  instead  of  being  capable  of  moving  anywhere  within  the  sphere,  it  is 
now  capable  of  moving  on  its  surface  only.  The  reasoning  used  in  the  last 
case  may  also  be  applied  here,  to  prove  that  when  the  centre  of  gravity  is  on 
either  side  of  the  perpendicular  P  F,  it  will  fall  toward  P  F,  and  oscillate,  and 
that,  if  it  be  placed  in  the  line  P  F,  it  will  remain  in  equilibrium.  But  in  this 
case  there  is  another  position,  in  which  the  centre  of  gravity  may  be  placed 
so  as  to  produce  equilibrium.  If  it  be  placed  at  the  highest  point  of  the  sphere 
in  which  it  moves,  the  whole  force  on  it  will  then  be  directed  on  the  point  of 
suspension,  perpendicularly  downward,  and  will  be  entirely  expended  in  pro- 
ducing pressure  on  that  point ;  consequently  the  body  will  in  this  case  be  in 
equilibrium.  But  this  state  of  equilibrium  is  of  a  character  very  different  from 
that  in  which  the  centre  of  gravity  was  at  the  lowest  part  of  the  sphere.  In 
the  present  case,  any  displacement,  however  slight,  of  the  centre  of  gravity, 
will  carry  it  to  a  lower  level,  and  the  force  of  gravity  will  then  prevent  its  re- 
turn to  its  former  state,  and  will  impel  it  downward  until  it  attain  the  lowest 
point  of  the  sphere,  and  round  that  point  it  will  oscillate. 

The  two  stales  of  equilibrium  which  have  been  just  noticed  are  called  stable 
and  instable  equilibrium.  The  character  of  the  former  is,  that  any  disturbance 
of  the  state  produces  oscillation  about  it ;  but  any  disturbance  of  the  latter  state 
produces  a  total  overthrow,  and  finally  causes  oscillation  around  the  state  of 
stable  equilibrium. 

Let  A  B,  ficr.  10,  be  an  elliptical  board  resting  on  its  edge  on  a  horizontal 
plane.     In  the  position  here  represented,  the  extremity  P  of  the  lesser  axis 


Fig.  10. 


being  the  point  of  support,  the  board  is  in  stable  equilibrium  ;  for  any  motion 
on  etther  side  must  cause  the  centre  of  gravity  C  to  ascend  in  the  directions 
C  0,  and  oscillation  will  ensue.  If,  however,  it  rest  upon  the  smaller  end,  as 
in  fig.  11,  the  position  would  still  be  a  state  of  equilibrium,  because  the  centre 

Fig.  11. 


of  gravity  is  directly  above  the  point  of  support ;  but  it  would  be  instable  equi- 
librium, because  the  slightest  displacement  of  the  centre  of  gravity  would  cause 
it  to  descend. 

Thus  an  egg  or  a  lemon  may  be  balaiiced  on  the  end  ;  but  the  least  distur- 
bance will  overthrow  it.  On  the  contrary,  it  will  easily  rest  on  the  side,  and 
any  disturbance  will  produce  oscillation. 

When  the  circumstances  under  which  the  body  is  placed  allow  the  centre 
of  gravity  to  move  only  in  a  horizontal  line,  the  body  is  in  a  state  which  may 
be  called  neutral  equilibrium.  The  slightest  force  will  move  the  centre  of 
gravity,  but  will  neither  produce  oscillation  nor  overthrow  the  body,  as  in  the 
last  two  cases. 

An  example  of  this  state  is  furnished  by  a  cylinder  placed  upon  a  horizontal 
plane.  As  the  cylinder  is  railed  upon  the  plane,  the  centre  of  gravity  C,  fig. 
12,  moves  in  a  line  parallel  to  the  plane  A  B,  and  distant  from  it  by  the  radius 

Fig.  12. 


of  the  cylinder.  The  body  will  thus  rest  indifl'erently  in  any  position,  because 
the  Une  of  direction  always  falls  upon  a  point  P  at  which  the  body  rests  upon 
the  plane. 

If  the  plane  were  inclined,  as  in  fig.  13,  a  body  might  be  so  shaped,  that, 


230 


CENTRE  OF  GRAVITY. 


Fi?.  13. 


while  it  would  roll,  the  centre  of  gravity  would  move  horizontally.  In  this 
case,  the  body  would  rest  indifTerenlly  on  any  part  of  the  plane,  as  if  it  were 
horizontal,  provided  the  friction  be  sufficient  to  prevent  the  body  from  sliding 
down  the  plane. 

If  the  centre  of  gravity  of  a  cylinder  happen  not  to  coincide  with  its  centre, 
by  reason  of  the  want  of  uniformity  in  the  materials  of  which  it  is  composed, 
it  will  not  be  in  a  state  of  neutral  equilibrium  on  a  horizontal  plane,  as  in  fig. 
12.  In  this  case,  let  G,  fig.  14,  be  the  centre  of  gravity.  In  the  position  here 
represented,  where  the  centre  of  gravity  is  immediately  below  the  centre  C,  the 
state  will  be  stable  equilibrium,  because  a  motion  on  either  side  would  cause 
the  centre  of  gravity  to  ascend  ;  but  in  fig.  15,  where  G  is  immediately  above 


Fig.  14. 


Fig.  15. 


C,  the  state  is   instable  equilibrium,  because  a   motion  on  either  side  would 
cause  G  to  descend,  and  the  body  would  turn  into  the  position  fig.  14. 

A  cylinder  of  this  kind  will,  under  certain  circumstances,  roll  up  an  inclined 
plane.  Let  A  B,  fig.  16,  be  the  inclined  plane,  and  let  the  cylinder  be  so 
placed  that  the  line  of  direction  from  G  shall  be  above  the  point  P  at  which 
the  cylinder  rests  upon  the  plane.  The  whole  weight  of  the  body  acting  in 
the  direction  G  D  will  obviously  cause  the  cylinder  to  roll  toward  A,  provided 
the  friction  be  sufficient  to  prevent  sliding  ;  but  although  the  cylinder  in  this 
case  ascends,  the  centre  of  gravity  G  really  descends. 


Fig.  16. 


When  G  is  so  placed  that  the  line  of  direction  G  D  shall  fall  on  the  point 
P,  the  cylinder  will  be  in  equilibrium,  because  its  weight  acts  upon  the  point 
on  which  it  rests.  There  are  two  cases  represented  in  fig.  17  and  fig.  18,  in 
which  G  takes  this  position.  Fig.  17  represents  the  state  of  stable,  and  fig.  18 
of  instable  equilibrium. 


Fig.  18. 


When  a  body  is  placed  upon  a  base,  its  stability  depends  upon  the  position 
of  the  line  of  direction  and  the  height  of  the  centre  of  gravity  above  the  base. 
If  the  line  of  direction  fall  within  the  base,  the  body  will  stand  firm  ;  if  it  fall 
on  the  edge  of  the  base,  it  will  be  in  a  state  in  which  the  slightest  force  will 
overthrow  it  on  that  side  at  which  the  line  of  direction  falls  ;  and  if  the  line  of 
direction  fall  without  the  base,  the  body  must  turn  over  that  edge  which  is 
nearest  to  the  line  of  direction. 

In  fig.  19  and  fig.  20,  the  line  of  direction  G  P  falls  within  the  base,  and  it 


Fig.  19. 


Fiff.  20. 


G 

I 


is  obvious  that  the  body  will  stand  firm ;  for  any  attempt  to  turn  it  over  either 
edge  would  cause  the  centre  of  gravity  to  ascend.     But  in  fig.  21,  the  line  of 


232 


CENTRE  OF  GRAVITY. 


direction  falls  upon  the  edge,  and  if  the  body  be  turned  over,  the  centre  of 
gravity  immediately  commences  to  descend.  Until  it  be  turned  over,  however, 
the  centre  of  gravity  is  supported  by  the  edge. 

In  fig.  22,  the  line  of  direction  falls  outside  the  base,  the  centre  of  gravity 


Fig.  21. 


Fig- 


has  a  tendency  to  descend  from  G  toward  A,  and  the  body  will  accordingly 
fall  in  that  direction. 

When  the  line  of  direction  falls  within  the  base,  bodies  will  always  stand 
firm,  but  not  with  the  same  degree  of  stability.  In  general,  the  stability  de- 
pends on  the  height  through  which  the  centre  of  gravity  must  be  elevated  be- 
fore the  body  can  be  overthrown.  The  greater  this  height  is,  the  greater  in 
the  same  proportion  will  be  the  stability. 

Let  BAG,  fig.  23,  be  a  pyramid,  the  centre  of  gravity  being  at  G.     To 


Fig.  23. 


turn  this  over  the  edge  B,  the  centre  of  gravity  must  be  carried  over  the  arch 
G  E,  and  must  therefore  be  raised  through  the  height  H  E.  If,  however,  the 
pyramid  were  taller  relatively  to  its  base,  as  in  fig.  24,  the  height  H  E  would 
be  proportionally  less  ;  and  if  the  base  were  very  small  in  reference  to  the 
height,  as  in  fig.  25,  the  height  H  E  would  be  very  small,  and  a  slight  force 
would  throw  it  over  the  edge  B. 

It  is  obvious  that  the  same  observations  may  be  applied  to  all  figures  what- 
ever, the  conclusions  just  deduced  depending  only  on  the  distance  of  the  line 
of  direction  from  the  edge  of  the  base,  and  the  height  of  the  centre  of  gravity 
above  it. 

Hence  we  may  perceive  the  principle  on  which  the  stability  of  loaded  car- 
riages depends.  When  the  load  is  placed  at  a  considerable  elevation  above 
the  wheels,  the  centre  of  gravity  is  elevated,  and  the  carriage  becomes  pro- 
portionally insecure.     In  coaches  for  the  conveyance  of  passengers,  the  lug- 


CENTRE  OF  GRAVITY. 


233 


Fig.  24. 


gage  is  therefore  sometimes  placed  below  the  body  of  the  coach  ;  light  parcels 
of  large  bulk  may  be  placed  on  the  top  with  impunity. 

When  the  centre  of  gravity  of  a  carriage  is  much  elevated,  there  is  consid- 
erable danger  of  overthrow,  if  a  corner  be  turned  sharply  and  with  a  rapid  pace  ; 
for  the  centrifugal  force  then  acting  on  the  centre  of  gravity  will  easily  raise 
it  through  the  small  height  which  is  necessary  to  turn  the  carriage  over  the 
external  wheels. 

Fig.  25. 


The  same  wagon  will  have  greater  stability  when  loaded  with  a  heavy  sub- 
stance which  occupies  a  small  space,  such  as  metal,  than  when  it  carries  the 
same  weight  of  a  lighter  substance,  such  as  hay  ;  because  the  centre  of  gravity 
in  the  latter  case  will  be  much  more  elevated. 

If  a  large  table  be  placed  upon  a  single  leg  in  its  centre,  it  will  be  imprac- 
ticable to  make  it  stand  firm ;  but  if  the  pillar  on  which  it  rests  terminate  in  a 
tripod,  it  will  have  the  same  stability  as  if  it  had  three  legs  attached  to  the 
points  directly  over  the  places  where  the  feet  of  the  tripod  rest. 

When  a  solid  body  is  supported  by  more  points  than  one,  it  is  not  necessary 
for  its  stability  that  the  line  of  direction  should  fall  on  one  of  those  points. 
If  there  be  only  two  points  of  support,  the  line  of  direction  must  fall  between 
them.  The  body  is  in  this  case  supported  as  effectually  as  if  it  rested  on  an 
edge  coinciding  with  a  straight  line  drawn  from  one  point  of  support  to  the 


234 


CENTRE  OF  GRAVITY. 


Other.  If  there  be  three  points  of  support,- which  are  not  ranged  in  the  same 
straight  line,  the  body  will  be  supported  in  the  same  manner  as  it  would  be  by 
a  base  coinciding  with  the  triangle  formed  by  straight  lines  joining  the  three 
points  of  support.  In  the  same  manner,  whatever  be  the  number  of  points  on 
which  the  body  may  rest,  its  virtual  base  will  be  found  by  supposing  straight 
lines  drawn,  joining  the  several  points  successively.  When  the  line  of  direc- 
tion falls  within  this  base,  the  body  will  always  stand  firm,  and  otherwise  not. 
The  degre'e  of  stability  is  determined  in  the  same  manner  as  if  the  base  were 
a  continued  surface. 

Necessity  and  experience  teach  an  animal  to  adapt  its  postures  and  motions 
to  the  position  of  the  centre  of  gravity  of  his  body.  When  a  man  stands,  the 
line  of  direction  of  his  weight  must  fall  within  the  base  formed  by  his  feet.  If 
A  B  C  D,  fig.  26,  be  the  feet,  this  base  is  the  space  A  B  C  D.     It  is  evident 


Fig.  26. 

...M 


D 


that  the  more  his  toes  are  turned  outward,  the  more  contracted  the  base  will 
be  in  the  direction  E  F,  and  the  more  liable  he  will  be  to  fall  backward  or  for- 
ward. Also  the  closer  his  feet  are  together,  the  more  contracted  the  base 
will  be  in  the  direction  G  H,  and  the  more  liable  he  will  be  to  fall  toward 
either  side. 

When  a  man  walks,  the  legs  are  alternately  lifted  from  the  ground,  and  the 
centre  of  gravity  is  either  unsupported  or  thrown  from  the  one  side  to  the  other. 
The  body  is  also  thrown  a  little  forward,  in  order  that  the  tendency  of  the 
centre  of  gravity  to  fall  in  the  direction  of  the  toes  may  assist  the  muscular  ac- 
tion in  propelling  the  body.  This  forward  inclination  of  the  body  increases 
with  the  speed  of  the  motion. 

But  for  the  flexibility  of  the  knee-joint,  the  labor  of  walking  would  be  much 
greater  than  it  is ;  for  the  centre  of  gravity  would  be  more  elevated  by  each 
step.  The  line  of  motion  of  the  centre  of  gravity  in  walking  is  represented  by 
fig.  27,  and  deviates  but  little  from  a  regular  horizontal  line,  so  that  the  eleva- 

Fig.  27. 


tion  of  the  centre  of  gravity  is  subject  to  very  slight  variation.     But  if  there 
were  no  knee-joint,  as  when  a  man  has  wooden  legs,  the  centre  of  gravity 


CENTRE  OF  GRAVITY. 


235 


would  move  as  in  fig.  28,  so  that  at  eacH  step  the  weight  of  the  body  would 
be  lifted  through  a  considerable  height,  and  therefore  the  labor  of  walking 
would  be  much  increased. 


Fig.  28. 


If  a  man  stand  on  one  leg,  the  line  of  direction  of  his  weight  must  fall  within 
the  space  on  which  his  foot  treads.  The  smallness  of  this  space,  compared 
with  the  height  of  the  centre  of  gravity,  accounts  for  the  difficulty  of  this  feat. 

The  position  of  the  centre  of  gravity  of  the  body  changes  with  the  posture 
and  position  of  the  limbs.  If  the  arm  be  extended  from  one  side,  the  centre 
of  gravity  is  brought  nearer  to  that  side  than  it  was  when  the  arm  hung  per- 
pendicularly. When  dancers,  standing  on  one  leg,  extend  the  other  at  right 
angles  to  it,  they  must  incline  the  body  in  the  direction  opposite  to  that  in 
which  the  leg  is  extended,  in  order  to  bring  the  centre  of  gravity  over  the  foot 
which  supports  them. 

Fig.  29. 


When  a  porter  carries  a  load,  his  position  must  be  regulated  by  the  centre 
of  gravity  of  his  body  and  the  load  taken  together.     If  he  bore  the  load  on  his 


Fig.  30. 


236 


CENTRE  OF  GRAVITY. 


back,  fig.  29,  the  line  of  direction  would  pass  beyond  his  heels,  and  he  would  fall 
backward.  To  bring  the  centre  of  gravity  over  his  feet,  he  accordingly  leans 
forward,  fig.  30. 

If  a  nurse  carry  a  child  in  her  arms,  she  leans  back  for  a  like  reason. 

When  a  load  is  carried  on  the  head,  the  bearer  stands  upright,  that  the  centre 
of  gravity  may  be  over  his  feet.  In  ascending  a  hill,  we  appear  to  incline  for- 
ward, and  in  descending,  to  lean  backward  ;  but  in  truth  we  are  standing  up- 
right with  respect  to  a  level  plane.  This  is  necessary  to  keep  the  line  of 
direction  between  the  feet,  as  is  evident  from  fig.  31. 

Fig.  31. 


A  person  sitting  on  a  chair  which  has  no  back,  cannot  rise  from  it  without 
either  stooping  forward  to  bring  the  centre  of  gravity  over  the  feet,  or  drawing 
back  the  feet  to  bring  them  under  the  centre  of  gravity. 

•     A  quadruped  never  raises  both  feet  on  the  same  side  simultaneously,  for  the 
centre  of  gravity  would  then  be  unsupported.     Let  A  B  C  D,  fig.  32,  be  the 


Fig.  32. 


feet.  The  base  on  which  it  stands  is  A  B  C  D,  and  the  centre  of  gravity  is 
nearly  over  the  point  O,  where  the  diagonals  cross  each  other.  The  legs  A 
and  C  being  raised  together,  the  centre  of  gravity  is  supported  by  the  legs  B 
and  D,  since  it  falls  between  them  ;  and  when  B  and  D  are  raised,  it  is,  in 
like  manner,  supported  by  the  feet  A  and  C.     The  centre  of  gravity,  however. 


CENTRE  OF  GRAVITY. 


237 


is  often  unsupported  for  a  moment ;  for  the  leg  B  is  raised  from  the  ground 
before  A  comes  to  it,  as  is  plain  from  observing  the  track  of  a  horse's  feet,  the 
mark  of  A  being  upon  or  before  that  of  B.  In  the  more  rapid  paces  of  all 
animals  the  centre  of  gravity  is  at  intervals  unsupported. 

The  feats  of  rope-dancers  are  experiments  on  the  management  of  the  centre 
of  gravity.  The  evolution's  of  the  performer  are  found  to  be  facilitated  by 
holding  in  his  hand  a  heavy  pole.  His  security  in  this  case  depends,  not  on 
the  centre  of  gravity  of  his  body,  but  on  that  of  his  body  and  the  pole  taken 
together.  This  point  is  near  the  centre  of  the  pole,  so  that,  in  fact,  he  may  be 
said  to  hold  in  his  hands  the  point  on  the  position  of  which  the  facility  of  his 
feats  depends.  Without  the  aid  of  the  pole,  the  centre  of  gravity  would  be 
within  the  trunk  of  the  body,  and  its  position  could  not  be  adapted  to  circum- 
stances with  the  same  ease  and  rapidity. 

The  centre  of  gravity  of  a  mass  of  fluid  is  that  point  which  would  have  the 
properties  which  have  been  proved  to  belong  to  the  centre  of  gravity  of  a  solid, 
if  the  fluid  were  solidified  without  changing  in  any  respect  the  quantity  or  ar- 
rangement of  its  parts. 

The  centre  of  gravity  of  two  bodies  separated  from  one  another,  is  that  point 
which  would  possess  the  properties  ascribed  to  the  centre  of  gravity  if  the  two 
bodies  were  united  by  an  inflexible  line,  the  weight  of  which  might  be  neglected. 
To  find  this  point  mathematically  is  a  very  simple  problem.   Let  A  B,  fig.  33,  be 


Fig.  33. 


the  two  bodies,  and  a  and  b  their  centres  of  gravity.  Draw  the  right  line  a  h, 
and  divide  it  at  C,  in  such  a  manner  that  a  C  shall  have  the  same  proportion 
to  5  C  as  the  mass  of  the  body  B  has  to  the  mass  of  the  body  A. 

This  may  easily  be  verified  experimentally.  Let  A  and  B  be  two  bodies, 
whose  weight  is  considerable,  in  comparison  with  that  of  the  rod  a  b,  which 
joins  them.  Let  a  fine  silken  string,  with  its  ends  attached  to  them,  be  hung 
upon  a  pin,  and  on  the  same  pin  let  a  plumb-line  be  suspended.  In  whatever 
position  the  bodies  may  be  hung,  it  will  be  observed  that  the  plumb-line  will 
cross  the  rod  a  6  at  the  same  point,  and  that  point  will  divide  the  line  a  b  into 
parts  a  C  and  b  C,  which  are  in  the  proportion  of  the  mass  of  B  to  the  mass  of  A. 

The  centre  of  gravity  of  three  separate  bodies  is  defined  in  the  same  man- 
ner as  that  of  two,  and  may  be  found  by  first  determining  the  centre  of  gravity 
of  two,  and  then  supposing  their  masses  concentrated  at  that  point,  so  as  to 
form  one  body,  and  finding  the  centre  of  gravity  of  that  and  the  third. 

In  the  same  manner  the  centre  of  gravity  of  any  number  of  bodies  may  be 
determined. 

If  a  plate  of  uniform  thickness  be  bounded  by  straight  edges,  its  centre  of 
gravity  may  be  found  by  dividing  it  into  triangles  by  diagonal  lines,  as  in  fig. 
34,  and,  having  determined  the  centres  of  gravity  of  the  several  triangles,  the 
centre  of  gravity  of  the  whole  plate  will  be  their  common  centre  of  gravity 
found  as  above. 


238 


CENTRE  OP  GRAVITY. 


Fig.  34. 


Although  the  centre  of  gravity  takes  its  name  from  the  familiar  properties 
which  it  has  in  reference  to  detached  bodies  of  inconsiderable  magnitude, 
placed  on  or  near  the  surface  of  the  earth,  yet  it  possesses  properties  of  a 
much  more  general  and  not  less  important  nature.  One  of  the  most  remarka- 
ble of  these  is,  that  the  centre  of  gravity  of  any  number  of  separate  bodies  is 
never  affected  by  the  mutual  attraction,  impact,  or  other  influence  which  the 
bodies  may  transmit  from  one  to  another.  This  is  a  necessary  consequence 
of  the  equality  of  action  and  reaction  ;  for  if  A  and  B,  fig.  33,  attract  each 
other,  and  change  their  places  to  A'  B',  the  space  a  a'  will  have  to  b  b'  the 
same  proportion  as  B  has  to  A,  and  therefore,  by  what  has  just  been  proved  in 
fig.  33,  the  same  proportion  as  a  C  has  to  i  C.  It  follows  that  the  remainders 
a'  C  and  b'  C  will  be  in  the  proportion  of  B  to  A,  and  that  C  will  continue  to 
be  the  centre  of  gravity  of  the  bodies  after  ihey  have  approached  by  their 
mutual  attraction. 

Suppose,  for  example,  that  A  and  B  were  twelve  pounds  and  eight  pounds 
respectively,  and  that  a  b  were  forty  feet.  The  point  C  must  divide  a  b  into 
two  parts,  in  the  proportion  of  eight  to  twelve,  or  of  two  to  three.  Hence  it 
is  obvious  that  a  C  will  be  sixteen  feet,  and  b  C  twenty-four  feet.  Now,  sup- 
pose that  A  and  B  attract  each  other,  and  that  A  approaches  B  through  two 
feet.  Then  B  must  approach  A  through  three  feet.  Their  distances  from  C 
will  now  be  fourteen  feet  and  twenty-one  feet,  which,  being  in  the  proportion 
of  B  to  A,  the  point  C  will  still  be  their  centre  of  gravity. 

Hence  it  follows,  that  if  a  system  of  bodies,  placed  at  rest,  be  permitted  to 
obey  their  mutual  attractions,  although  the  bodies  will  thereby  be  severally 
moved,  yet  their  common  centre  of  gravity  must  remain  quiescent. 

When  one  of  two  bodies  is  moving  in  a  straight  line,  the  other  being  at  rest, 
their  common  centre  of  gravity  must  move  in  a  parallel  straight  line.  Let  A 
and  B,  fig.  35,  be  the  centres  of  gravity  of  the  bodies,  and  let  A  move  from  A 
to  a,  B  remaining  at  rest.  Draw  the  lines  A  B  and  a  B.  In  every  position 
which  the  body  B  assumes  during  its  motion,  the  centre  of  gravity  C  divides 
the  line  joining  them  into  parts  A  C,  B  C,  which  are  in  the  proportion  of  the 
mass  B  to  the  mass  A.  Now,  suppose  any  number  of  lines  drawn  from  B  to 
the  line  A  a ;  a  parallel  C  c  to  A  a  through  C  divides  all  these  lines  in  the 
same  proportion  ;  and  therefore,  while  the  body  A  moves  from  A  to  a,  the  com- 
mon centre  of  gravity  moves  from  C  to  c. 

If  both  the  bodies  A  and  B  moved  uniformly  in  straight  lines,  the  centre  of 
gravity  would  have  a  motion  compounded  of  the  two  motions  with  which  it 
would  be  affected,  if  each  moved  while  the  other  remained  at  rest.  In  the 
same  manner,  if  there  were  three  bodies,  each  moving  imiformly  in  a  straight 
line,  their  common  centre  of  gravity  would  have  a  motion  compounded  of  that 
^motion  which  it  would  have  if  one  remained  at  rest  while  the  other  two  moved, 


CENTRE  OF  GEAVITY. 


239 


Fi-.  35. 


and  that  which  the  motion  of  the  first  would  give  it  if  the  hist  two  remained  at 
rest  ;  and  in  the  same  manner  it  may  be  proved,  that  when  any  number  of 
bodies  move  each  in  a  straight  Hne,  their  common  centre  of  gravity  will  have 
a  motion  compounded  of  the  motions  which  it  receives  from  the  bodies  sev- 
erally. 

It  may  happen  that  the  several  motions  which  the  centre  of  gravity  receives 
from  the  bodies  of  the  system  will  neutralize  each  other  ;  and  this  does,  in  fact, 
take  place  for  such  motions  as  are  the  consequences  of  the  mutual  action  of 
the  bodies  upon  one  another. 

If  a  system  of  bodies  be  not  under  the  immediate  influence  of  any  forces, 
and  their  mutual  attraction  be  conceived  to  be  suspended,  they  must  severally  be 
either  at  rest  or  in  uniform  rectilinear  motion  in  virtue  of  their  inertia.  Hence 
their  common  centre  of  gravity  must  also  be  either  at  rest  or  in  uniform  recti- 
linear motion.  Now,  if  we  suppose  their  mutual  attractions  to  take  effect,  the 
state  of  their  common  centre  of  gravity  will  not  be  changed,  but  the  bodies 
will  severally  receive  motions  compounded  of  their  previous  uniform  rectilinear 
motions  and  those  which  result  from  their  mutual  attractions.  The  combined 
effects  will  cause  each  body  to  revolve  in  an  orbit  round  the  common  centre  of 
gravity,  or  will  precipitate  it  toward  that  point.  But  still  that  point  will  main- 
tain its  former  state  undisturbed. 

This  constitutes  one  of  the  general  laws  of  mechanical  science,  and  is  of 
great  importance  in  physical  astronomy.  It  is  known  by  the  title  "the  con- 
servation of  the  motion  of  the  centre  of  gravity." 

The  solar  system  is  an  instance  of  the  class  of  phenomena  to  which  we 
have  just  referred.  All  the  motions  of  the  bodies  which  compose  it  can  be 
traced  to  certain  uniform  rectilinear  motions,  received  from  some  former  im- 
pulse, or  from  a  force  whose  action  has  been  suspended,  and  those  motions 
which  necessarily  result  from  the  principle  of  gravitation.  But  we  shall  not 
here  insist  further  on  this  subject,  which  more  properly  belongs  to  another 
department  of  the  science. 

If  a  solid  body  suffer  an  impact  in  the  direction  of  a  line  passing  through  its 
centre  of  gravity,  all  the  particles  of  the  body  will  be  driven  forward  with  the 
same  velocity  in  lines  parallel  to  the  direction  of  the  impact,  and  the  whole 
force  of  the  motion  will  be  equal  to  that  of  the  impact.  The  impelling  force 
being  equally  distributed  among  all  the  parts,  the  velocity  will  be  found  by 
dividing  the  numerical  value  of  that  force  by  the  number  expressing  the  mass. 

If  any  number  of  impacts  be  given  simultaneously  to  different  points  of  a 
body,  a  certain  complex  motion  will  generally  ensue.  The  mass  will  have  a 
relative  motion  round  the  centre  of  gravity  as  if  it  were  fixed,  while  that  point 
will  move  forward  uniformly  in  a  straight  line,  carrying  the  body  with  it.    The 


240 


CENTRE  OF  GRAVITY. 


relative  motion  of  the  mass  round  the  centre  of  gravity  may  be  found  by  con- 
sidering the  centre  of  gravity  as  a  fixed  point,  round  which  the  mass  is  free  to 
move,  and  then  determining  the  motion  which  the  applied  forces  would  pro- 
duce. This  motion  being  supposed  to  continue  uninterrupted,  let  all  the  forces 
be  imagined  to  be  applied  in  their  proper  directions  and  quantities  to  the  centre 
of  gravity.  By  the  principles  for  the  composition  of  force  they  will  be  me- 
chanically equivalent  to  a  single  force  through  that  point.  In  the  direction  of 
this  single  force  the  centre  of  gravity  will  move,  and  have  the  same  velocity 
as  if  the  whole  mass  were  there  concentrated  and  received  the  impelling 
forces. 

These  general  properties,  which  are  entirfely  independent  of  gravity,  render 
the  "  centre  of  gravity"  an  inadequate  title  for  this  important  point.  Some 
physical  writers  have  consequently  called  it  the  "  centre  of  inertia."  The 
"  centre  of  gravity,"  however,  is  the  name  by  which  it  is  still  generally  desig- 
nated. 


THE  LEYER  AND  VHEEL¥ORK. 


Simple  Machine.  —  Statics. — Djmamics.  —  Force.  —  Power.  —  Weight.  —  Lever.  —  Cord. — Inclined 
Plane.  —  Arms.  —  Fulcrum.  —  Three  kinds  of  Lever.  —  Crow^-Bar. — Handspike. — Oar. — Nut- 
Crackers. — Turning  Lathe. — Steelyard. — Rectangular  Lever. — Hammer. — Load  between  two 
Bearers. — Combination  of  Levers. — Equivalent  Lever. — Wheel  and  Axle, — Thickness  of  the 
Rope. — Ways  of  applying  the  Power. — Projecting  Pins. — Windlass. — Winch. — Axle. — Horizon- 
tal Wheel. — Tread-mill. — Cranes. — Water- Wheels. — Paddle-Wheel. — Rachet-Wheel. — Rack. — 
Spring  of  a  Watch. — Fusee. — Straps  or  Cords. — Examples  of. — Turning  Lathe. — Revolving 
Shafts. — Spinning  Machinery. — Saw-Mill. — Pinion. — Leaves. — Crane. — Spur- Wheels. — Crown- 
wheels.— Bevelled  Wheels. — Hunting-Cog. — Chronometers. — Hair-Spring. — Balance- Wheel. 


VOL,.  II.— 16 


THE  LEVER  AND  WHEELWORK. 


243 


THE  LEVEE  AID  WHEELWORK. 


A  MACHINE  is  an  instrument  by  which  force  or  motion  may  be  transmitted 
and  modified  as  to  its  quantity  and  direction.  There  are  two  ways  in  which 
a  machine  may  be  applied,  and  which  give  rise  to  a  division  of  mechanical 
science  into  parts  denominated  statics  and  dynamics ;  the  one  including  the 
theory  of  equilibrium,  and  the  other  the  theory  of  motion.  When  a  machine  is 
considered  statically,  it  is  viewed  as  an  instrument  by  which  forces  of  deter- 
minate quantities  and  directions  are  made  to  balance  other  forces  of  other 
quantities  and  other  directions.  If  it  be  viewed  dynamically,  it  is  considered 
as  a  means  by  which  certain  motions  of  determinate  quantity  and  direction  may 
be  made  to  produce  other  motions  in  other  directions  and  quantities.  It  will 
not  be  convenient,  however,  in  the  present  treatise,  to  follow  this  division  of 
the  subject.  We  shall,  on  the  other  hand,  as  hitherto,  consider  the  phenomena 
of  equilibrium  and  motion  together. 

The  effects  of  machinery  are  too  frequently  described  in  such  a  manner  as 
to  invest  them  with  the  appearance  of  paradox,  and  to  excite  astonishment  at 
what  appears  to  contradict  the  results  of  the  most  common  experience.  It  will 
be  our  object  here  to  take  a  different  course,  and  to  attempt  to  show  that  those 
effects  which  have  been  held  up  as  matters  of  astonishment  are  the  necessary, 
natural,  and  obvious  results  of  causes  adapted  to  produce  them  in  a  manner 
analogous  to  the  objects  of  most  familiar  experience. 

In  the  application  of  a  machine  there  are  three  things  to  be  considered  : 
1.  The  force  or  resistance  which  is  required  to  be  sustained,  opposed,  or  over- 
come. 2.  The  force  which  is  used  to  sustain,  support,  or  overcome,  that  re- 
sistance. 3.  The  machine  itself,  by  which  the  effect  of  this  latter  force  is 
transmitted  to  the  former.  Of  whatever  nature  be  the  force  or  the  resistance 
which  is  to  be  sustained  or  overcome,  it  is  technically  called  the  weight,  since, 
whatever  it  be,  a  weight  of  equivalent  effect  may  always  be  found.  The 
force  which  is  employed  to  sustain  or  overcome  it  is  technically  called  the 
power. 


244 


THE  LEVER  AND  WHEELWORK. 


In  expressing  the  effect  of  machinery,  it  is  usual  to  say  that  the  power  sus- 
tains the  weight ;  but  this,  in  fact,  is  not  the  case,  and  hence  arises  that  ap- 
pearance of  paradox  which  has  already  been  alluded  to.  If,  for  example,  it  is 
said  that  a  power  of  one  ounce  sustains  the  weight  of  one  ton,  astonishment  is 
not  unnaturally  excited,  because  the  fact,  as  thus  stated,  if  the  terms  be  literally 
interpreted,  is  physically  impossible.  No  power  less  than  a  ton  can,  in  the  or- 
dinary acceptation  of  the  word,  support  the  weight  of  a  ton.  It  will,  however, 
be  asked  how  it  happens  that  a  machine  appears  to  do  this  1  how  it  happens 
that  by  holding  a  silken  thread,  which  an  ounce  weight  would  snap,  many  hun- 
dred weight  may  be  sustained  ?  To  explain  this,  it  will  only  be  necessary 
to  consider  the  effect  of  a  machine,  when  the  power  and  weight  are  in  equi- 
librium. 

In  every  machine  there  are  some  fixed  points  or  props ;  and  the  arrange- 
ment of  the  parts  is  always  such  that  the  pressure,  excited  by  the  power  or 
weight,  or  both,  is  distributed  among  these  props.  If  the  weight  amount  to 
twenty  hundred,  it  is  possible  so  to  distribute  it  that  any  proportion,  however 
great,  of  it  mav  be  thrown  on  the  fixed  points  or  props  of  the  machine  ;  the  re- 
maining part  only  can  properly  be  said  to  be  supported  by  the  power  ;  and  this 
part  can  never  be  greater  than  the  power.  Considering  the  effect  in  this  way, 
it  appears  that  the  power  supports  just  so  much  of  the  weight,  and  no  more,  as 
is  equal  to  its  own  force,  and  that  all  the  remaining  part  of  the  weight  is  sus- 
tained by  the  machine. 

The  force  of  these  observations  will  be  more  apparent  when  the  nature  and 
properties  of  the  mechanic  powers  and  other  machines  have  been  explained. 

When  a  machine  is  used  dynamically,  its  effects  are  explained  on  different 
principles.  It  is  true  that,  in  this  case,  a  very  small  power  may  elevate  a  very 
great  weight ;  but,  nevertheless,  in  so  doing,  whatever  be  the  machine  used, 
the  total  expenditure  of  power,  in  raising  the  weight  through  any  height,  is 
never  less  than  that  which  would  be  expended  if  the  power  were  immediately 
applied  to  the  weight  without  the  intervention  of  any  machine.  This  circum- 
stance arises  from  a  universal  property  of  machines,  by  which  the  velocity  of 
the  weight  is  always  less  than  that  of  the  power,  in  exactly  the  same  propor- 
tion as  the  power  itself  is  less  than  the  weight ;  so  that,  when  a  certain  power 
is  applied  to  elevate  a  weight,  the  rate  at  which  the  elevation  is  effected  is  al- 
ways slow  in  the  same  proportion  as  the  weight  is  great.  From  a  due  consid- 
eration of  this  remarkable  law,  it  will  easily  be  understood  that  a  machine  can 
never  diminish  the  total  expenditure  of  power  necessary  to  raise  any  weight  or 
to  overcome  any  resistance.  In  such  cases,  all  that  a  machine  ever  does,  or 
ever  can  do,  is  to  enable  the  power  to  be  expended  at  a  slow  rate,  and  in  a 
more  advantageous  direction  than  if  it  were  immediately  applied  to  the  weight 
or  the  resistance. 

Let  us  suppose  that  P  is  a  power  amounting  to  an  ounce,  and  that  W  is  a 
weight  amounting  to  50  ounces,  and  that  P  elevates  W  by  means  of  a  machine. 
In  virtue  of  the  property  already  stated,  it  follows  that,  while  P  moves  through 
50  feet,  W  will  be  moved  through  1  foot  ;  but  in  moving  P  through  50  feet,  50 
distinct  efforts  are  made,  by  each  of  which  1  ounce  is  moved  through  1  foot, 
and  by  which  collectively  50  distinct  ounces  might  be  successively  raised 
through  1  foot.  But  the  weight  W  is  50  ounces,  and  has  been  raised  through 
1  foot ;  whence  it  appears  that  the  expenditure  of  power  is  equal  to  that 
which  would  be  necessary  to  raise  the  weight  without  the  intervention  of  any 
machine. 

This  important  principle  may  be  presented  under  another  aspect,  which  will 
perhaps  render  it  more  apparent.  Suppose  the  weight  W  were  actually  divided 
into  50  equal  parts,  or  suppose  it  were  a  vessel  of  liquid  weighing  50  ounces, 


THE  LEVER  AND  WHEELWORK. 


245 


and  containing  50  equal  measures  ;  if  these  50  measures  were  successively 
lifted  through  a  height  of  1  foot,  the  efforts  necessary  to  accomplish  this  would 
be  the  same  as  those  used  to  move  the  power  P  through  50  feet,  and  it  is  ob- 
vious that  the  total  expenditure  of  force  would  be  the  same  as  that  which  would 
be  necessary  to  lift  the  entire  contents  of  the  vessel  through  1  foot. 

When  the  nature  and  properties  of  the  mechanic  powers  and  other  machines 
have  been  explained,  the  force  of  these  observations  will  be  more  distinctly 
perceived.  The  effects  of  props  and  fixed  points  in  sustaining  part  of  the 
weight,  and  sometimes  the  whole,  both  of  the  weight  and  power,  will  then  be 
manifest,  and  every  machine  will  furnish  a  verification  of  the  remarkable  pro- 
portion between  the  velocities  of  the  weight  and  power,  which  has  enabled  us 
to  explain  what  might  otherwise  be  paradoxical  and  difficult  of  compre- 
hension. 

The  most  simple  species  of  machines  are  those  which  are  commonly  denom- 
inated the  mechanic  powers.  These  have  been  differently  enumerated  by  differ- 
ent writers.  If,  however,  the  object  be  to  arrange  in  distinct  classes,  and  in 
the  smallest  possible  number  of  them,  those  machines  which  are  alike  in  prin- 
ciple, the  mechanic  powers  may  be  reduced  to  three : — 

1.  The  lever. 

2.  The  cord. 

3.  The  inclined  plane.  .  , 

To  one  or  other  of  these  classes  all  simple  machines  whatever  may  be  re- 
duced, and  all  complex  machines  may  be  resolved  into  simple  elements  which 
come  under  them. 

The  first  class  includes  every  machine  which  is  composed  of  a  solid  body 
revolving  on  a  fixed  axis,  although  the  name  lever  has  been  commonly  confined 
to  cases  where  the  machine  affects  certain  particular  forms.  The  power  and 
weight  are  always  supposed  to  be  applied  in  directions  at  right  angles  to  the 
axis.  If  lines  be  drawn  from  the  axis  perpendicular  to  the  directions  of  power 
and  weight,  equilibrium  will  subsist,  provided  the  power,  multiplied  by  the  per- 
pendicular distance  of  its  direction  from  the  axis,  be  equal  to  the  weight  multi- 
plied by  the  perpendicular  distance  of  its  direction  from  the  axis.  This  is  a 
principle  to  which  we  shall  have  occasion  to  refer  in  explaining  the  various 
machines  of  this  class. 

If  the  moment  of  the  power  be  greater  than  that  of  the  weight,  the  effect  of 
the  power  will  prevail  over  that  of  the  weight,  and.  elevate  it;  but  if,  on  the 
other  hand,  the  moment  of  the  power  be  less  than  that  of  the  weight,  the  power 
will  be  insufficient  to  support  the  weight,  and  will  allow  it  to  fall. 

The  second  class  of  simple  machines  includes  all  those  cases  in  which  force 
is  transmitted  by  means  of  flexible  threads,  ropes,  or  chains.  The  principle 
by  which  the  effects  of  these  machines  are  estimated  is,  that  the  tension 
throughout  the  whole  length  of  the  same  cord,  provided  it  be  perfectly  flexible, 
and  free  from  the  effects  of  friction,  must  be  the  same.  Thus,  if  a  force  acting 
at  one  end  be  balanced  by  a  force  acting  at  the  other  end,  however  the  cord 
may  be  bent,  or  whatever  course  it  may  be  compelled  to  take,  by  any  causes 
which  may  affect  it  between  its  ends,  these  forces  must  be  equal,  provided  the 
cord  be  free  to  move  over  any  obstacles  which  may  deflect  it. 

Within  this  class  of  machines  are  included  all  the  various  forms  o^  -pulleys. 

The  third  class  of  simple  machines  includes  all  those  cases  in  which  the 
weight  or  resistance  is  supported  or  moved  on  a  hard  surface  inclined  to  the 
vertical  direction. 

The  effects  of  such  machines  are  estimated  by  resolving  the  whole  weight  ^ 
of  the  body  into  two  elements  by  the  parallelogram  of  forces.     One  of  these  \ 


246 


THE  LEVER  AND  WHEELWORK. 


elements  is  perpendicular  to  the  surface,  and  supported  by  its  resistance;  the  | 
other  is  parallel  to  the  surface,  and  supported  by  the  power.     The  proportion, 
therefore,  of  the  power  to  the  weight  will  always  depend  on  the  obliquity  of  the  [ 
surface  to  the  direction  of  the  weight. 

Under  this  class  of  machines  come  the  inclined  plane,  commonly  so  called, 
the  wedge,  the  screw,  and  various  others. 

In  order  to  simplify  the  development  of  the  elementary  theory  of  machines, 
it  is  expedient  to  omit  the  consideration  of  many  circumstances,  of  which, 
however,  a  strict  account  must  be  taken  before  any  practically  useful  applica- 
tion of  that  theory  can  be  attempted.  A  machine,  as  we  must  for  the  present 
contemplate  it,  is  a  thing  which  can  have  no  real  or  practical  existence.  Its 
various  parts  are  considered  to  be  free  from  friction  :  all  surfaces  which  move 
in  contact,  are  supposed  to  be  infinitely  smooth  and  polished.  The  solid  parts 
are  conceived  to  be  absolutely  inflexible.  The  weight  and  inertia  of  the  ma- 
chine itself  are  wholly  neglected,  and  we  reason  upon  it  as  if  it  were  divested 
of  these  qualities.  Cords  and  ropes  are  supposed  to  have  no  stiffness,  to  be 
infinitely  flexible.  The  machine,  when  it  moves,  is  supposed  to  sufl^er  no  re- 
sistance from  the  atmosphere,  and  to  be  in  all  respects  circumstanced  as  if  it 
were  in  vacuo. 

It  is  scarcely  necessary  to  state  that,  all  these  suppositions  being  false,  none 
of  the  consequences  deduced  from  them  can  be  true.  Nevertheless,  as  it  is  the 
business  of  art  to  bring  machines  as  near  to  this  state  of  ideal  perfection  as 
possible,  the  conclusions  which  are  thus  obtained,  though  false  in  a  strict  sense, 
yet  deviate  from  the  truth  in  but  a  small  degree.  Like  the  first  outline  of  a 
picture,  they  resemble,  in  their  general  features,  that  truth  to  which,  after  many 
subsequent  corrections,  they  must  finally  approximate. 

After  a  first  approximation  has  been  made  on  the  several  false  suppositions 
which  have  been  mentioned,  various  effects,  which  have  been  previously  neg- 
lected, are  successively  taken  into  account.  Roughness,  rigidity,  imperfect 
flexibility,  the  resistance  of  air  and  other  fluids,  the  effects  of  the  weight  and 
inertia  of  the  machine,  are  'severally  examined,  and  their  laws  and  properties 
detected.  The  modifications  and  corrections,  thus  suggested  as  necessary  to 
be  introduced  into  our  former  conclusions,  are  applied,  and  a  second  approxi- 
mation, but  still  only  an  approximation,  to  truth  is  made.  For,  in  investigating 
the  laws  which  regulate  the  several  effects  just  mentioned,  we  are  compelled 
to  proceed  upon  a  new  group  of  false  suppositions.  To  determine  the  laws 
which  regulate  the  friction  of  surfaces,  it  is  necessary  to  assume  that  every 
part  of  the  surfaces  of  contact  is  uniformly  rough ;  that  the  solid  parts  which 
are  imperfectly  rigid,  and  the  cords  which  are  imperfectly  flexible,  are  consti- 
tuted throughout  their  entire  dimensions  of  a  uniform  material  ;  so  that  the  im- 
perfection does  not  prevail  more  in  one  part  than  another.  Thus  all  irregular- 
ity is  left  out  of  account,  and  a  general  average  of  the  effects  taken.  It  is 
obvious,  therefore,  that  by  these  means  we  have  still  failed  in  obtaining  a  re- 
sult exactly  conformable  to  the  real  state  of  things  ;  but  it  is  equally  obvious 
that  we  have  obtained  one  much  more  comformable  to  that  state  than  had 
been  previously  accomplished,  and  sufficiently  near  it  for  most  practical  pur- 
poses. 

This  apparent  imperfection  in  our  instruments  and  powers  of  investigation  is 
not  peculiar  to  mechanics  ;  it  pervades  all  departments  of  natural  science.  In 
astronomy,  the  motions  of  the  celestial  bodies,  and  their  various  changes  and 
appearances,  as  developed  by  theory,  assisted  by  observation  and  experience, 
are  only  approximations  to  the  real  motions  and  appearances  which  take  place 
in  nature.  It  is  true  that  these  approximations  are  susceptible  of  almost  unlim- 
'  ited  accuracy  ;  but  still  they  are,  and  ever  will  continue  to  be,  only  approxima- 


THE  LEVER  AND  WHEELWORK. 


247 


tioTis.     Optics  and  all  other  branches  of  natural  science  are  liable  to  the  same 
observations. 

THE    LEVER. 

An  inflexible,  straight  bar,  turning  on  an  axis,  is  commonly  called  a  lever. 
The  arms  of  the  lever  are  those  parts  of  the  bar  which  extend  on  each  side 
of  the  axis. 

The  axis  is  called  the  fulcrum  or  prop. 

Levers  are  commonly  divided  into  three  kinds,  according  to  the  relative  po- 
sitions of  the  power,  the  weight,  and  the  fulcrum. 

In  a  lever  of  the  first  kind,  as  in  fig.  1,  the  fulcrum  is  between  the  power 
and  weight. 

Fig.  1. 

T       dr      F  ar-w_ 


In  a  lever  of  the  second  kind,  as  in  fig.  2,  the  weight  is  between  the  fulcrum 
and  power. 

In  a  lever  of  the  third  kind,  as  in  fig.  3,  the  power  is  between  the  fulcrum 

and  weight. 

Fig.  2. 


In  all  these  cases  the  power  will  sustain  the  weight  in  equilibrium,  provided 
its  moment  be  equal  to  that  of  the  weight.  But  the  moment  of  the  power  is, 
in  this  case,  equal  to  the  product  obtained  by  multiplying  the  power  by  its  dis- 
tance from  the  fulcrum,  and  the  moment  of  the  weight,  by  multiplying  the 
weight  by  its  distance  from  the  fulcrum.  Thus,  if  the  number  of  ounces  in  P, 
being  multiplied  by  the  number  of  inches  in  P  F,  be  equal  to  the  number  of 
ounces  in  W,  multiplied  by  the  number  of  inches  in  W  F,  equihbrium  will  be 
established.  It  is  evident  from  this,  that,  as  the  distance  of  the  power  from 
the  fulcrum  increases  in  comparison  to  the  distance  of  the  weight  from  the 
fulcrum,  in  the  same  degree  exactly  will  the  proportion  of  the  power  to  the 
weight  diminish.  In  other  words,  the  proportion  of  the  power  to  the  weight 
will  be  always  the  same  as  that  of  their  distances  from  the  fulcrum  taken  in  a 
reverse  order. 

In  cases  where  a  small  power  is  required  to  sustain  or  elevate  a  great  weight, 
it  will  therefore  be  necessary  either  to  remove  the  power  to  a  great  distance 
from  the  fulcrum,  or  to  bring  the  weight  very  near  it. 

Numerous  examples  of  levers  of  the  first  kind  may  be  given.  A  crowbar, 
applied  to  elevate  a  stone  or  other  weight,  is  an  instance.  The  fulcrum  is  an- 
other stone  placed  near  that  which  is  to  be  raised,  and  the  power  is  the  hand 
placed  at  the  other  end  of  the  bar. 

A  handspike  is  a  similar  example. 

A  poker  applied  to  raise  fuel  is  a  lever  of  the  first  kind,  the  fulcrum  being 
the  bar  of  the  grate. 


248 


THE  LEVEH  AND  WHEELWORK. 


I       Scissors,  shears,  nippers,  pincers,  and  other  similar  instruments,  are  com- 

'  posed  of  two  levers  of  the  first  kind  ;  the  fulcrum  being  the  joint  or  pivot,  and 

]  the  weight  the  resistance  of  the  substance  to  be  cut  or  seized  ;  the  power  being 

'  the  fingers  applied  at  the  other  end  of  the  levers. 

!       The  brake  of  a  pump  is  a  lever  of  the  first  kind ;  the  pump-rods  and  piston 

'  being  the  weight  to  be  raised. 

,       Examples  of  levers  of  the  second  kind,  though  not  so  frequent  as  those  just 

I  mentioned,  are  not  uncommon. 

An  oar  is  a  lever  of  the  second  kind  :  the  reaction  of  the  water  against  the 
'  blade  is  the  fulcrum  ;  the  boat  is  the  weight,  and  the  hand  of  the  boatman  the 
power. 

The  rudder  of  a  ship  or  boat  is  an  example  of  this  kind  of  lever,  and  explain- 
ed in  a  similar  way. 

The  chipping-knife  is  a  lever  of  the  second  kind.  The  end  attached  to  the 
bench  is  the  fulcrum,  and  the  weight  the  resistance  of  the  substance  to  be  cut, 
placed  beneath  it. 

A  door  moved  upon  its  hinges  is  another  example. 

Nut-crackers  are  two  levers  of  the  second  kind  ;  the  hinge  which  unites 
them  being  the  fulcrum,  the  resistance  of  the  shell  placed  between  them  being 
the  weight,  and  the  hand  applied  to  the  extremity  being  the  power. 

A  wheelbarrow  is  a  lever  of  the  second  kind ;  the  fulcrum  being  the  point 
at  which  the  wheel  presses  on  the  ground;  and  the  weight  being  that  of  the 
barrow  and  its  load,  collected  at  their  centre  of  gravity. 

The  same  observation  may  be  applied  to  all  two-wheeled  carriages,  which 
are  partly  sustained  by  the  animal  which  draws  them. 

In  a  lever  of  the  third  kind,  the  weight,  being  more  distant  from  the  fulcrum 
than  the  power,  must  be  proportionably  less  than  it.  In  this  instrument,  there- 
fore, the  power  acts  upon  the  weight  to  a  mechanical  disadvantage,  inasmuch 
as  a  greater  power  is  necessary  to  support  or  move  the  weight  than  would  be 
required  if  the  power  were  immediately  applied  to  the  weight,  without  the  in- 
tervention of  a  machine.  We  shall,  however,  hereafter  show  that  the  advan- 
tage which  is  lost  in  force  is  gained  in  despatch,  and  that,  in  proportion  as  the 
weight  is  less  than  the  power  which  moves  it,  so  will  the  speed  of  its  motion 
be  greater  than  that  of  the  power. 

Hence  a  lever  of  the  third  kind  is  only  used  in  cases  where  the  exertion 
of  great  power  is  a  consideration  subordinate  to  those  of  rapidity  and  despatch. 
The  most  striking  example  of  levers  of  the  third  kind  is  found  in  the  animal 
economy.  The  limbs  of  animals  are  generally  levers  of  this  description.  The 
socket  of  the  bone  is  the  fulcrum ;  a  strong  muscle  attached  to  the  bone  near 
the  socket  is  the  power  ;  and  the  weight  of  the  limb,  together  with  whatever 
resistance  is  opposed  to  its  motion,  is  the  weight.  A  slight  contraction  of  the 
muscle  in  this  case  gives  a  considerable  motion  to  the  limb  :  this  efiect  is  par- 
ticularly conspicuous  in  the  motion  of  the  arms  and  legs  in  the  human  body  ; 
a  very  inconsiderable  contraction  of  the  muscles  at  the  shoulders  and  hips  giv- 
ing the  sweep  to  the  limbs  from  which  the  body  derives  so  much  activity. 

The  treddle  of  the  turning-lathe  is  a  lever  of  the  third  kind.  The  hinge 
which  attaches  it  to  the  floor  is  the  fulcrum,  the  foot  applied  to  it  near  the 
hinge  is  the  power,  and  the  crank  upon  the  axis  of  the  fly-wheel,  with  which 
its  extremity  is  connected,  is  the  weight. 

Tongs  are  levers  of  this  kind,  as  also  the  shears  used  in  shearing  sheep.    In 
these  cases,  the  power  is  the  hand  placed  immediately  below  the  fulcrum,  or  \ 
point  where  the  two  levers  are  connected. 

When  the  power  is  said  to  support  the  weight  by  means  of  a  lever,  or  any  \ 
other  machine,  it  is  only  meant  that  the  power  keeps  the  machine  in  equilib-  ' 


THE  LEVER  AND,  WHEELWORK. 


249 


rium,  and  thereby  enables  it  to  sustain  the  weight.  It  is  necessary  to  attend 
to  this  distinction,  to  remove  the  difficulty  which  may  arise  from  the  paradox 
of  a  small  power  sustaining  a  great  weight. 

In  a  lever  of  the  first  kind,  the  fulcrum  F,  fig.  1,  or  axis,  sustains  the  united 
forces  of  the  power  and  weight. 

In  a  lever  of  the  second  kind,  if  the  power  be  supposed  to  act  over  a  wheel, 
R,  fig.  2,  the  fulcrum  F  sustains  a  pressure  equal  to  the  difl^erence  between  the 
power  and  weight,  and  the  axis  of  the  wheel  R  sustains  a  pressure  equal  to 
twice  the  power  ;  so  that  the  total  pressures  on  F  and  R  are  equivalent  to  the 
united  forces  of  the  power  and  weight. 

In  a,  lever  of  the  third  kind  similar  observations  are  applicable.  The  wheel 
R,  fig.  3,  sustains  a  pressure  equal  to  twice  the  power,  and  the  fulcrum  F  sus- 
tains a  pressure  equal  to  the  diflerence  between  the  power  and  weight. 

These  facts  may  be  experimentally  established  by  attaching  a  string  to  the 
lever  immediately  over  the  fulcrum,  and  suspending  the  lever  by  that  string  from 
the  arm  of  a  balance.  The  counterpoising  weight,  when  the  fulcrum  is  re- 
moved, will,  in  the  first  case,  be  equal  to  the  sum  of  the  weight  and  power,  and 
in  the  last  two  cases  equal  to  their  diff"erence. 

We  have  hitherto  omitted  the  consideration  of  the  effect  of  the  weight  of  the 
lever  itself.  If  the  centre  of  gravity  of  the  lever  be  in  the  vertical  line  through 
the  axis,  the  weight  of  the  instrument  will  have  no  other  eff'ect  than  to  increase 
the  pressure  on  the  axis  by  its  own  amount.  But  if  the  centre  of  gravity  be 
on  the  same  side  of  the  axis  with  the  weight,  as  at  G,  it  will  oppose  the  eff'ect 
of  the  power,  a  certain  part  of  which  must  therefore  be  allowed  to  support  it. 
To  ascertain  what  part  of  the  power  is  thus  expended,  it  is  to  be  considered 
that  the  moment  of  the  weight  of  the  lever  collected  at  G,  is  found  by  multi- 
plying that  weight  by  the  distance  G  F.  The  moment  of  that  part  of  the  power 
which  supports  this  must  be  equal  to  it ;  therefore,  it  is  only  necessary  to  find 
how  much  of  the  power  multiplied  by  P  F  will  be  equal  to  the  weight  of  the 
lever  multiplied  by  G  F.     This  is  a  question  in  common  arithmetic. 

If  the  centre  of  gravity  of  the  lever  be  at  a  different  side  of  the  axis- from  the 
weight,  as  at  G',  the  weight  of  the  instrument  will  co-operate  with  the  power 
in  sustaining  the  weight  W.  To  determine  what  portion  of  the  weight  W  is 
thus  sustained  by  the  weight  of  the  lever,  it  is  only  necessary  to  find  how 
much  of  W,  multiplied  by  the  distance  W  F,  is  equal  to  the  weight  of  the  lever 
multiplied  by  G'  F. 

In  these  cases,  the  pressure  on  the  fulcrum,  as  already  estimated,  will  always 
be  increased  by  the  weight  of  the  lever. 

The  sense  in  which  a  small  power  is  said  to  sustain  a  great  weight,  and  the 
manner  of  accomplishing  this,  being  explained,  we  shall  now  consider  how  the 
power  is  applied  in  moving  the  weight.     Let  P  W,  fig.  4,  be  the  places  of  the 


Fig.  4. 


power  and  weight,  and  F  that  of  the  fulcrum,  and  let  the  power  be  depressed 
to  P'  while  the  weight  is  raised  to  W.  The  space  P  P'  evidently  bears  the 
same  proportion  to  W  W,  as  the  arm  P  F  to  W  F.  Thus,  if  P  F  be  ten  times 
W  F,  P  P'  will  be  ten  times  W  W.  A  power  of  one  pound  at  P,  being  moved 
from  P  to  P',  will  carry  a  weight  of  ten  pounds  from  W  to  W.  But  in  this 
case  it  ought  not  to  be  said  that  a  lesser  weight  moves  a  greater,  for  it  is  not 


THE  LEVER  AND  WHEELWORK. 


difficult  to  show  that  the  total  expenditure  of  force  in  the  motion  of  one  poimd 
from  P  to  P'  is  exactly  the  same  as  in  the  motion  of  ten  pounds  from  W  to  W. 
If  the  space  P  P'  be  ten  inches,  the  space  W  W  will  be  one  inch.  A  weight 
of  one  pound  is  therefore  moved  through  ten  successive  inches,  and  in  each 
inch  the  force  expended  is  that  which  would  be  sufficient  to  move  one  pound 
through  one  inch.  The  total  expenditure  of  force  from  P  to  P'  is  ten  times  the 
force  necessary  to  move  one  pound  through  one  inch,  or,  what  is  the  same,  it 
is  that  which  would  be  necessary  to  move  ten  pounds  through  one  inch.  But 
this  is  exactly  what  is  accomplished  by  the  opposite  end,  W,  of  the  lever  ;  for 
the  weight  W  is  ten  pounds,  and  the  space  W  W  is  one  inch. 

If  the  weight  W  of  ten  pounds  could  be  conveniently  divided  into  ten  equal 
parts  of  one  pound  each,  each  part  might  be  separately  raised  through  one  inch, 
without  the  intervention  of  the  lever  or  any  other  machine.  In  this  case,  the 
same  quantity  of  power  would  be  expended,  and  expended  in  the  same  manner 
as  in  the  case  just  mentioned. 

It  is  evident,  therefore,  that  when  a  machine  is  applied  to  raise  a  weight,  or 
to  overcome  resistance,  as  much  force  must  be  really  used  as  if  the  power  were 
immediately  applied  to  the  weight  or  resistance.  All  that  is  accomplished  by 
the  machine  is  to  enable  the  power  to  do  that  by  a  succession  of  distinct  efforts 
which  should  be  otherwise  performed  by  a  single  effort.  These  observations 
will  be  found  to  be  applicable  to  all  other  machines. 

Weighing-machines  of  almost  every  kind,  whether  used  for  commercial  or 
philosophical  purposes,  are  varieties  of  the  lever.  The  common  balance, 
which,  of  all  weighing-machines,  is  the  most  perfect,  and  best  adapted  for  or- 
dinary use,  whether  in  commerce  or  experimental  philosophy,  is  a  lever  with 
equal  arms.  In  the  steelyard,  one  weight  serves  as  a  counterpoise  and  meas- 
ure of  others  of  different  amount,  by  receiving  a  leverage  variable  according  to 
the  varying  amount  of  the  weight  against  which  it  acts. 

We  have  hitherto  considered  the  power  and  weight  as  acting  on  the  lever,  in 
directions  perpendicular  to  its  length,  and  parallel  to  each  other.  This  does 
not  always  happen.    Let  A  B,  fig.  5,  be  a  lever  whose  fulcrum  is  F,  and  let  A 

J  Fig.  5. 


R  be  the  direction  of  the  power,  and  B  S  the  direction  of  the  weight.  If  the 
lines  R  A  and  S  B  be  continued,  and  perpendiculars  F  C  and  F  D  drawn  from 
the  fulcrum  to  those  lines,  the  moment  of  the  power  will  be  found  by  multiply- 
ing the  power  by  the  line  F  C,  and  the  moment  of  the  weight  by  multiplying 
the  weight  by  F  D.  If  these  moments  be  equal,  the  power  will  sustain  the 
weight  in  equilibrium. 

It  is  evident  that  the  same  reasoning  will  be  applicable  when  the  arms  of 
the  lever  are  not  in  the  same  direction.  These  arms  may  be  of  any  figure  or 
shape,  and  may  be  placed  relatively  to  each  other  in  any  position. 

In  the  rectangular  lever  the  arms  are  perpendicular  to  each  other,  and  the 
fulcrum  F,  fig.  6,  is  at  the  right  angle.  The  moment  of  the  power,  in  this  case, 
is  P  multiplied  by  A  F,  and  that  of  the  weight  W  multiplied  by  B  F.  When 
the  instrument  is  in  equilibrium  these  moments  must  be  equal. 

When  the  hammer  is  used  for  drawing  a  nail,  it  is  a  lever  of  this  kind.    The 


THE  LEVER  AND  ^VHEELWORK. 


251 


claw  of  the  hammer  is  the  shorter  arm  ;  the  resistance  of  the  nail  is  the  weight ; 
and  the  hand  applied  to  the  handle  the  power. 


Fig.  7. 


When  a  beam  rests  on  two  props,  A  B,  fig.  7,  and  supports  at  some  interme- 
diate place,  C,  a  weight,  W,  this  weight  is  distributed  between  the  props  in  a 
manner  which  may  be  determined  by  the  principles  already  explained.  If  the 
pressure  on  the  prop  B  be  considered  as  a  power  sustaining  the  weight  W,  by 
means  of  the  lever  of  the  second  kind,  B  A,  then  this  power  multiplied  by  B  A 
must  be  equal  to  the  weight  multiplied  by  C  A.  Hence  the  pressure  on  B  will 
be  the  same  fraction  of  the  weight  as  the  part  A  C  is  of  A  B.  In  the  same 
manner  it  may  be  proved,  that  the  pressure  on  A  is  the  same  fraction  of  the 
weight  as  B  C  is  of  B  A.  Thus,  if  A  C  be  one  third,  and  therefore  B  C  two 
thirds  of  B  A,  the  pressure  on  B  will  be  one  third  of  the  weight,  and  the  pres- 
sure on  A  two  thirds  of  the  weight. 

It  follows  from  this  reasoning  that,  if  the  weight  be  in  the  middle,  equally 
distant  from  B  and  A,  each  prop  will  sustain  half  the  weight.  The  effect  of 
the  weight  of  the  beam  itself  may  be  determined  by  considering  it  to  be  col- 
lected at  its  centre  of  gravity.  If  this  point,  therefore,  be  equally  distant  from 
the  props,  the  weight  of  the  beam  will  be  equally  distributed  between  them. 

According  to  these  principles,  the  manner  in  which  a  load  borne  on  poles 
between  two  bearers  is  distributed  between  them  may  be  ascertained.  As  the 
efforts  of  the  bearers  and  the  direction  of  the  weight  are  alwaj's  parallel,  the 
position  of  the  poles  relatively  to  the  horizon  makes  no  difference  in  the  distri- 
bution of  the  weights  between  the  bearers.  Whether  they  ascend  or  descend, 
or  move  on  a  level  plane,  the  weight  will  be  similarly  shared  between  them. 

If  the  beam  extend  beyond  the  prop,  as  in  fig.  8,  and  the  weight  be  suspend- 

Fig.  8. 


ed  at  a  point  not  placed  between  them,  the  props  must  be  applied  at  "different 
sides  of  the  beam.  The  pressures  which  they  sustain  may  be  calculated  in 
the  same  manner  as  in  the  former  case.  The  pressure  of  the  prop  B  may  be 
considered  as  a  power  sustaining  the  weight  W  by  means  of  the  lever  B  C. 
Hence  the  pressure  of  B,  multiplied  by  B  A,  must  be  equal  to  the  weight  W 
multiplied  by  A  C.     Therefore  the  pressure  on  B  bears  the  same  proportion  to 


252 


THE  LEVER  AND  WHEELWORK. 


the  weight  as  A  C  does  to  A  B.  In  the  same  manner,  considering  B  as  a  fid- 
crum,  and  the  pressure  of  the  prop  A  as  the  power,  it  may  be  proved  that  the 
pressure  of  A  bears  the  same  proportion  to  the  weight  as  the  line  B  C  does  to 
A  B.  It  therefore  appears  that  the  pressure  on  the  prop  A  is  greater  than  the 
weight. 

When  great  power  is  required,  and  it  is  inconvenient  to  construct  a  long 
lever,  a  combination  of  levers  may  be  used.    In  fig.  9,  such  a  system  of  levers 


is  represented,  consisting  of  three  levers  of  the  first  kind.  The  manned  in 
which  the  effect  of  the  power  is  transmitted  to  the  weight  may  be  investigated 
by  considering  the  effect  of  each  lever  successively.  The  power  at  P  produces 
an  upvvard  force  at  P',  which  bears  to  P  the  same  proportion  as  P'  F  to  P  F. 
Therefore  the  effect  at  P'  is  as  many  times  the  power  as  the  line  P  F  is  of  P' 
F.  Thus,  if  P  F  be  ten  times  P'  F,  the  upward  force  at  P'  is  ten  times  the 
power.  The  arm,  P'  F',  of  the  second  lever  is  pressed  upward  by  a  force 
equal  to  ten  times  the  power  at  P.  In  the  same  manner  this  may  be  shown  to 
produce  an  effect  at, P"  as  many  times  greater  than  P'  as  P'  F^  is  greater  than 
F"  F\  Thus,  if  P''  F'  be  twelve  times  F"  F',  the  effect  at  P"  will  be  twelve 
times  that  of  P'.  But  this  last  was  ten  times  the  power,  and  therefore  the  P^' 
will  be  one  hundred  and  twenty  times  the  power.  In  the  same  manner  it  may 
be  shown  that  the  weight  is  as  many  times  greater  than  the  effect  at  P"  as  P" 
F"  is  greater  than  W  F'^  If  F"  F"  be  five  times  W  F",  the  weight  will  be 
five  times  the  effect  at  P".  But  this  effect  is  one  hundred  and  twenty  times 
the  power,  and  "therefore  the  weight  would  be  six  hundred  times  the  power. 

In  the  same  manner  the  effect  of  any  compound  system  of  levers  may  be 
ascertained  by  taking  the  proportion  of  the  weight  to  the  power  in  each  lever 
separately,  and  multiplying  these  numbers  together.  In  the  example  given, 
these  proportions  are  10,  12,  and  5,  which,  multiplied  together,  give  600.  In 
fig.  9,  the  levers  composing  the  system  are  of  the  first  kind  ;  but  the  principles 
of  the  calculation  will  not  be  altered  if  they  be  of  the  second  or  third  kind,  or 
some  of  one  kind  and  some  of  another. 

That  number  which  expresses  the  proportion  of  the  weight  to  the  equilibra- 
ting power  in  any  machine  we  shall  call  the  power  of  the  machine.  Thus,  if, 
in  a  lever,  a  power  of  one  pound  support  a  weight  of  ten  pounds,  the  power  of 
the  machine  is  ten.  If  a  power  of  2  lbs.  support  a  weight  of  11  lbs.,  the  power 
of  the  machine  is  5^,  2  being  contained  in  11  S-j  times. 

As  the  distances  of  the  power  and  weight  from  the  fulcrum  of  a  lever  may 
be  varied  at  pleasure,  and  any  assigned  proportion  given  to  them,  a  lever  may 
always  be  conceived  having  a  power  equal  to  that  of  any  given  machine.  Such 
a  lever  may  be  called,  in  relation  to  that  machine,  the  equivalent  lever. 

As  every  complex  machine  consists  of  a  number  of  simple  machines  acting 
one  upon  another,  and  as  each  simple  machine  may  be  represented  by  an  equiv- 
alent lever,  the  complex  machine  will  be  represented  by  a  compound  system 
of  equivalent  levers.  From  what  has  been  proved  in  fig.  9,  it  therefore  fol- 
lows that  the  power  of  a  complex  machine  may  be  calculated  by  multiplying 
together  the  povvers  of  the  several  simple  machines  of  which  it  is  composed. 


THE'  LEVER  AND  WHEEL"WORK. 


W'HEELWORK. 

When  a  lever  is  applied  to  raise  a  weight,  or  overcome  a  resistance,  the 
space  through  which  it  acts  at  any  one  time  is  small,  and  the  work  must  be  ac- 
complished by  a  succession  of  short  and  intermitting  efforts.  In  fig.  4,  after 
the  weight  has  been  raised  from  W  to  W^,  the  lever  must  again  return  to  its 
first  position,  to  repeat  the  action.  During  this  return  the  motion  of  the  weight 
is  suspended,  and  it  will  fall  downward  unless  some  provision  be  made  to  sus- 
tain it.  The  common  lever  is,  therefore,  only  used  in  cases  where  weights  are 
required  to  be  raised  through  small  spaces,  and  under  these  circumstances  its 
great  simplicity  strongly  recommends  it.  But  where  a  continuous  motion  is  to 
be  produced,  as  in  raising  ore  from  the  mine,  or  in  weighing  the  anchor  of  a 
vessel,  some  contrivance  must  be  adopted  to  remove  the  intermitting  action  of 
the  lever,  and  render  it  continual.  The  various  forms  given  to  the  lever,  with 
a  view  to  accomplish  this,  are  generally  denominated  the  wheel  and  axle. 

In  fig.  10,  A  B  is  a  horizontal  axle,  which  rests  in  pivots  at  its  extremities, 


Fia-.  10. 


or  is  supported  in  gudgeons,  and  capable  of  revolving.  Round  this  axis  a  rope 
is  coiled,  which  sustains  the  weight  W.  On  the  same  axis  a  wheel,  C,  is 
fixed,  round  which  a  rope  is  coiled  in  a  contrary  direction,  to  which  is  append- 
ed the  power  P.  The  moment  of  the  power  is  found  by  multiplying  it  by  the 
radius  of  a  wheel,  and  the  moment  of  the  weight  by  multiplying  it  by  the  radius 
of  its  axle.  If  these  moments  be  equal,  the  machine  will  be  in  equilibrium. 
Whence  it  appears  that  the  power  of  the  machine  is  expressed  by  the  propor- 
tion which  the  radius  of  the  wheel  bears  to  the  radius  of  the  axle  ;  or,  what  is 
the  same,  of  the  diameter  of  the  wheel  to  the  diameter  of  the  axle.  This  prin- 
ciple is  applicable  to  the  wheel  and  axle  in  every  variety  of  form  under  which 
it  can  be  presented. 

It  is  evident  that,  as  the  power  descends  continually,  and  the  rope  is  uncoiled 
from  the  wheel,  the  weight  will  be  raised  continually,  the  rope  by  which  it  is 
suspended  being  at  the  same  time  coiled  upon  the  axle. 

When  the  machine  is  in  equilibrium,  the  forces  of  both  the  weight  and  power 
are  sustained  by  the  axle,  and  distributed  betwen  its  props,  in  the  manner  ex- 
plained in  fig.  7. 

When  the  machine  is  applied  to  raise  a  weight,  the  velocity  with  which  the 
power  moves  is  as  many  times  greater  than  that  with  which  the  weight  rises,  as 
the  weight  itself  is  greater  than  the  power.  This  is  a  principle  which  has  already 
been  noticed,  and  which  is  common  to  all  machines  whatsoever.  It  may  hence 
be  proved  that,  in  the  elevation  of  the  weight,  a  quantity  of  power  is  expended 
equal  to  that  which  would  be  necessary  to  elevate  the  weight  if  the-  power  were 
immediately  applied  to  it,  without  the  intervention  of  any  machine.  This  has 
been  explained  in  the  case  of  the  lever,  and  may  be  explained  in  the  present 
instance  in  nearly  the  same  words. 


THE  LEVER  AND  WHEELWORK. 


In  one  revolution  of  the  machine  the  length  of  rope  uncoiled  from  the  wheel 
is  equal  to  the  circumference  of  the  wheel,  and  through  this  space  the  power 
must  therefore  move.  At  the  same  time  the  length  of  rope  coiled  upon  the  axle 
is  equal  to  the  circumference  of  the  axle,  and  through  this  space  the  weight 
must  be  raised.  The  spaces,  therefore,  through  which  the  power  and  weight 
move  in  the  same  time,  are  in  the  proportion  of  the  circumferences  of  the 
wheel  and  axle  ;  but  these  circumferences  are  in  the  same  proportion  as 
their  diameters.  Therefore  the  velocity  of  the  power  will  bear  to  the 
velocity  of  the  weight  the  same  proportion  as  the  diameter  of  the  wheel  bears 
to  the  diameter  of  the  axle,  or,  what  is  the  same,  as  the  weight  bears  to  the 
power. 

We  have  here  omitted  the  consideration  of  the  thickness  of  the  rope.  When 
this  is  considered,  the  force  must  be  conceived  as  acting  in  the  direction  of  the 
centre  of  the  rope,  and  therefore  the  thickness  of  the  rope  which  supports  the 
power  ought  to  be  added  to  the  diameter  of  the  wheel,  and  the  thickness  of  the 
rope  which  supports  the  weight  to  the  diameter  of  the  axle.  It  is  the  more 
necessary  to  attend  to  this  circumstance,  as  the  strength  of  the  rope  necessary 
to  support  the  weight  causes  its  thickness  to  bear  a  considerable  proportion  to 
the  diameter  of  the  axle  ;  while  the  rope  which  sustains  the  power  not  requir- 
ing the  same  strength,  and  being  applied  to  a  larger  circle,  bears  a  very  incon- 
siderable proportion  to  its  diameter. 

In  numerous  forms  of  the  wheel  and  axle,  the  weight  or  resistance  is  applied 
by  a  rope  coiled  upon  the  axle ;  but  the  manner  in  which  the  power  is  applied 
is  very  various,  and  not  often  by  means  of  a  rope.  The  circumference  of  a 
wheel  sometimes  carries  projecting  pins,  as  represented  in  fig.  10,  to  which 
the  hand  is  applied  to  turn  the  machine.  An  instance  of  this  occurs  in  the 
wheel  used  in  the  steerage  of  a  vessel. 

In  the  common  windlass  the  power  is  applied  by  means  of  a  winch,  which  is 
a  rectangular  lever,  as  represented  in  fig.  11.     The  arm  B  C  of  the  winch 

■  Fig.  11. 


MTU 


represents  the  radius  of  the  wheel,  and  the  power  is  applied  to  C  D  at  right 
angles  to  B  C. 

In  some  cases  no  wheel  is  attached  to  the  axle  ;  but  it  is  pierced  with  holes 
directed  toward  its  centre,  in  which  long  levers  are  incessantly  inserted,  and  a 
continuous  action  produced  by  several  men  working  at  the  same  time  ;  so  that, 
while  some  are  transferring  the  levers  from  hole  to  hole,  others  are  working 
the  windlass. 

The  axle  is  sometimes  placed  in  a  vertical  position,  the  wheel  or  levers 
being  moved  horizontally.  The  capstan  is  an  example  of  this  :  a  vertical  axis 
is  fixed  in  the  deck  of  the  ship  ;  the  circumference  is  pierced  with  holes  pre- 
sented toward  its  centre.  These  holes  receive  long  levers,  as  represented  in 
fig.  12.  The  men  who  work  the  capstan  walk  continually  round  the  axle, 
pressing  forward  the  levers  near  their  extremities. 


THE  LEVER  AND  WHEELWORK. 


255 


In  some  cases  the  wheel  is  turned  by  the  weight  of  animals  placed  at  its 
circumference,  who  move  forward  as  fast  as  the  wheel  descends,  so  as  to  main- 


Fig.  13. 


tain  their  position  continually  at  the  extremity  of  the  horizontal  diameter.    The 
treadmill,  fig.  13,  and  certain  crane^',  such  as  fig.  14,  are  examples  of  this. 


Fig.  14. 


In  water-wheels,  the  power  is  the  weight  of  water  contained  in  buckets  at 
the  circumference,  as  in  fig.  15,  which  is  called  an  overshot  wheel ;  and  some- 
times the  impulse  of  water  against  float-boards  at  the  circumference,  as  in  the 


Fig.  15. 


Fig.  17. 


undershot  wheel,  fig.  16.  Both  these  principles  act  in  the  breast- wheel, 
fig.  17. 

In  the  paddle-wheel  of  a  steamboat,  the  power  is  the  resistance  which  the 
water  offers  to  the  motion  of  the  paddle-boards. 

In  windmills,  the  power  is  the  force  of  the  wind  acting  on  various  parts  of 
the  arms,  and  may  be  considered  as  difierent  powers  simultaneously  acting  on 
diff'erent  wheels  having  the  same  axle. 

In  most  cases  in  which  the  wheel  and  axle  is  used,  the  action  of  the  powei 
is  liable  to  occasional  suspension  or  intermission,  in  which  case  some  contri- 
vance is  necessary  to  prevent  the  recoil  of  the  weight.  A  ratchet-wheel,  R, 
fig.  10,  is  provided  for  this  purpose,  which  is  a  contrivance  which  permits  the 
wheel  to  turn  in  one  direction  ;  but  a  catch  which  falls  between  the  teeth  of  a 
fixed  wheel  prevents  its  motion  in  the  other  direction.  The  effect  of  the  power 
or  weight  is  sometimes  transmitted  to  the  whet  I  or  axle  by  means  of  a  straight 


256 


THE  LEVER  AND  WHEELWORK. 


bar,  on  the  edge  of  which  teeth  are  raised,  which  engage  themselves  in  cor- 
responding teeth  on  the  wheel  or  axle.  Such  a  bar  is  called  a  rack  ;  and  an 
instance  of  its  use  may  be  observed  in  the  manner  of  working  the  pistons  of  an 
air-pump. 

The  power  of.  the  wheel  and  axle  being  expressed  by  the  number  of  times 
the  diameter  of  the  axle  is  contained  in  that  of  the  wheel,  there  are  obviously 
only  two  ways  by  which  this  power  may  be  increased,  viz.,  either  by  dimin- 
ishing the  diameter  of  the  axle,  or  increasing  that  of  the  wheel.  In  cases 
where  great  power  is  required,  each  of  these  methods  is  attended  with  practi- 
cal inconvenience  and  difficulty.  If  the  diameter  of  the  wheel  be  considerably 
enlarged,  the  machine  will  become  unwieldy,  and  the  power  will  work  through 
an  unmanageable  space.  If,  on  the  other  hand,  the  power  of  the  machine  be 
increased  by  reducing  the  thickness  of  the  axle,  the  strength  of  the  axle  will 
become  insufficient  for  the  support  of  that  weight,  the  magnitude  of  which  had 
rendered  the  increase  of  the  power  of  the  machine  necessary.  To  combine 
the  requisite  strength  with  moderate  dimensions  and  great  mechanical  power 
is,  therefore,  impracticable  in  the  ordinary  form  of  the  wheel  and  axle.  This 
has,  however,  been  accomplished  by  giving  different  thicknesses  to  different 
parts  of  the  axle,  and  carrying  a  rope,  which  is  coiled  on  the  thinner  part, 
through  a  wheel  attached  to  the  weight,  and  coiling  it  in  the  opposite  direction 
on  the  thicker  part,  as  in  fig.  18.     To  investigate  the  proportion  of  the  power 


Fig.  19. 


Fig.  18. 


to  the  weight  in  this  case,  let  fig.  19  represent  a  section  of  the  apparatus  at 
right  angles  to  the  axis.  The  weight  is  equally  suspended  by  the  two  parts 
of  the  rope,  S  and  S',  and  therefore  each  part  is  stretched  by  a  force  equal  to 
half  the  weight.  The  moment  of  the  force  which  stretches  the  rope  S  is  half 
the  weight  multiplied  by  the  radius  of  the  thinner  part  of  the  axle.  This  force, 
being  at  the  same  side  of  the  centre  with  the  power,  co-operates  with  it  in  sup- 
porting the  force  which  stretches  S',  and  which  acts  at  the  other  side  of  the 
centre.  The  moments  of  P  and  S  are  equal  to  that  of  S' ;  and  therefore,  if  P 
be  multiplied  by  the  radius  of  the  wheel,  and  added  to  half  the  weight  multi- 
plied by  the  radius  of  the  thinner  part  of  the  axle,  we  must  obtain  a  sum  equal 
to  half  the  weight  multiplied  by  the  radius  of  the  thicker  part  of  the  axle. 
Hence  it  is  easy  to  perceive,  that  the  power  multiplied  by  the  radius  of  the 
wheel  is  equal  to  half  the  weight  multiplied  by  the  difference  of  the  radii  of  the 
thicker  and  thinner  parts  of  the  axle  ;  or,  what  is  the  same,  the  power  multi- 
plied by  the  diameter  of  the  wheel  is  equal  to  the  weight  multiplied  by  half  the 
difference  of  the  diameters  of  the  thinner  and  thicker  parts  of  the  axle. 

A  wheel  and  axle  constructed  in  this  manner  is  equivalent  to  an  ordinary 
one,  in  which  the  wheel  has  the  same  diameter,  and  whose  axle  has  a  diame- 
ter equal  to  half  the  difference  of  the  diameters  of  the  thicker  and  thinner 
parts.  The  power  of  the  machine  is  expressed  by  the  proportion  which  the 
diameter  of  the  wheel  bears  ij  half  the  difference  of  these  diameters  :  and 


THE  LEVER  AND  WHEELWOHK. 


257 


therefore 


power,  when  the  diameter  of  the  wheel  is  given,  does  not,  as  in 
the  ordinary  wheel  and  axle,  depend  on  the  smallness  of  the  axle,  but  on  the 
smallness  of  the  difference  of  the  thinner  and  thicker  parts  of  it.  The  axle 
may,  therefore,  be  constructed  of  such  a  thickness  as  to  give  it  all  the  requisite 
strength,  and  yet  the  difference  of  the  diameters  of  its  different  parts  may  be  so 
small  as  to  give  it  all  the  requisite  power. 

It  often  happens  that  a  varying  weight  is  to  be  raised,  or  resistance  over- 
come, by  uniform  power.  If,  in  such  a  case,  the  weight  be  raised  by  a  rope 
coiled  upon  a  uniform  axle,  the  action  of  the  power  would  not  be  uniform,  but 
would  vary  with  the  weight.  It  is,  however,  in  most  cases  desirable  or  neces- 
sary that  the  weight  or  resistance,  even  though  it  vary,  shall  be  moved  uni- 
formly. This  will  be  accomplished  if  by  any  means  the  leverage  of  the  weight 
is  made  to  increase  in  the  same  proportion  as  the  weight  diminishes,  and  to 
diminish  in  the  same  proportion  as  the  weight  increases  ;  for  in  that  case  the 
moment  of  the  weight  will  never  vary,  whatever  it  gains  by  the  increase  of 
weight  being  lost  by  the  diminished  leverage,  and  whatever  it  loses  by  the 
diminished  weight  being  gained  by  the  increased  leverage.  An  axle,  the  sur- 
face of  which  is  curved  in  such  a  manner  that  the  thickness  on  which  the  rope 
is  coiled  continually  increases  or  diminishes  in  the  same  proportion  as  the 
weight  or  resistance  diminishes  or  increases,  will  produce  this  effect. 

It  is  obvious  that  all  that  has  been  said  respecting  a  variable  weight  or  re- 
sistance is  also  applicable  to  a  variable  power,  which,  therefore,  may,  by  the 
same  means,  be  made  to  produce  a  uniform  effect.  An  instance  of  this  occurs 
in  a  watch,  which  is  moved  by  a  spiral  spring.  When  the  watch  has  been 
wound  up,  this  spring  acts  with  its  greatest  intensity,  and,  as  the  watch  goes 
down,  the  elastic  force  of  the  spring  gradually  loses  its  energy.  This  spring 
is  connected  by  a  chain  with  an  axle  of  varying  thickness,  called  a  fusee. 
When  the  spring  is  at  its  greatest  intensity,  the  chain  acts  upon  the  thinnest 
part  of  the  fusee,  and,  as  it  is  uncoiled,  it  acts  upon  a  part  of  the  fusee  which  is 
continually  increasing  in  thickness,  the  spring  at  the  same  time  losing  its  elas- 
tic power  in  exactly  the  same  proportion.  A  representation  of  the  fusee,  and 
the  cylindrical  box  which  contains  the  spring,  is  given  in  fig.  20,  and  of*  the 
spring  itself  in  fig.  21. 


Fig.  20. 


Fig.  21. 


When  great  power  is  required,  wheels  and  axles  may  be  combined  in  a 
manner  analogous  to  a  compound  system  of  levers,  explained  in  fig.  9.  In  this 
case  the  power  acts  on  the  circumference  of  the  first  wheel,  and  its  effect  is 
transmitted  to  the  circumference  of  the  first  axle.  That  circumference  is  placed 
in  connexion  with  the  circumference  of  the  second  wheel,  and  the  effect  is 
thereby  transmitted  to  the  circumference  of  the  second  axle,  and  so  on.  It  is 
obvious,  from  what  was  there  shown,  that  the  power  of  such  a  combination 
of  wheels  and  axles  will  be  found  by  multiplying  together  the  powers  of  the 
several  wheels  of  which  it  is  composed.  It  is  sometimes  convenient  to  com- 
pute this  power  by  numbers,  expressing  the  proportions  of  the  circumferences 
or  diameters  of  the  several  wheels,  to  the  circumferences  or  diameters  of  the 
several  axles  respectively.  This  computation  is  made  by  first  multiplying  the 
numbers  together  which  express  the  circumferences  or  diameters  of  the  wheels, 

VOL,.  II.— 17 


258 


THE  LEVER  AND  WHEELWORE. 


and  then  multiplying  together  the  numbers  which  express  the  circumferences 
or  diameters  of  the  several  axles.  The  proportion  of  the  two  products  will 
express  the  power  of  the  machine.  Thus,  if  the  circumferences  or  diameters 
be  as  the  numbers  10,  14,  and  15,  their  product  will  be  2,100;  and  if  the  cir- 
cumferences or  diameters  of  the  axles  be  expressed  by  the  numbers  3,  4,  and 
5,  their  product  will  be  60,  and  the  power  of  the  machine  will  be  expressed  by 
the  proportion  of  2,100  and  60,  or  35  to  1. 

The  manner  in  which  the  circumferences  of  the  axles  act  upon  the  circum- 
ferences of  the  wheels  in  compound  wheelwork  is  various.  Sometimes  a  strap 
or  cord  is  applied  to  a  groove  in  the  circumference  of  the  axle,  and  carried 
round  a  similar  groove  in  the  circumference  of  the  succeeding  wheel.  The 
friction  of  this  cord  or  strap  with  the  groove  is  sufficient  to  prevent  its  sliding, 
and  to  communicate  the  force  from  the  axle  to  the  wheel,  or  vice  versa.  This 
method  of  connecting  wheelwork  is  represented  in  fig.  22. 


Fig.  22. 


Numerous  examples  of  wheels  and  axles  driven  by  straps  or  cords  occur  m 
machinery,  applied  to  almost  every  department  of  the  arts  and  manufactures. 
In  the  turning-lathe,  the  wheel  worked  by  the  treddle  is  connected  with  the 
mandrel  by  a  catgut  cord  passing  through  grooves  in  the  wheel  and  axle.  In 
all  great  factories  revolving  shafts  are  carried  along  the  apartments,  on  which, 
at  certain  intervals,  straps  are  attached,  passing  round  their  circumferences, 
and  carried  round  the  wheels  which  give  motion  to  the  several  machines.  If 
the  wheels,  connected  by  straps  or  cords,  are  required  to  revolve  in  the  same 
direction,  these  cords  are  arranged  as  in  fig.  22  ;  but  if  they  are  required  to  re- 
volve in  contrary  directions,  they  are  applied  as  in  fig.  23. 

Fig.  23. 


One  of  the  chief  advantages  of  the  method  of  transmitting  motion  between 
wheels  and  axles  by  straps  or  cords  is,  that  the  wheel  and  axle  may  be  placed 
at  any  distance  from  each  other  which  may  be  found  convenient,  and  may  be 
made  to  turn  either  in  the  same  or  contrary  directions. 

When  the  circumference  of  the  wheel  acts  immediately  on  the  circumfer- 
ence of  the  succeeding  axle,  some  means  must  necessarily  be  adopted  to  pre- 
vent the  wheel  from  moving  in  contact  with  the  axle  without  compelling  the 
latter  to  turn.  If  the  surfaces  of  both  were  perfectly  smooth,  so  that  all  fric- 
tion were  removed,  it  is  obvious  that  either  would  slide  over  the  surface  of  the 
other  without  communicating  motion  to  it.  But,  on  the  other  hand,  if  there 
were  any  asperities,  however  small,  upon  these  surfaces,  they  would  become 
mutually  inserted  among  each  other,  and  neither  the  wheel  nor  axle  could 
move  without  causing  the  asperities  with  which  its  edge  is  studded  to  encoun- 


THE  LEVER  AND  WHEELWORK. 


259 


ter  those  asperities  which  project  from  the  surface  of  the  other ;  and  thus,  un- 
til these  projections  should  be  broken  off,  both  wheel  and  axle  must  be  moved 
at  the  same  time.  It  is  on  this  account  that,  if  the  surfaces  of  the  wheels  and 
axles  are  by  any  means  rendered  rough,  and  pressed  together  with  sufficient 
force,  the  motion  of  either  will  turn  the  other,  provided  the  load  or  resistance 
be  not  greater  than  the  force  necessary  to  break  off  these  small  projections 
which  produce  the  friction. 

In  cases  where  great  power  is  not  required,  motion  is  communicated  in  this 
way  through  a  train  of  wheelwork,  by  rendering  the  surface  of  the  wheel  and 
axle  rough,  either  by  facing  them  with  buff  leather,  or  with  wood  cut  across 
the  grain.  This  method  is  sometimes  used  in  spinning  machinery,  where  one 
large  buffed  wheel,  placed  in  a  horizontal  position,  revolves  in  contact  with 
several  small  buffed  rollers,  each  roller  communicating  motion  to  a  spindle. 
The  position  of  the  wheel  W,  and  the  rollers  R  R,  &c.,  are  represented  in 
fig.  24.  Each  roller  can  be  thrown  out  of  contact  with  the  wheel,  and  restored 
to  it  at  pleasure. 

The  communication  of  motion  between  wheels  and  axles  by  friction  has  the 
advantage  of  great  smoothness  and  evenness,  and  of  proceeding  with  little 
noise  ;  but  this  method  can  only  be  used  in  cases  where  the  resistance  is  not 
very  considerable,  and  therefore  is  seldom  adopted  in  works  on  a  large  scale. 
Dr.  Gregory  mentions  an  instance  of  a  sawmill  at  Southampton,  England,  where 


the  wheels  act  upon  each  other  by  the  contact  of  the  end  grain  of  wood.  The 
machinery  makes  very  little  noise,  and  wears  very  well,  having  been  used  not 
less  than  twenty  years. 

The  most  usual  method  of  transmitting  motion  through  a  train  of  wheelwork 
is  by  the  formation  of  teeth  upon  their  circumferences,  so  that  these  indentures 
of  each  wheel  fall  between  the  corresponding  ones  of  that  in  which  it  works, 
and  insure  the  action  so  long  as  the  strain  is  not  so  great  as  to  fracture  the 
tooth. 

In  the  formation  of  teeth,  very  minute  attention  must  be  given  to  their  fio-ure, 
in  order  that  the  motion  may  be  communicated  from  wheel  to  wheel  with 
smoothness  and  uniformity.  This  can  only  be  accomplished  by  shaping  the 
teeth  according  to  curves  of  a  peculiar  kind,  which  mathematicians  have  in- 
vented, and  assigned  rules  for  drawing.  The  ill  consequences  of  neglecting 
this  will  be  very  apparent,  by  considering  the  nature  of  the  action  which  would 
be  produced  if  the  teeth  were  formed  of  square  projecting  pins,  as  in  fio-.  25. 
When  the  tooth  A  comes  into  contact  with  B,  it  acts  obliquely  upon  it,  and,  as 
it  moves,  the  corner  of  B  sUdes  upon  the  plane  surface  of  A  in  such  a  manner 
as  to  produce  much  friction,  and  to  grind  away  the  side  of  A  and  the  end  of  B. 
As  they  approach  the  position  C  D,  they  sustain  a  jolt  the  moment  their  sur- 


260 


THE  LEVER  AND  WHEELWORK. 


faces  come  into  full  contact ;  and  after  passing  the  position  of  C  D,  the  same 
scraping  and  grinding  efl'ect  is  produced  in  the  opposite  direction,  until,  by  the 


Fig.  25. 


revolution  of  the  wheels,  the  teeth  become  disengaged.      These  effects  are 
avoided  by  giving  to  the  teeth  the  curved  forms  represented  in  fig.  26      By 

-       •  Fig.  26. 


such  means  the  surfaces  of  the  teeth  roll  upon  each  other  with  very  inconsid- 
erable friction,  and  the  direction  in  which  the  pressure  is  excited  is  always 
that  of  a  line,  M  N,  touching  the  two  wheels,  and  at  right  angles  to  the  radii. 
Thus  the  pressure,  being  always  the  same,  and  acting  with  the  same  leverage, 
produces  a  uniform  effect. 

When  wheels  work  together,  their  teeth  must  necessarily  be  of  the  same 
size,  and  therefore  the  proportion  of  their  circumferences  may  always  be  esti- 
mated by  the  number  of  teeth  which  they  carry.  Hence  it  follows  that,  in 
computing  the  power  of  compound  wheelwork,  the  number  of  teeth  may  al- 
ways be  used  to  express  the  circumferences  respectively,  or  the  diameters 
which  are  proportional  to  these  circumferences.  When  teeth  are  raised  upon 
an  axle,  it  is  generally  called  a  pinion,  and  in  that  case  the  teeth  are  called 
leaves.  The  rule  for  computing  the  train  of  wheelwork,  given  in  fig.  9,  will  be 
expressed  as  follows  :  When  the  wheel  and  axle  carry  teeth,  multiply  together 
the  number  of  teeth  in  each  of  the  wheels,  and  next  the  number  of  leaves  in 
each  of  the  pinions  ;  the  proportion  of  the  two  products  will  express  the  power 
of  the  machine.  If  some  of  the  wheels  and  axles  carry  teeth,  and  others  not, 
this  computation  may  be  made  by  using  for  those  circumferences  which  do  not 
bear  teeth  the  number  of  teeth  which  would  fill  them.  Fig.  27  represents  a 
train  of  three  wheels  and  pinions.  The  wheel  F,  which  bears  the  power,  and 
the  axle  which  bears  the  weight,  have  no  teeth  ;  but  it  is  easy  to  find  the  num- 
ber of  teeth  which  they  would  carry. 

It  is  evident  that  each  pinion  revolves  much  more  frequently  in  a  given  time 
than  the  wheel  which  it  drives.  Thus,  if  the  pinion  C  be  furnished  with  ten 
teeth,  and  the  wheel  E,  which  it  drives,  have  sixty  teeth,  the  pinion  C  must 
turn  six  times,  in  order  to  turn  the  wheel  E  once  round.  The  velocities  of 
revolution  of  every  wheel  and  pinion  which  work  in  one  another  will,  there- 
fore, have  the  same  proportion  as  their  number  of  teeth  taken  in  a  reverse  or- 


THE  LEVER  AND  WHBELWORK. 


261 


der,  and  by  this  means  the  relative  velocity  of  wheels  and  pinions  may  be  de- 
termined according  to  any  proposed  rate. 

Wheelwork,  like  all  other  machinery,  is  used  to  transmit  and  modify  force 


Fig.  27. 

XI 

HP 


m  every  department  of  the  arts  and  manufactures  ;  but  it  is  also  used  in  cases 
where  motion  alone,  and  not  force,  is  the  object  to  be  attained.  The  most  re- 
markable example  of  this  occurs  in  watch  and  clock  work,  where  the  object 
is  merely  to  produce  uniform  motions  of  rotation,  having  certain  proportions, 
and  without  any  regard  to  the  elevation  of  weights,  or  the  overcoming  of  resist- 
ances. 

A  crane  is  an  example  of  combination  of  wheelwork  used  for  the  purpose  of 
raising  or  lowering  great  weights.     Fig.  28  represents  a  machine  of  this  kind. 

Fig.  28. 


A  B  is  a  strong  vertical  beam,  resting  on  a  pivot,  and  secured  in  its  position 
by  beams  in  the  floor.  It  is  capable,  however,  of  turning  on  its  axis,  being 
confined  between  rollers  attached  to  the  beams  and  fixed  in  the  floor.  C  D  is 
a  projecting  arm,  called  a  gib,  formed  of  beams  which  are  mortised  into  A  B. 
The  wheelwork  is  mounted  in  two  casiiron  crosses,  bolted  on  each  side  of  the 
beams,  one  of  which  appears  at  E  F  G  H.  The  winch  at  which  the  power  is 
applied  is  at  I.  This  carries  a  pinion  immediately  behind  H.  This  pinion 
works  in  a  wheel,  K,  which  carries  another  pinion  upon  its  axle.  This  last 
pinion  works  in  a  larger  wheel,  L,  which  carries  upon  its  axis  a  barrel,  M,  on 


262 


THE  LEVER  AND  WHEELWOHK. 


which  a  chain  or  rope  is  coiled.  The  chain  passes  over  a  pulley,  D,  at  the 
top  of  the  gib.  At  the  end  of  the  chain  a  hook,  0,  is  attached,  to  support  the 
weight  W.  During  the  elevation  of  the  weight,  it  is  convenient  that  its  recoil 
should  be  hindered  in  case  of  any  occasional  suspension  of  the  power.  This 
is  accomplished  by  a  ratchet-wheel  attached  to  the  barrel  M,  as  illustrated  in 
fig.  10  ;  but  when  the  weight  W  is  to  be  lowered,  the  catch  must  be  removed 
from  this  ratchet-wheel.  In  this  case,  the  too-rapid  descent  of  the  weight  is 
in  some  cases  checked  by  pressure  excited  on  some  part  of  the  wheelwork,  so 
as  to  produce  sufficient  friction  to  retard  the  descent  in  any  required  degree,  or 
even  to  suspend  it,  if  necessary.  The  vertical  beam  at  B  resting  on  a  pivot, 
and  being  fixed  between  rollers,  allows  the  gib  to  be  turned  round  in  any  direc- 
tion ;  so  that  a  weight  raised  from  one  side  of  the  crane  may  be  carried  round 
and  deposited  on  another  side,  at  any  distance  within  the  range  of  the  gib. 
Thus,  if  a  crane  be  placed  upon  a  wharf  near  a  vessel,  weights  may  be  raised, 
and,  when  elevated,  the  gib  may  be  turned  round  so  as  to  let  them  descend 
into  the  hold. 

The  power  of  this  machine  may  be  computed  upon  the  principles  already 
explained.  The  magnitude  of  the  circle,  in  which  the  power  at  I  moves,  may 
be  determined  by  the  radius  of  the  winch,  and  therefore  the  number  of  teeth 
which  a  wheel  of  that  size  would  carry  may  be  found.  In  like  manner,  we 
may  determine  the  number  of  leaves  in  a  pinion  whose  magnitude  would  be 
equal  to  the  barrel  M.  Let  the  first  number  be  multiplied  by  the  number  of 
teeth  in  the  wheel  K,  and  that  product  by  the  number  of  teeth  in  the  wheel  L. 
Next,  let  a  number  of  leaves  in  the  pinion  H  be  multiplied  by  the  number  of- 
leaves  in  the  pinion  attached  to  the  axle  of  the  wheel  K,  and  let  that  product 
be  multiplied  by  the  number  of  leaves  in  a  pinion  whose  diameter  is  equal  to 
that  of  the  barrel  M.  These  two  products  will  express  the  power  of  the  ma- 
chine. 

Toothed  wheels  are  of  three  kinds,  distinguished  by  the  position  which  the 
teeth  bear  with  respect  to  the  axis  of  the  wheel.  When  they  are  raised  upon 
the  edge  of  the  wheel,  as  in  fig.  27,  they  are  called  spur  wheels,  or  spur  gear. 
When  they  are  raised  parallel  to  the  axis,  as  in  fig.  29,  it  is  called  a  crown 


Pig.  29. 


wheel.     When  the  teeth  are  raised  on  a  surface  inclined  to  the  plane  of  the 
wheel,  as  in  fig.  30,  they  are  called  bevelled  wheels. 


Fig.  30. 


If  a  motion  round  one  axis  is  to  be  communicated  to  another  axis  parallel  to 


THE  LEVER  AND  WHEELWORK. 


263 


it,  spur  gear  is  generally  used.  Thus,  in  fig.  27,  the  three  axes  are  parallel 
to  each  other.  If  a  motion  round  one  axis  is  to  be  communicated  to  another 
at  right  angles  to  it,  a  crown  wheel,  working  in  a  spur  pinion,  as  in  fig.  29, 
will  serve  ;  or  the  same  object  may  be  obtained  by  two  bevelled  wheels,  as  in 
fig.  30. 

If  a  motion  round  one  axis  is  required  to  be  communicated  to  another  in- 
clined to  it  at  any  proposed  angle,  two  bevelled  wheels  can  always  be  used. 
In  fig.  31,  let  A  B  and  A  C  be  the  two  axles  ;  two  bevelled  wheels,  such  as 


D  E  and  E  F,  on  these  axles  will  transmit  the  motion  or  rotation  from  one  to 
the  other,  and  the  relative  velocity  may,  as  usual,  be  regulated  by  the  propor- 
tional magnitude  of  the  wheels. 

In  order  to  equalize  the  wear  of  the  teeth  of  a  wheel  and  pinion,  which  work 
in  one  another,  it  is  necessary  that  every  leaf  of  the  pinion  should  work  in 
succession  through  every  tooth  of  the  wheel,  and  not  continually  act  upon  the 
same  set  of  teeth.  If  the  teeth  coidd  be  accurately  shaped  according  to  math- 
ematical principles,  and  the  materials  of  which  they  are  formed  be  perfectly 
uniform,  this  precaution  would  be  less  necessary  ;  but,  as  slight  inequalities, 
both  of  material  and  form,  must  necessarily  exist,  the  effects  of  these  should  be 
as  far  as  possible  equalized,  by  distributing  them  through  every  part  of  the 
wheel.  For  this  purpose,  it  is  usual,  especially  in  millwork,  where  considera- 
ble force  is  used,  so  to  regulate  the  proportion  of  the  number  of  teeth  in  the 
wheel  and  pinion,  that  the  same  leaf  of  the  pinion  shall  not  be  engaged  twice 
with  any  one  tooth  of  the  wheel  until  after  the  action  of  a  number  of  teeth,  ex- 
pressed by  the  product  of  the  number  of  teeth  in  the  wheel  and  pinion.  Let 
us  suppose  that  the  pinion  contains  ten  leaves,  which  we  shall  denominate  by 
the  numbers  1,  2,  3,  &c.,  and  that  the  wheel  contains  60  teeth  similarly  de- 
nominated. At  the  commencement  of  the  motion,  suppose  the  leaf  1  of  the 
pinion  engages  the  tooth  1  of  the  wheel ;  then,  after  one  revolution,  the  leaf  1 
of  the  pinion  will  engage  the  tooth  11  of  the  wheel,  and  after  two  revolutions 
the  leaf  1  of  the  pinion  will  engage  the  tooth  21  of  the  wheel,  and  in  like  man- 
ner, after  three,  four,  and  five  revolutions  of  the  pinion,  the  leaf  1  will  engage 
successively  the  teeth  31,  41,  and  51  of  the  wheel.  After  the  sixth  revolution, 
the  leaf  1  of  the  pinion  will  engage  the  tooth  1  of  the  wheel.  Thus  it  is  evi- 
dent that,  in  the  case  here  supposed,  the  leaf  1  of  the  pinion  will  continually 
be  engaged  with  the  teeth  1,  11,21,  31,  41,  and  51  of  the  wheel,  and  no  oth- 
ers. The  like  may  be  said  of  every  leaf  of  the  pinion.  Thus  the  leaf  2  of  the 
pinion  will  be  successively  engaged  with  the  teeth  2,  12,  22,  32,  42,  and  52 
of  the  wheel,  and  no  others.  Any  accidental  inequalities  of  these  teeth  will 
therefore  continually  act  upon  each  other,  until  the  circumference  of  the  wheel 
be  divided  into  parts  of  ten  teeth  each,  unequally  worn.  This  effect  would  be 
avoided  by  giving  either  the  wheel  or  pinion  one  tooth  more  or  one  tooth  less. 
Thus,  suppose  the  wheel,  instead  of  having  60  teeth,  had  61,  then,  after  six 
revolutions  of  the  pinion,  the  leaf  1  of  the  pinion  would  be  engaged  with  the 


264 


THE  LEVER  AND  WHEELWORK. 


tooth  61  of  the  wheel ;  and,  after  one  revolution  of  the  wheel,  the  leaf  2  of  the 
pinion  would  be  engaged  with  the  tooth  1  of  the  wheel.  Thus,  during  the  first 
revolution  of  the  wheel,  the  leaf  1  of  the  pinion  would  be  successively  engaged 
with  the  teeth  1,  11,  21,  31,  41,  51,  and  61  of  the  wheel  ;  at  the  commence- 
ment of  the  second  revolution  of  the  wheel  the  leaf  2  of  the  pinion  would  be 
engaged  with  the  tooth  1  of  the  wheel  ;  and,  during  the  second  revolution  of 
the  wheel,  the  leaf  1  of  the  pinion  would  be  successively  engaged  with  the 
teeth  10,  20,  30,  40,  50,  and  60  of  the  wheel.  In  the  same  manner  it  may  be 
shown  that,  in  the  third  revolution  of  the  wheel,  the  leaf  1  of  the  pinion  would 
be  successively  engaged  with  the  teeth  9,  19,  29,  39,  49,  and  59  of  the  wheel  ; 
during  the  fourth  revolution  of  the  wheel,  the  leaf  1  of  the  pinion  would  be 
successively  engaged  with  the  teeth  8,  18,  28,  38,  48,  and  58  of  the  wheel. 
By  continuing  this  reasoning  it  will  appear  that,  during  the  tenth  revolution  of 
the  wheel,  the  leaf  1  of  the  pinion  will  be  engaged  successively  with  the  teeth 
2,  12,  22,  32,  42,  and  52  of  the  wheel.  At  the  commencement  of  the  eleventh 
revolution  of  the  wheel  the  leaf  1  of  the  pinion  will  be  engaged  with  the  tooth 
1  of  the  wheel,  as  at  the  beginning  of  the  motion.  It  is  evident,  therefore,  that, 
during  the  first  ten  revolutions  of  the  wheel,  each  leaf  of  the  pinion  has  been 
successively  engaged  with  every  tooth  of  the  wheel,  and  that  during  these  ten 
revolutions  the  pinion  has  revolved  61  times.  Thus  the  leaves  of  the  pinion 
have  acted  610  times  upon  the  teeth  of  the  wheel,  before  two  teeth  can  have 
acted  twice  upon  each  other. 

The  odd  tooth  which  produces  this  effect  is  called  by  millwrights  the  hunt- 
ing-cog. 

The  most  familiar  case  in  which  wheelwork  is  used  to  produce  and  regulate 
motion  merely,  without  any  reference  to  weights  to  be  raised  or  resistances  to 
be  overcome,  is  that  of  chronometers.  In  watch  and  clock  work,  the  object  is 
to  cause  a  wheel  to  revolve  with  a  uniform  velocity,  and  at  a  certain  rate.  The 
motion  of  this  wheel  is  indicated  by  an  index  or  hand  placed  upon  its  axis,  and 
carried  round  with  it.  In  proportion  to  the  length  of  the  hand,  the  circle  over 
which  its  extremity  plays  is  enlarged,  and  its  motion  becomes  more  percepti- 
ble. This  circle  is  divided,  so  that  very  small  fractions  of  a  revolution  of  the 
hand  may  be  accurately  observed.  In  most  chronometers  it  is  required  to  give 
motion  to  two  hands,  and  sometimes  to  three.  These  motions  proceed  at  dif- 
ferent rates,  according  to  the  subdivisions  of  time  generally  adopted.  One 
wheel  revolves  in  a  minute,  bearing  a  hand  which  plays  round  a  circle  divided 
into  sixty  equal  parts  ;  the  motion  of  the  hand  over  each  part  indicating  one 
second,  and  a  complete  revolution  of  the  hand  being  performed  in  one  minute. 
Another  wheel  revolves  once,  while  the  former  revolves  sixty  times  ;  conse- 
quently the  hand  carried  by  this  wheel  revolves  once  in  sixty  minutes,  or  one 
hour.  The  circle  on  which  it  plays  is,  like  the  former,  divided  into  sixty 
equal  parts,  and  the  motion  of  the  hand  over  each  division  is  performed  in 
one  minute.  This  is  generally  called  the  minute  hand,  and  the  former  the  sec- 
ond hand. 

A  third  wheel  revolves  once,  while  that  which  carries  the  minute  hand  re- 
volves twelve  times  ;  consequently  this  last  wheel,  which  carries  the  hour  hand, 
revolves  at  a  rate  twelve  times  less  than  that  of  the  minute  hand,  and  therefore 
seven  hundred  and  twenty  times  less  than  the  second  hand.  We  shall  now 
endeavor  to  explain  the  manner  in  which  these  motions  are  produced  and  reg- 
ulated. Let  A,  B,  C,  D,  E,  fig.  32,  represent  a  train  of  wheels,  and  a,  b,  c,  d, 
represent  their  pinions,  e  being  a  cylinder  on  the  axis  of  the  wheel  E,  round 
which  a  rope  is  coiled,  sustaining  a  weight,  W.  Let  the  effect  of  this  weight, 
transmitted  through  the  train  of  wheels,  be  opposed  by  a  power,  P,  acting  upon 
the  wheel  A,  and  let  this  power  be  supposed  to  be  of  such  a  nature  as  to  cause 


THE  LEVER  AND  WHEELWORK. 


265 


the  weight  W  to  descend  with  a  uniform  velocity,  and  at  any  proposed  rate. 
The  wheel  E  carries  on  its  circumference  eighty-four  teeth.  The  wheel  D 
carries  eighty  teeth  ;  the  wheel  C  is  also  furnished  with  eighty  teeth,  and  the 

Pig.  32. 


wheel  B  with  seventy-five.  The  pinions  d  and  c  are  each  furnished  with 
twelve  leaves,  and  the  pinions  b  and  a  with  ten. 

If  the  power  at  P  be  so  regulated  as  to  allow  the  wheel  A  to  revolve  once 
in  a  minute,  with  a  uniform  velocity,  a  hand  attached  to  the  axis  of  this  wheel 
will  serve  as  the  second  hand.  The  pinion  a,  carrying  ten  teeth,  must  revolve 
seven  times  and  a  half  to  produce  one  revolution  of  B,  consequently  fifteen 
revolutions  of  the  wheel  A  will  produce  two  revolutions  of  the  wheel  B  ;  the 
wheel  B  therefore  revolves  twice  in  fifteen  minutes.  The  pinion  b  must  re- 
volve eight  times  to  produce  one  revolution  of  the  wheel  C,  and  therefore  the 
wheel  C  must  revolve  once  in  four  quarters  of  an  hour,  or  in  one  hour.  If  a 
hand  be  attached  to  the  axis  of  this  wheel,  it  will  have  the  motion  necessary 
for  the  minute  hand.  The  pinion  c  must  revolve  six  and  two  thirds  times  to 
produce  one  revolution  of  the  wheel  D,  and  therefore  this  wheel  must  revolve 
once  in  six  and  two  thirds  hours.  The  pinion  d  revolves  seven  times  for  one 
revolution  of  the  wheel  E,  and  therefore  the  wheel  E  will  revolve  once  in 
forty-six  and  two  thirds  hours. 

On  the  axis  of  the*  wheel  C  a  second  pinion  may  be  placed,  furnished  with 
seven  leaves,  which  may  lead  a  wheel  of  eighty-four  teeth,  so  that  this  wheel 
shall  turn  once  during  twelve  turns  of  the  wheel  C.  If  a  hand  be  fixed  upon 
the  axis,  this  hand  will  revolve  once  for  twelve  revolutions  of  the  minute  hand 
fixed  upon  the  axis  of  the  wheel  C  ;  that  is,  it  will  revolve  once  in  twelve 
hours.  If  it  play  upon  a  dial  divided  into  twelve  equal  parts,  it  will  move  over 
each  part  in  an  hour,  and  will  serve  the  purpose  of  the  hour  hand  of  the  chro- 
nometer. 

We  have  here  supposed  that  the  second  hand,  the  minute  hand,  and  the  hour 
hand,  move  on  separate  dials.  This,  however,  is  not  necessary.  The  axis  of 
the  hour  hand  is  commonly  a  tube,  enclosing  within  it  that  of  the  minute  hand, 
so  that  the  same  dial  serves  for  both.  The  second  hand,  however,  is  generally 
furnished  with  a  separate  dial. 

We  shall  now  explain  the  manner  in  which  a  power  is  applied  to  the  wheel 
A,  so  as  to  regulate  and  equalize  the  eff'ect  of  the  weight  W.  Suppose  the 
wheel  A  furnished  with  thirty  teeth,  as  in  fig.  33  ;  if  nothing  check  the  mo- 
tion, the  weight  W  would  descend  with  an  accelerated  velocity,  and  would 
communicate  an  accelerated  motion  to  the  wheel  A.  This  efl'ect,  however,  is 
interrupted  by  the  following  contrivance  :  L  M  is  a  pendulum  vibrating  on 
the  centre  L,  and  so  regulated  that  the  time  of  its  oscillation  is  one  second. 
The  pallets  I  and  K  are  connected  with  the  pendulum,  so  as  to  oscillate  with 
it.  In  the  position  of  the  pendulum  represented  in  the  figure,  the  pallet  I  stops 
the  motion  of  the  wheel  A,  and  entirely  suspends  the  action  of  the  weight  W, 


266 


THE  LEVER  AND  WHEELWORK. 


fig.  32,  SO  that  for  a  moment  the  entire  machine  is  motionless.  The  weio^ht 
M,  however,  falls  by  its  gravity  toward  the  lowest  position,  and  disengages  the 
pallet  I  from  the  tooth  of  the  wheel.     The  Weight  W  begins  then  to  take  ef- 


fect, and  the  wheel  A  turns  from  A  toward  B.  Meanwhile  the  pendulum  M 
oscillates  to  the  other  side,  and  the  pallet  K  falls  under  a  tooth  of  the  wheel 
A,  and  checks  for  a  moment  its  further  motion.  On  the  returning  vibration, 
the  pallet  K  becomes  again  disengaged,  and  allows  the  tooth  of  the  wheel  to 
escape,  and  by  the  influence  of  the  weight  W  another  tooth  passes  be- 
fore the  motion  of  the  wheel  A  is  again  checked  by  the  interposition  of  the 
pallet  I. 

From  this  explanation  it  will  appear  that,  in  two  vibrations  of  the  pendulum, 
one  tooth  of  the  wheel  A  passes  the  pallet  I,  and  therefore,  if  the  wheel  A  be 
furnished  with  30  teeth,  it  will  be  allowed  to  make  one  revolution  during  60 
vibrations  of  the  pendulum.  If,  therefore,  the  pendulum  be  regulated  so-  as 
to  vibrate  seconds,  this  wheel  will  revolve  once  in  a  minute.  From  the  ac- 
tion of  the  pallets  in  checking  the  motion  of  the  wheel  A,  and  allowing  its 
teeth  alternately  to  escape,  this  has  been  called  the  escapement  wheel ; 
and  the  wheel  and  pallets  together  are  generally  called  the  escapement,  or 
^scapement. 

.  We  have  already  explained  that,  by  reason  of  the  friction  on  the  points 
of  support,  and  other  causes,  the  swing  of  the  pendulum  would  gradually 
diminish,  and  its  vibration  at  length  cease.  This,  however,  is  prevented 
by  the  action  of  the  teeth  of  the  'scapement  wheel  upon  the  pallets,  which 
is  just  sufficient  to  communicate  that  quantity  of  force  to  the  pendulum  which 
is  necessary  to  counteract  the  retarding  effects,  and  to  maintain  its  mo- 
tion. It  thus  appears  that,  although  the  effect  of  the  gravity  of  the  weight 
W  in  giving  motion  to  the  machine  is  at  intervals  suspended,  yet  this  part  of 
the  force  is  not  lost,  being,  during  these  intervals,  employed  in  giving  to  the 
pendulum  all  that  motion  which  it  would  lose  by  the  resistances  to  which  it  is 
inevitably  exposed. 

In  stationary  clocks,  and  in  other  cases  in  which  the  bulk  of  the  machine 
is  not  an  objection,  a  descending  weight  is  used  as  the  moving  power.  But  in 
watches  and  portable  chronometers,  this  would  be  attended  with  evident  incon- 
venience.    In  such  cases,  a  spiral  spring,  called  the  main-spring,  is  the  mov- 


THE  LEVER  AND  WHEELWORK. 


267 


ing  power.  The  manner  in  which  this  spring  communicates  rotation  to  an 
axis,  and  the  ingenious  method  of  equalizing  the  effect  of  its  variable  elasticity 
by  giving  to  it  a  leverage,  which  increases  as  the  elastic  force  diminishes,  has 
been  already  explained. 

A  similar  objection  lies  against  the  use  of  a  pendulum  in  portable  chronom- 
eters. A  spiral  spring  of  a  similar  kind,  but  infinitely  more  delicate,  called  a 
hair-spring,  is  substituted  in  its  place.  This  spring  is  connected  with  a  nicely- 
balanced  wheel,  called  the  balance-wheel,  which  plays  in  pivots.  When  this 
wheel  is  turned  to  a  certain  extent  in  one  direction,  the  hair-spring  is  coiled 
up,  and  its  elasticity  causes  the  wheel  to  recoil,  and  return  to  a  position  in 
which  the  energy  of  the  spring  acts  in  the  opposite  direction.  The  balance 
whe6l  then  returns,  and  continually  vibrates  in  the  same  manner.  The  axis 
of  this  wheel  is  furnished  with  pallets  similar  to  those  of  the  pendulum,  which 
are  alternately  engaged  with  the  teeth  of  a  crown  wheel,  which  takes  the  place 
of  the  'scapement  wheel  already  described. 

A  general  view  of  the  work  of  a  common  watch  is  represented  in  fig.  34. 


Fig.  34. 


A  is  the  balance  wheel,  bearing  pallets  p  p  upon  its  axis  ;  C  is  the  crown 
wheel,  whose  teeth  are  suffered  to  escape  alternately  by  those  pallets  in  the 
manner  already  described  in  the  'scapement  of  a  clock.  On  the  axis  of  the 
crown  wheel  is  placed  a  pinion,  d,  which  drives  another  crown  wheel,  K.  On 
the  axis  of  this  is  placed  the  pinion  c,  which  plays  in  the  teeth  of  the  third 
wheel  L.  The  pinion  h,  on  the  axis  of  L,  is  engaged  with  the  wheel  M, 
called  the  centre  wheel.  The  axle  of  this  wheel  is  carried  up  through  the 
centre  of  the  dial.  A  pinion,  a,  is  placed  upon  it,  which  works  in  the  great 
wheel  N.  On  this  wheel  the  main-spring  immediately  acts.  O  P  is  the 
main-spring  stripped  of  its  barrel.  The  axis  of  the  wheel  M,  passing  through 
the  centre  of  the  dial,  is  squared  at  the  end  to  receive  the  minute-hand.  A 
second  pinion,  Q,  is  placed  upon  this  axle,  which  drives  a  wheel,  T.  On  the 
axle  of  this  wheel  a  pinion,^,  is  placed,  which  drives  the  hour-wheel  V.  This 
wheel  is  placed  upon  a  tubular  axis,  which  encloses  within  it  the  axis  of  the 
wheel  M.  This  tubular  axis,  passing  through  the  centre  of  the  dial,  carries 
the  hour-hand. 

The  wheels  A,  B,  C,  D,  E,  fig.  32,  correspond  to  the  wheels  C,  K,  L,  M, 
N,  fig.  34  ;  and  the  pinions  a,  b,  c,  d,  e,  fig.  32,  correspond  to  the  pinions  d,  c, 
b,  a,  fig.  34.  From  what  has  already  been  explained  of  these  wheels,  it  will 
be  obvious  that  the  wheel  M,  fig.  34,  revolves  once  in  an  hour,  causing  the 
minute-hand  to  move  round  the  dial  once  in  that  time.  This  wheel  at  the 
same  time  turns  the  pinion  Q,  which  leads  the  wheel  T.     This  wheel  again 


268 


THE  LEVER  AND  WHEELWORK. 


turns  the  pinion^,  which  leads  the  hour  wheel  V.  The  leaves  and  teeth  of 
these  pinions  and  wheels  are  proportioned,  as  already  explained,  so  that  the 
wheel  V  revolves  once  during  twelve  revolutions  of  the  wheel  M.  The  hour- 
hand,  therefore,  which  is  carried  by  the  tubular  axle  of  the  wheel  V,  moves 
once  round  the  dial  in  twelve  hours. 

Our  object  here  has  not  been  to  give  a  detailed  account  of  watch  and  clock 
work.  Such  a  general  account  has  only  been  attempted  as  may  explain  how 
tooth-and-pinion  work  may  be  applied  to  regulate  motion. 


THE  PULLEY. 


Cord. — Sheave. — Fixed  Pulley. — Fire-Escapes. — Single  moveable  Pulley. — Systems  of  PuUeys.- 
Smeaton's  Tackle. — White's  Pulley. — Advantage  of. — Runner. — Spanish  Bartons. 


THE  PULLEY. 


271 


THE   PULLEY, 


The  class  of  simple  machines  which  present  themselves  to  our  attention  at 
this  time,  is  that  which  is  called  the  cord.  If  a  rope  were  perfectly  flexible, 
and  were  capable  of  being  bent  over  a  sharp  edge,  and  of  moving  upon  it  with- 
out friction,  we  should  be  enabled  by  its  means  to  make  a  force  in  any  one  direc- 
tion overcome  resistance,  or  communicate  motion  in  any  other  direction.  Thus 
if  P,  fig.  1,  be  such  an  edge,  a  perfectly  flexible  rope  passing  over  it  would  be 

Fig.  1. 


capable  of  transmitting  a  force  S  F  to  a  resistance  Q  B,  so  as  to  support  or 
overcome  B,  or  by  a  motion  in  the  direction  of  S  F  to  produce  another  motion 
in  the  direction  B  Q.  But  as  no  materials  of  which  ropes  can  be  constructed 
can  give  them  perfect  flexibility,  and  as,  in  proportion  to  the  strength  by  which 
they  are  enabled  to  transmit  force,  their  rigidity  increases,  it  is  necessary,  in 
practice,  to  adopt  means  to  remove  or  mitigate  those  effects  which  attend  im- 
perfect flexibility,  and  which  would  otherwise  render  cords  practically  inappli- 
cable as  machines. 

When  a  cord  is  used  to  transmit  a  force  from  one  direction  to  another,  its 
stiffness  renders  some  force  necessary  in  bending  it  over  the  angle  P,  which 
the  two  directions  form  ;  and  if  the  angle  be  sharp,  the  exertion  of  such  a  force 
may  be  attended  with  the  rupture  of  the  cord.  If,  instead  of  bending  the  rope 
at  one  point  over  a  single  angle,  the  change  of  direction  were  produced  by  suc- 
cessively deflecting  it  over  several  angles,  each  of  which  would  be  less  sharp 


272 


THE  PULLEY. 


than  a  single  one  could  be,  tbe  force  requisite  for  the  deflection,  as  well  as  the 
liability  of  rupturing  the  cord,  would  be  considerably  diminished.  But  this 
end  will  be  still  more  perfectly  attained  if  the  deflection  of  the  cord  be  pro- 
duced by  bending  it  over  the  surface  of  a  curve. 

If  a  rope  were  applied  only  to  sustain,  and  not  to  move  a  weight,  this  would 
be  sufficient  to  remove  the  inconveniences  arising  from  its  rigidity.  But  when 
motion  is  to  be  produced,  the  rope,  in  passing  over  the  curved  surface,  would 
be  subject  to  excessive  friction,  and  consequently  to  rapid  wear.  This  incon- 
venience is  removed  by  causing  the  surface  on  which  the  rope  runs  to  move 
with  it,  so  that  no  more  friction  is  produced  than  would  arise  from  the  curved 
surface  rolling  upon  the  rope. 

All  these  ends  are  attained  by  the  common  pulley,  which  consists  of  a  wheel 
called  a  sheave,  fixed  in  a  block  and  turning  on  pivots.  A  groove  is  formed  in 
the  edge  of  the  wheel,  in  which  the  rope  runs,  the  wheel  revolving  with  it. 
Such  an  apparatus  is  represented  in  fig.  2. 

Fig.  2. 


We  shall,  for  the  present,  omit  the  consideration  of  that  part  of  the  effects 
of  the  stiflfness  and  friction  of  the  machine  which  is  not  removed  by  the  con- 
trivance just  explained,  and  shall  consider  the  rope  as  perfectly  flexible,  and 
moving  without  friction. 

From  the  definition  of  a  flexible  cord,  it  follows  that  its  tension,  or  the  force 
by  which  it  is  stretched  throughout  its  entire  length,  must  be  uniform.  From 
this  principle,  and  this  alone,  all  the  mechanical  properties  of  pulleys  may  be 
derived. 

Although,  as  already  explained,  the  whole  mechanical  efficacy  of  this  ma- 
chine depends  on  the  qualities  of  the  cord,  and  not  on  those  of  the  block  and 
sheave,  which  are  Only  introduced  to  remove  the  accidental  effects  of  stiflfness 
and  friction,  yet  it  has  been  usual  to  give  the  name  pulley  to  the  block  and 
sheave,  and  a  combination  of  blocks,  sheaves,  and  ropes,  is  called  a  tackle. 

When  the  rope  passes  over  a  single  wheel,  which  is  fixed  in  its  position,  as 
in  fig.  2,  the  machine  is  called  du  fixed  pulley .  Since  the  tension  of  the  cord  is 
uniform  throughout  its  length,  it  follows  that  in  this  machine  the  power  and 
weight  are  equal.  For  the  weight  stretches  that  part  of  the  cord  which  is 
between  the  weight  and  pulley,  and  the  power  stretches  that  part  between  the 
power  and  the  pulley  ;  and  since  the  tension  throughout  the  whole  length  is 
the  same,  the  weight  must  be  equal  to  the  power. 

Hence  it  appears  that  no  mechanical  advantage  is  gained  by  this  machine. 
Nevertheless,  there  is  scarcely  any  engine,  simple  or  complex,  attended  with 
more  convenience.  In  the  application  of  power,  whether  of  men  or  animals, 
or  arising  from  natural  forces,  there  are  always  some  directions  in  which  it 
may  be  exerted  to  much  greater  convenience  and  advantage  than  others,  and 
in  many  cases  the  exertion  of  these  powers  is  limited  to  a  single  direction.  A 
machine,  therefore,  which  enables  us  to  give  the  most  advantageous  direction 
to  the  moving  power,  whatever  be  the  direction  of  the  resistance  opposed  to 
it,  contributes  as  much  practical  convenience  as  one  which  enables  a  small 


power  to  balance  or  overcome  a  great  weight.  In  directing  the  power  against 
the  resistance,  it  is  often  necessary  to  use  two  fixed  pulleys.  Thus,  in  eleva- 
ting a  weight  A,  fig.  3,  to  the  summit  of  a  building,  by  the  strength  of  a  horse 
moving  below,  two  fixed  pulleys,  B  and  C,  may  be  used.  The  rope  is  carried 
from  A  over  the  pulley  B  ;  the  rope  passes,  and  returning  downward,  is  brought 
under  C,  and  finally  drawn  by  the  animal  on  the  horizontal  plane.  In  the  same 
manner  sails  are  spread,  and  flags  hoisted  on  the  yards  and  masts  of  a  ship,  by 
sailors  pulling  a  rope  on  the  deck. 


Fis.  3. 


By  means  of  the  fixed  pulley  a  man  may  raise  himself  to  a  considerable 
height,  or  descend  to  any  proposed  depth.  If  he  be  placed  in  a  chair  or 
bucket  attached  to  one  end  of  a  rope,  which  is  carried  over  a  fixed  pulley,  by 
laying  hold  of  this  rope  on  the  other  side,  as  represented  in  fig.  4,  he  may,  at 
will,  descend  to  a  depth  equal  to  half  of  the  entire  length  of  the  rope,  by  con- 
tinually yielding  rope  on  the  one  side,  and  depressing  the  bucket  or  chair  by 
his  weight  on  the  other.  Fire-escapes  have  been  constructed  on  this  principle, 
the  fixed  pulley  being  attached  to  some  part  of  the  building. 

A  single  moveable  pulley  is  represented  in  fig.  5.  A  cord  is  carried  from  a 
fixed  point  F,  and,  passing  through  a  block  B,  attached  to  a  weight  W,  passes 
over  a  fixed  pulley  C,  the  power  being  applied  at  P.  We  shall  first  suppose 
the  parts  of  the  cord  on  each  side  the  wheel  B  to  be  parallel ;  in  this  case,  the 
whole  weight  W  being  sustained  by  the  parts  of  the  cords  B  C  and  B  F, 
and  these  parts  being  equally  stretched,  each  must  sustain  half  the  weight, 
which  is  therefore  the  tension  of  the  cord.  This  tension  is  resisted  by  the 
power  at  P,  which  must  therefore  be  equal  to  half  the  weight.  In  this  ma- 
chine, therefore,  the  weight  is  twice  the  power. 


Fig.  5. 


Fig.  6. 


fS 


yfi 


If  the  parts  of  the  cord  B  C  and  B  F  be  not  parallel,  as  in  fig.  6,  a  greater 
power  than  half  the  weight  is  therefore  necessary  to  sustain  it.  To  determine 
the  power  necessary  to  support  a  given  weight,  in  this  case  take  the  line  B 
A  in  the  vertical  direction,  consisting  of  as  many  inches  as  the  weight  consists 
of  ounces  ;  from  A  draw  A  D  parallel  to  B  C,  and  A  E  parallel  to  B  F ;  the 

VOli.  II.— 18 


274 


THE  PULLEY. 


force  of  the  weight  represented  by  A  B  will  be  equivalent  to  two  forces  repre- 
sented by  B  D  and  B  E.  The  number  of  inches  in  these  lines  respectively 
will  represent  the  number  of  ounces  which  are  equivalent  to  the  tensions  of 
the  parts  B  b'  and  B  C  of  the  cord.  But  as  these  tensions  are  equal,  B  D 
and  B  E  must  be  equal,  and  each  will  express  the  amount  of  the  power  P, 
which  stretches  the  cord  at  P  C. 

It  is  evident  that  the  four  lines,  A  E,  E  B,  B  D,  and  D  A,  are  equal.  And 
as  each  of  them  represents  the  power,  the  weight  which  is  represented  by 
A  B  must  be  less  than  twice  the  power  which  is  represented  by  A  E  and  E  B 
taken  together.  It  follows,  therefore,  that  as  parts  of  the  ropes  which  support 
the  weight  depart  from  parallelism,  the  machine  becomes  less  and  less  effica- 
cious ;  and  there  are  certain  obliquities  at  which  the  equilibrating  power  would 
be  much  greater  than  the  weight. 

The  mechanical  power  of  pulleys  admits  of  being  almost  indefinitely  in- 
creased by  combination.  Systems  of  pulleys  may  be  divided  into  two  classes  : 
those  in  which  a  single  rope  is  used,  and  those  which  consist  of  several  dis- 
tinct ropes.     Figs.  7  and  8,  represent  two  systems  of  pulleys,  each  having  a 

Fig.  8. 


single  rope.  The  weight  is  in  each  case  attached  to  a  moveable  block  B,  in 
which  are  fixed  two  or  more  wheels  ;  A  is  a  fixed  block,  and  ihe  rope  is  suc- 
cessively passed  over  the  wheels  above  and  below,  and,  after  passing  over  the 
last  wheel  above,  is  attached  to  the  power.  The  tension  of  thai  part  of  the 
cord  to  which  the  power  is  attached  is  produced  by  the  power,  and  therefore 
equivalent  to  it,  and  the  same  tension  must  extend  throughout  its  whole  length. 
The  weight  is  sustained  by  all  those  parts  of  the  cord  which  pass  from  the 
lower  block,  and,  as  the  force  which  stretches  them  all  is  the  same,  viz.,  that 
of  the  power,  the  effect  of  the  weight  must  be  equally  distributed  among  them, 
their  directions  being  supposed  to  be  parallel.  It  will  be  evident,  from  this 
reasoning,  that  the  weight  will  be  as  many  times  greater  than  the  power,  as 
the  number  of  cords  which  support  the  lower  block.  Thus,  if  there  be  six 
cords,  each  cord  will  support  a  sixth  part  of  the  weight — that  is,  the  weight 
will  be  six  times  the  tension  of  the  cord,  or  six  times  the  power.  In  fig.  7, 
the  cord  is  represented  as  being  finally  attached  to  a  hook  on  the  upper  block. 
But  it  may  be  carried  over  an  additional  wheel  fixed  in  that  block,  and  finally 
attached  to  a  hook  in  the  lower  block,  as  in  fig.  8,  by  which  one  will  be  added 
to  the  power  of  the  machine,  the  number  of  cords  at  the  lower  block  being  in- 
creased by  one.  In  the  system  represented  in  fig.  7,  the  wheels  are  placed 
in  the  blocks  one  above  the  other  ;  in  fig.  8  they  are  placed  side  by  side.     In 


THE  PULLEY. 


275 


all  systems  of  pulleys  of  this  class,  the  weight  of  the  lower  block  is  to  be  con- 
sidered as  a  part  of  the  weight  to  be  raised,  and,  in  estimating  the  power  of 
the  machine,  this  should  always  be  attended  to. 

When  the  power  of  the  machine,  and  therefore  the  number  of  wheels,  is 
considerable,  some  difficulty  arises  in  the  arrangement  of  the  wheels  and 
cords.  The  celebrated  Smeaton  contrived  a  tackle,  which  takes  its  name 
from  him,  in  which  there  are  ten  wheels  in  each  block  :  five  large  wheels 
placed  side  by  side,  and  five  smaller  ones  similarly  placed  above  them  in  the 
lower  block,  and  below  them  in  the  upper.     Fig.  9  represents  Smeaton'e  blocks 


without  the  rope.  The  wheels  are  marked  with  the  numbers  1,  2,  3,  &c.,  in 
the  order  in  which  the  rope  is  to  be  passed  over  them.  As  in  this  pulley, 
twenty  distinct  parts  of  the  rope  support  the  lower  block,  the  weight,  including 
the  lower  block,  will  be  twenty  times  the  equilibrating  power. 

In  all  these  systems  of  pulleys,  every  wheel  has  a  separate  axle,  and  there 
is  a  distinct  wheel  for  every  turn  of  the  rope  at  each  block.  Each  wheel  is 
attended  with  friction  on  its  axle,  and  also  with  friction  between  the  sheave 
and  block.  The  machine  is  by  this  means  robbed  of  a  great  part  of  its  efficacy, 
since,  to  overcome  the  friction  alone,  a  considerable  power  is  in  most  cases 
necessary. 

An  ingenious  contrivance  has  been  suggested,  by  which  all  the  advantage 
of  a  large  number  of  wheels  may  be  obtained  without  the  multiplied  friction  of 
distinct  sheaves  and  axles.  To  comprehend  the  excellence  of  this  contrivance 
it  will  be  necessary  to  consider  the  rate  at  which  the  rope  passes  over  the  sev- 
eral wheels  of  such  a  system,  as  fig.  7.  If  one  foot  of  the  rope  G  F  pass  over 
the  pulley  F,  two  feet  must  pass  over  the  pulley  E,  because  the  distance  be- 
tween F  and  E  being  shortened  one  foot,  the  total  length  of  the  rope  G  F  E 
must  be  .shortened  two  feet.  These  two  feet  of  rope  must  pass  in  the  direc- 
tion E  D  ;  and  the  wheel  D,  rising  one  foot,  three  feet  of  rope  must  conse- 
quently pass  over  it.  These  three  feet  of  rope  passing  in  the  direction  D  C 
and  the  rope  D  C  being  also  shortened  one  foot  by  the  ascent  of  the  lower 
block,  four  feet  of  rope  must  pass  over  the  wheel  C.  In  the  same  way  it  may 
be  shown  that  five  feet  must  pass  over  B,  and  six  feet  over  A.  Thus,  what- 
ever be  the  number  of  wheels  in  the  upper  and  lower  blocks,  the  parts  of  the 
rope  which  pass  in  the  same  time  over  the  wheels  in  the  lower  block  are  in 
the  proportion  of  the  odd  numbers  1,  3,  5,  &c.  ;  and  those  which  pass  over  the 


276 


THE  PULLEY. 


wheels  in  the  upper  block  in  the  same  time,  are  as  the  even  numbers  2,  4,  6, 
&c.  If  the  wheels  were  all  of  equal  size,  as  in  fig.  8,  they  would  revolve 
with  velocities  proportional  to  the  rate  at  which  the  rope  passes  over  them  :  so 
that,  while  the  first  wheel  below  revolves  once,  the  first  wheel  above  will  re- 
volve twice  ;  the  second  wheel  below  three  times  ;  the  second  wheel  above 
four  times,  and  so  on.  If,  however,  the  wheels  differed  in  size  in  proportion 
to  the  quantity  of  rope  which  must  pass  over  them,  they  would  evidently  re- 
volve in  the  same  time.  Thus,  if  the  first  wheel  above  were  twice  the  size 
of  the  first  wheel  below,  one  revolution  would  throw  off  twice  the  quantity  of 
rope.  Again,  if  the  second  wheel  below  were  thrice  the  size  of  the  first  wheel 
below,  it  would  throw  off  in  one  revolution  thrice  the  quantity  of  rope,  and  so 
on.  Wheels  thus  proportioned,  revolving  in  exactly  the  same  time,  might  be 
all  placed  on  one  axle,  and  would  partake  of  one  common  motion,  or,  what  is 
to  the  same  effect,  several  grooves  might  be  cut  upon  the  face  of  one  solid 
wheel,  with  diameters  in  the  proportion  of  the  odd  numbers  1,  3,  5,  &c.,  for 
the  lower  pulley,  and  corresponding  grooves  on  the  face  of  another  solid  wheel 
represented  by  the  even  numbers  2,  4,  6,  &c.,  for  the  upper  pulley.  The  rope, 
being  passed  successively  over  the  grooves  of  such  wheels,  would  be  thrown 
off  exactly  in  the  same  manner  as  if  every  groove  were  upon  a  separate  wheel, 
and  every  wheel  revolved  independently  of  the  others.  Such  is  White's  pul- 
ley, represented  in  fig.  10.  v 


The  advantage  of  this  machine,  when  accurately  constructed,  is  very  con- 
siderable. The  friction,  even  when  great  resistances  are  to  be  opposed,  is 
very  trifling  ;  but,  on  the 'Other  hand,  it  has  corresponding  disadva.ntages  which 
greatly  circumscribe  its  practical  utility.  In  the  workmanship  of  the  grooves, 
great  difficulty  is  found  in  giving  them  the  exact  proportions  ;  in  doing  which, 
the  thickness  of  the  rope  must  be  accurately  allowed  for ;  and  consequently  it 
follows  that  the  same  pulley  can  never  act,  except  with  a  rope  of  a  particular 
diameter.  A  very  slight  deviation  from  the  true  proportion  of  the  grooves  will 
cause  the  rope  to  be  unequally  stretched,  and  will  throw  on  some  parts  of  it  an 


undue  proportion  of  the  weight,  while  other  parts  become  nearly,  and  some- 
times altogether  slack.  Besides  these  defects,  the  rope  is  so  liable  to  derange- 
ment by  being  thrown  out  of  the  grooves,  that  the  pulley  can  scarcely  be  con- 
sidered portable. 

For  these  and  other  reasons,  this  machine,  ingenious  as  it  unquestionably 
is,  has  never  been  extensively  used. 

In  the  several  systems  of  pulleys  just  explained,  the  hook  to  which  the  fixed 
block  is  attached  supports  the  entire  of  both  the  power  and  weight.  When 
the  machine  is  in  equilibrium,  the  power  only  supports  so  much  of  the  weight 
as  is  equal  to  the  tension  of  the  cord,  all  the  remainder  of  the  weight  being 
thrown  on  the  fixed  point. 

If  the  power  be  moved  so  as  to  raise  the  weight,  it  will  move  with  a  velocity 
as  many  times  greater  than  that  of  the  weight,  as  the  weight  itself  is  greater 
than  the  power.  Thus  in  fig.  7,  if  the  weight  attached  to  the  lower  block  as- 
cend one  foot,  six  feet  of  line  will  pass  over  the  pulley  A,  according  to  what 
has  been  already  proved.  Thus  the  power  will  descend  through  six  feet, 
while  the  weight  rises  one  foot.  But,  in  this  case,  the  weight  is  six  times  the 
power. 

When  two  or  more  ropes  are  used,  pulleys  may  be  combined  in  various  ways 
so  as  to  produce  any  degree  of  mechanical  effect.  If  to  any  of  the  .systems 
already  described,  a  single  moveable  pulley  be  added,  the  power  of  the  ma- 
chine would  be  doubled.  In  this  case,  the  second  rope  is  attached  to  the  hook 
of  the  lower  block,  as  in  fig.  11,  and,  being  carried  through  a  moveable  pulley 


Fig.  U. 


Fig.  12. 


attached  to  the  weight,  it  is  finally  brought  up  to  a  fixed  point.  The  tension 
of  the  sFecond  cord  is  equal  to  half  the  weight ;  and  therefore  the  power  P,  by 
means  of  the  first  cord,  will  have  only  half  the  tension  which  it  would  have  if 
the  weight  were  attached  to  the  lower  block.  A  moveable  pulley  thus  applied 
is  called  a  runner. 

Two  systems  of  pulleys,  called  Spanish  bartons,  having  each  two  ropes,  are 
represented  in  fig.  12.  The  tension  of  the  rope  P  A  B  C  in  the  first  system 
is  equal  to  the  power  ;  and  therefore  the  parts  B  A  and  B  C  support  a  portion 
of  the  weight  equal  to  twice  the  -power.  The  rope  E  A  supports  the  tensions 
of  A  P  and  A  B  ;  and  therefore  the  tension  of  A  E  D  is  twice  the  power. 
Thus  the  united  tensions  of  the  ropes  which  support  the  pulley  B  is  four  times 


278 


THE  PULLEY. 


the  power,  which  is  therefore  the  amount  of  the  weight.  In  the  second  sys- 
tem, the  rope  P  A  D  is  stretched  by  the  power.  The  rope  A  E  B  C  acts 
against  the  united  tensions  A  P  and  A  D  ;  and  therefore  the  tension  of  A  E  or 
E  B  is  twice  the  power.  Thus  the  weight  acts  against  three  tensions  :  two 
of  which  are  equal  to  twice  the  power,  and  the  remaining  one  is  equal  to  the 
power.     The  weight  is  therefore  equal  to  five  times  the  power. 

A  single  rope  may  be  so  arranged  with  one  moveable  pulley  as  to  support  a 
weight  equal  to  three  times  the  power.  In  fig.  13,  this  arrangement  is  repre- 
sented, where  the  numbers  sufficiently  indicate  the  tension  of  the  rope,  and 
the  proportion  of  the  weight  and  power.  In  fig.  14,  another  method  of  produ- 
cing the  same  effect  with  two  ropes  is  represented. 


Fig.  13. 


Fig.  14. 


If  several  single  moveable  pulleys  be  made  successively  to  act  upon  each 
other,  the  effect  is  doubled  by  every  additional  pulley  :  such  a  system  as  this 
is  represented  in  fig.  15.  The  tension  of  the  first  rope  is  equal  to  the  power  ; 
the  second  rope  acts  against  twice  the  tension  of  the  first,  and  therefore  it  is 
stretched  with  a  force  equal  to  twice  the  power ;  the  third  rope  acts  against 
twice  this  tension,  and  therefore  it  is  stretched  with  a  force  equal  to  four  times 
the  power,  and  so  on. 

In  this  system,  it  is  obvious  that  the  ropes  will  require  to  have  diflferent  de- 
grees of  strength,  since  the  tension  to  which  they  are  subject  increases  in  a 
double  proportion  from  the  power  to  the  weight. 

If  each  of  the  ropes,  instead  of  being  attached  to  fixed  points  at  the  top,  are 
carried  over  fixed  pulleys,  and  attached  to  the  several  moveable  pulleys  re- 
spectively, as  in  fig.  16,  the  power  of  the  machine  will  be  greatly  increased  ; 
for  in  that  case  the  forces  which  stretch  the  successive  ropes  increase  in  a 


Fig.  16. 


Pig.  17. 


THE  PULLEY. 


279 


treble  instead  of  a  double  proportion,  as  will  be  evident  by  attending  to  the 
numbers  which  express  the  tensions  in  the  figure.  One  rope  would  render 
the  weight  three  times  the  power  ;  two  ropes  nine  times  ;  three  ropes  twenty- 
seven  times,  and  so  on.  An  arrangement  of  pulleys  is  represented  in  fig.  17, 
by  which  each  rope,  instead  of  being  finally  attached  to  a  fixed  point,  as  in 
fig.  15,  is  attached  to  the  weight.  The  weight  is  in  this  case  supported  by 
three  ropes  :  one  stretched  with  a  force  equal  to  the  power  ;  another  with  a 
force  equal  to  twice  the  power  ;  and  a  third  with  a  force  equal  to  four  times 
the  power.     The  weight  is,  therefore,  in  this  case,  seven  times  the  power. 

If  the  ropes,  instead  of  being  attached  to  the  weight,  pass  through  wheels, 
as  in  fig.  18,  and  are  finally  attached  to  the  pulleys  above,  the  power  of  the 
machine  will  be  considerably  increased.  In  the  system  here  represented,  the 
weight  is  twenty-six  times  the  power. 

In  considering  these  several  combinations  of  pulleys,  we  have  omitted  to 
estimate  the  effects  produced  by  the  weights  of  the  sheaves  and  blocks.  With- 
out entering  into  the  details  of  this  computation,  it  may  be  observed  generally 
that  in  the  systems  represented  in  figs.  15  and  16,  the  weight  of  the  wheel  and 
blocks  acts  against  the  power;  but  that  in  figs.  17  and  18,  they  assist  the  power 
in  supporting  the  weight.  In  the  systems  represented  in  fig.  12,  the  weight 
of  the  pulleys,  to  a  certain  extent,  neutralize  each  other. 

It  will  in  all  cases  be  found  that  that  quantity  by  which  the  weight  exceeds 
the  power,  is  supported  by  fixed  points  ;  and  therefore,  although  it  be  commonly 
stated  that  a  small  power  supports  a  great  weight,  yet,  in  the  pulley,  as  in  all 
other  machines,  the  power  supports  no  more  of  the  weight  than  is  exactly  equal 
to  its  own  amount.  It  will  not  be  necessary  to  establish  this  in  each  of  the 
examples  which  have  been  given  ;  having  explained  it  in  one  instance,  the  stu- 
dent will  find  no  difficulty  in  applying  the  same  reasoning  to  others.  In  fig. 
15,  the  fixed  pulley  sustains  a  force  equal  to  twice  the  power,  and  by  it  the 
power  giving  tension  to  the  first  rope  sustains  a  part  of  the  weight  equal  to 
itself.  The  first  hook  sustains  a  portion  of  the  weight  equal  to  the  tension  of 
the  first  string,  or  to  the  power.  The  second  hook  sustains  a  force  equal  to 
twice  the  power  ;  and  the  third  hook  sustains  a  force  equal  to  four  times  the 
power.  The  three  hooks  therefore  sustain  a  portion  of  the  weight  equal  to 
seven  times  the  power  ;  and  the  weight  itself  being  eight  times  the  power,  it 
is  evident  that  the  part  of  the  weight  which  remains  to  be  supported  by  the 
power,  is  equal  to  the  power  itself. 

When  a  weight  is  raised  by  any  of  the  systems  of  pulleys  which  have  been 
last  described,  the  proportion  between  the  velocity  of  the  weight  and  the  ve- 
locity of  the  power,  so  frequently  noticed  in  other  machines,  will  always  be 
observed.  In  the  system  of  pulleys  represented  in  fig.  15,  the  weight  being 
eight  times  the  power,  the  velocity  of  the  power  will  be  eight  times  that  of 
the  weight.  If  the  power  be  moved  through  eight  feet,  that  part  of  the  rope 
between  the  fixed  pulley  and  the  first  moveable  pulley  will  be  shortened  by 
eight  feet.  And  since  the  two  parts  which  lie  above  the  first  moveable  pulley 
must  be  equally  shortened,  each  will  be  diminished  by  four  feet ;  therefore  the 
first  pulley  will  rise  through  four  feet,  while  the  power  moves  through  eight 
feet.  In  the  same  way  it  may  be  shown  that  while  the  first  pulley  moves 
through  four  feet,  the  second  moves  through  two  ;  and  while  the  second  moves 
through  two,  the  third,  to  which  the  weight  is  attached,  is  raised  through  one 
foot.  While  the  power,  therefore,  is  carried  through  eight  feet,  the  weight  is 
moved  through  one  foot. 

By  reasoning  similar  to  this  it  may  be  shown  that  the  space  through  which 
the  power  is  moved  in  every  case  is  as  many  times  greater  than  the  height 
through  which  the  weight  is  raised,  as  the  weight  is  greater  than  the  power. 


280 


THE  PULLEY. 


From  its  portable  form,  cheapness  of  construction,  and  the  facility  with 
which  it  may  be  applied  in  almost  every  situation,  the  pulley  is  one  of  the 
most  useful  of  the  simple  machines.  The  mechanical  advantage,  however, 
which  it  appears  in  theory  to  possess,  is  considerably  diminished  in  practice, 
owing  to  the  stiffness  of  the  cordage  and  the  friction  of  the  wheels  and  blocks. 
By  this  means,  it  is  computed  that  in  most  cases  so  great  a  proportion  as  two 
thirds  of  the  power  is  lost.  The  pulley  is  much  used  in  building,  where 
weights  are  to  be  elevated  to  great  heights.  But  its  most  extensive  applica- 
tion is  found  in  the  rigging  of  ships,  where  almost  every  motion  is  accom- 
plished by  its  means. 

In  all  the  examples  of  pulleys,  we  have  supposed  the  parts  of  the  rope  sus- 
taining the  weight,  and  each  of  the  moveable  pulleys,  to  be  parallel  to  each 
other.  If  they  be  subject  to  considerable  obliquity,  the  relative  tensions  of 
the  different  ropes  must  be  estimated  according  to  the  principle  applied  in 
figure  6. 


THE  IICLIIED  PLAIE,  VEDGE, 
AID  SCRE¥. 


Inclined  Plane. — Effect  of  a  Weight  on. — Power  of. — Roads. — Power  oblique  to  tl:e  Plane. — Plane 
sometimes  moves  under  the  Weight. — Wedge. — Sometimes  formed  of  two  inclined  Planes. — More 
powerful  as  its  Angle  is  acute. — Where  used. — Limits  to  the  Angle. — Screw. — Hunter's  Screw. — 
Examples. — Micrometer  Screw. 


THE  INCLIIED  PLANE,  WEDGE 
AND  SCRE¥. 


The  inclined  plane  is  the  most  simple  of  all  machines.  It  is  a  hard  plane 
surface  forming  some  angle  with  a  horizontal  plane,  that  angle  not  being  a 
right  angle.  When  a  weight  is  placed  on  such  a  plane,  a  twofold  effect  is 
produced.  A  part  of  the  effect  of  the  weight  is  resisted  by  the  plane  and  pro- 
duces a  pressure  upon  it ;  and  the  remainder  urges  the  weight  down  the  plane, 
and  would  produce  a  pressure  against  any  surface  resisting  its  motion  placed 
in  a  direction  perpendicular  to  the  plane. 

Let  A  B,  fig.  1,  be  such  a  plane,  B  C  its  horizontal  base,  A  C  its  height, 
and  A  B  C  its  angle  of  elevation.     Let  W  be  a  weight  placed  upon  it.     This 


Fig.  1. 


...,-A. 


weight  acts  in  the  A^ertical  direction  W  D,  and  is  equivalent  to  two  forces — 
W  F  perpendicular  to  the  plane,  and  W  E  directed  down  the  plane.  If  a 
plane  be  placed  at  right  angles  to  the  inclined  plane  below  W,  it  will  resist  the 
descent  of  the  weight,  and  sustain  a  pressure  expressed  by  WE.  Thus,  the 
weight  W  resting  in  the  corner,  instead  of  producing  one  pressure  in  the  di- 
rection W  D,  will  produce  two  pressures  :   one  expressed  by  W  F  upon  the 


284 


THE  INCLINED  PLANE,  WEDGE,  AND  SCREW. 


inclined  plane,  and  the  other  expressed  by  W  E  upon  the  resisting  plane. 
These  pressures  respectively  have  the  same  proportion  to  the  entire  weight  as 
W  F  and  W  E  have  to  W  D,  or  as  D  E  and  W  E  have  to  W  D,  because  D  E 
is  equal  to  W  F.  Now  the  triangle  W  E  D  is  in  all  respects  similar  to  the 
triangle  A  B  C,  the  one  differing  from  the  other  only  in  the  scale  on  which  it 
is  constructed.  Therefore  the  three  lines  A  C,  C  B,  and  B  A,  are  in  the  same 
proportion  to  each  other  as  the  lines  W  E,  E  D,  and  W  D.  Hence  A  B  has 
to  A  C  the  same  proportion  as  the  whole  weight  has  to  the  pressure  directed 
toward  B,  and  A  B  has  to  B  C  the  same  proportion  as  the  whole  weight  has 
to  the  pressure  on  the  inclined  plane. 

We  have  here  supposed  the  weight  to  be  sustained  upon  the  inclined  plane, 
by  a  hard  plane  fixed  at  right  angles  to  it.  But  the  power  necessary  to  sus- 
tain the  weight  will  be  the  same,  in  whatever  way  it  is  applied,  provided  it  act 
in  the  direction  of  the  plane.  Thus  a  cord  may  be  attached  to  the  weight,  and 
stretched  toward  A,  or  the  hands  of  men  may  be  applied  to  the  weight  below 
it,  so  as  to  resist  its  descent  toward  B.  But  in  whatever  way  it  be  applied, 
the  amount  of  the  power  will  be  determined  in  the  same  manner.  Suppose  the 
weight  to  consist  of  as  many  pounds  as  there  are  inches  in  A  B,  then  the  power 
requisite  to  sustain  it  upon  the  plane  will  consist  of  as  many  pounds  as  there 
are  inches  in  A  C,  and  the  pressure  on  the  plane  will  amount  to  as  many  pounds 
as  there  are  inches  in  B  C. 

From  what  has  been  stated,  it  may  easily  be  inferred  that  the  less  the  ele- 
vation of  the  plane  is,  the  less  will  be  the  power  requisite  to  sustain  a  given 
weight  upon  it,  and  the  greater  will  be  the  pressure  upon  it.  Suppose  the  in- 
clined plane  A  B  to  turn  upon  a  hinge  at  B,  and  to  be  depressed  so  that  its 
angle  of  elevation  shall  be  diminished,  it  is  evident  that  as  this  angle  decreases, 
the  height  of  the  plane  decreases,  and  its  base  increases.  Thus,  when  it  takes 
the  position  B  A',  the  height  A'  C  is  less  than  the  former  height  A  C,  while  the 
base  B  C  is  greater  than  the  former  base  B  C.  The  power  requisite  to  support 
the  weight  upon  the  plane  in  the  position  B  A'  is  represented  by  A'  C,  and 
is  as  much  less  than  the  power  requisite  to  sustain  it  upon  the  plane  A  B,  as 
the  height  A'  C  is  less  than  the  height  A  C.  On  the  other  hand,  the  pressure 
upon  the  plane  in  the  position  B  A'  is  as  much  greater  than  the  pressure  upon 
the  plane  B  A,  as  the  base  B  C  is  greater  than  the  base  B  C. 

The  power  of  an  inclined  plane,  considered  as  a  machine,  is  therefore  esti- 
mated by  the  proportion  which  the  length  bears  to  the  height.  This  power  is 
always  increased  by  diminishing  the  elevation  of  the  plane. 

Roads  which  are  not  level  may  be  regarded  as  inclined  planes,  and  loads 
drawn  upon  them  in  carriages,  considered  in  reference  to  the  powers  which 
impel  them,  are  subject  to  all  the  conditions  which  have  been  established  for 
inclined  planes.  The  inclination  of  the  road  is  estimated  by  the  height  cor- 
responding to  some  proposed  length.  Thus  it  is  said  to  rise  one  foot  in  fifteen, 
one  foot  in  twenty,  &c.,  meaning  that  if  fifteen  or  twenty  feet  of  the  road  be 
taken  as  the  length  of  an  inclined  plane,  such  as  A  B,  the  corresponding 
height  will  be  one  foot.  Or  the  same  may  be  expressed  thus  :  that  if  fifteen 
or  twenty  feet  be  measured  upon  the  road,  the  difference  of  the  levels  of  the 
two  extremities  of  the  distance  measured  is  one  foot.  According  to  this  method 
of  estimating  the  inclination  of  roads,  the  power  requisite  to  sustain  a  load  upon 
them  (setting  aside  the  effect  of  friction)  is  always  proportional  to  that  eleva- 
tion. Thus,  if  a  road  rise  one  foot  in  twenty,  a  power  of  one  ton  will  be  suffi- 
cient to  sustain  twenty  tons,  and  so  on. 

On  a  horizontal  plane,  the  only  resistance  which  the  power  has  to  overcome, 
is  the  friction  of  the  load  with  the  plane,  and  the  consideration  of  this  being 
for  the  present  omitted,  a  weight  once  put  in  motion  would  continue  moving 


THE  INCLINED  PLANE,  WEDGE,  AND  SCREW. 


285 


for  ever,  without  any  further  action  of  the  power.  But  if  the  plane  be  inclined, 
the  power  will  be  expended  in  raising  the  weight  through  the  perpendicular 
height  of  the  plane.  Thus,  in  a  road  which  rises  one  foot  in  ten,  the  power 
is  expended  in  raising  the  weight  through  one  perpendicular  foot  for  every  ten 
feet  of  the  road  over  which  it  is  moved.  As  the  expenditure  of  power  depends 
upon  the  rate  at  which  the  weight  is  raised  perpendicularly,  it  is  evident  that 
the  greater  the  inclination  of  the  road  is,  the  slower  the  motion  must  be  with 
the  same  force.  If  the  energy  of  the  power  be  such  as  to  raise  the  weight  at 
the  rate  of  one  foot  per  minute,  the  weight  may  be  moved  in  each  minute 
through  that  length  of  the  road  which  corresponds  to  a  rise  of  one  foot.  Thus 
if  two  roads  rise,  one  at  the  rate  of  a  foot  in  fifteen  feet,  and  the  other  at  the 
rate  of  one  foot  in  twenty  feet,  the  same  expenditure  of  power  will  move  the 
weight  through  fifteen  feet  of  the  one,  and  twenty  feet  of  the  other  at  the  same 
rate. 

From  such  considerations  as  these,  it  will  readily  appear  that  it  may  often 
be  more  expedient  to  carry  a  road  through  a  circuitous  route  than  to  continue 
it  in  the  most  direct  course  ;  for,  though  the  measured  length  of  road  may  be  con- 
siderably greater  in  the  former  case,  yet  more  maybe  gained  in  speed  with  the 
same  expenditure  of  power,  than  is  lost  by  the  increase  of  distance.  By  at- 
tending to  these  circumstances,  modern  road-makers  have  greatly  facilitated 
and  expedited  the  intercourse  between  distant  places. 

If  the  power  act  oblique  to  the  plane,  it  will  have  a  twofold  effect :  a  part 
being  expended  in  supporting  or  drawing  the  weight,  and  a  part  in  diminishing 
or  increasing  the  pressure  upon  the  plane.  Let  W  P,  fig.  1,  be  the  power. 
This  will  be  equivalent  to  two  forces,  W  F',  perpendicular  to  the  plane,  and 
W  E',  in  the  direction  of  the  plane.  In  order  that  the  power  should  sustain 
the  weight,  it  is  necessary  that  that  part  W  E^  of  the  power  which  acts  in  the 
direction  of  the  plane,  should  be  equal  to  that  part  W  E,  fig.  1,  of  the  weight 
which  acts  down  the  plane.  The  other  part  W  F,  of  the  power  acting  perpen- 
dicular to  the  plane,  is  immediately  opposed  to  that  part  W  F  of  the  weight 
which  produces  pressure.  The  pressure  upon  the  plane  will  therefore  be  di- 
minished by  the  amount  of  W  F'.  The  amount  of  the  power,  which  will 
equilibrate  with  the  weight,  may,  in  this  case,  be  found  as  follows  :  Take  W  E' 
equal  to  W  E,  and  draw  E'  P  perpendicular  to  the  plane,  and  meeting  the 
direction  of  the  power.  The  proportion  of  the  power  to  the  weight  will  be 
that  of  W  P  to  W  D.  And  the  proportion  of  the  pressure  to  the  weight  will 
be  that  of  the  difference  between  W  F  and  W  F'  to  W  D.  If  the  amount  of 
the  power  have  a  less  proportion  to  the  weight  than  W  P  has  to  W  D,  it  will 
not  support  the  body  on  the  plane,  but  will  allow  it  to  descend.  And  if  it  had 
a  greater  proportion,  it  will  draw  the  weight  up  the  plane  toward  A. 

It  sometimes  happens  that  a  weight  upon  one  inclined  plane  is  raised  or 
supported  by  another  weight  upon  another  inclined  plane.  Thus,  if  A  B  and 
A  B',  fig.  2,  be  two  inclined  planes,  forming  an  angle  at  A,  and  W  W  be  two 
weights  placed  upon  these  planes,  and  connected  by  a  cord  passing  over  a 
pulley  at  A,  the  one  weight  will  either  sustain  the  other,  or  one  will  descend, 
drawing  the  other  up.     To  determine  the  circumstances  under  which  these 


Fig.  2. 


286 


THE  INCLINED  PLANE,  WEDGE,  AND  SCREW. 


effects  will  ensue,  draw  the  lines  W  D  and  W  D'  in  the  vertical  direction,  and  ! 
take  upon  them  as  many  inches  as  there  are  ounces  in  the  weights  respectively. 
W  D  and  W  D'  being  the  lengths  thus  taken,  and  therefore  representing  the 
weights,  the  lines  W  E  and  W  E^  will  represent  the  effects  of  these  weights 
respectively  down  the  planes.  If  W  E  and  W  E'  be  equal,  the  weights  will 
sustain  each  other  without  motion.  But  if  W  E  be  greater  than  W  E',  the 
weight  W  will  descend,  drawing  the  weight  W  up.  And  if  W  E'  be  greater 
than  W  E,  the  weight  W  will  descend,  drawing  the  weight  W  up.  In  every 
case,  the  lines  W  F  and  W  F'  will  represent  the  pressures  upon  the  planes 
respectively. 

It  is  not  necessary  for  the  effect  just  described,  that  the  inclined  planes 
should,  as  represented  in  the  figure,  form  an  angle  with  each  other.  They 
may  be  parallel,  or  in  any  other  position,  the  rope  being  carried  over  a  suffi- 
cient number  of  wheels  placed  so  as  to  give  it  the  necessary  deflection.  This 
method  of  moving  loads  is  frequently  applied  in  great  public  works  where  rail- 
roads are  used.  Loaded  wagons  descend  one  inclined  plane,  while  other 
wagons,  either  empty  or  so  loaded  as  to  permit  the  descent  of  those  with  which 
"they  are  connected,  are  drawn  up  the  other. 

In  the  application  of  the  inclined  plane,  which  we  have  hitherto  noticed, 
the  machine  itself  is  supposed  to  be  fixed  in  its  position,  while  the  weight  or 
load  is  moved  upon  it.  But  it  frequently  happens  that  resistances  are  to  be 
overcome  which  do  not  admit  to  be  thus  moved.  In  such  cases,  instead  of 
moving  the  load  upon  the  plane,  the  plane  is  to  be  moved  under  or  against  the 
load.     Let  D  E,  fig.  3,  be  a  heavy  beam  secured  in  a  vertical  position  be- 

Fig.  3. 


iween  guides,  F  G  and  H  I,  so  that  it  is  free  to  move  upward  or  downward, 
but  not  laterally.  Let  A  B  C  be  an  inclined  plane,  the  extremity  of  which  is 
placed  beneath  the  end  of  the  beam.  A  force  applied  to  the  back  of  this  plane 
A  C,  in  the  direction  C  B,  will  urge  the  plane  under  the  beam,  so  as  to  raise 
the  beam  to  the  position  represented  in  fig.  4.  Thus,  while  the  inclined  plane 
is  moved  through  the  distance  C  B,the  beam  is  raised  through  the  height  C  A. 

Pig.  4. 


THE  INCLINED  PLANE,  WEDGE,  AND  SCREW. 


287 


When  the  inclined  plane  is  applied  in  this  manner,  it  is  called  a  wedge.  And 
if  the  power  applied  to  the  back  were  a  continued  pressure,  its  proportion  to 
the  weight  would  be  that  of  A  C  to  C  B.  It  follows,  therefore,  that  the  more 
acute  the  angle  B  is,  the  more  powerful  will  be  the  wedge. 

In  some  cases  the  wedge  is  formed  of  two  inclined  planes,  placed  base  to 
base,  as  represented  in  fig.  5.     The  theoretical  estimation  of  the  power  of 


Eig.  5. 


this  machine  is  not  applicable  in  practice  with  any  degree  of  accuracy.     This  ( 
is  in  part  owing  to  the  enormous  proportion  which  the  friction  in  most  cases   ) 
bears  to  the  theoretical  value  of  the  power,  but  still  more  to  the  nature  of  the  < 
power  generally  used.     The  force  of  a  blow  is  of  a  nature  so  wholly  different  S 
from  continued  forces,  such  as  the  pressure  of  weights,  or  the  resistance  of-  I 
fered  by  the  cohesion  of  bodies,  that  they  admit  of  no  numerical  comparison.  ) 
Hence  we  cannot  properly  state   the  proportion  which  the  force  of  a  blow  ( 
bears  to  the  amount  of  a  weight  or  resistance.     The  wedge  is  almost  invaria-  i 
bly  urged  by  percussion,  while  the  resistances  which  it  has  to  overcome  are 
as  constantly  forces  of  the  other  kind.     Although,  however,  no  exact  numeri- 
cal comparison  can  be  made,  yet  it  may  be  stated  in  a  general  way  that  the 
wedge  is  more  and  more  povverful  as  its  angle  is  more  acute. 

In  the  arts  and  manufactures,  wedges  are  used  where  enormous  force  is  to 
be  exerted  through  a  very  small  space.  Thus  it  is  resorted  to  for  splitting 
masses  of  timber  or  stone.  Ships  are  raised  in  docks  by  wedges  driven  under 
their  keels.  The  wedge  is  the  principal  agent  in  the  oil-mill.  The  seeds 
from  which  the  oil  is  to  be  extracted  are  introduced  into  hair  bags,  and  placed 
between  planes  of  hard  wood.  Wedges  inserted  between  the  bags  are  driven 
by  allowing  heavy  beams  to  fall  on  them.  The  pressure  thus  excited  is  so  in- 
tense, that  the  seeds  in  the  bags  are  formed  into  a  mass  nearly  as  solid  as 
wood. 

Instances  have  occurred  in  which  the  wedge  has  been  used  to  restore  a  tot- 
tering edifice  to  its  perpendicular  position.  All  cutting  and  piercing  instru- 
ments, such  as  knives,  razors,  scissors,  chisels,  &c.,  nails,  pins,  needles,  awls, 
&c.,  are  wedges.  The  angle  of  the  wedge,  in  these  cases,  is  more  or  less 
acute,  according  to  the  purpose  to  which  it  is  to  be  applied.  In  determining 
this,  two  things  are  to  be  considered — the  mechanical  power,  which  is  in- 
creased by  diminishing  the  angle  of  the  wedge,  and  the  strength  of  the  tool, 
which  is  always  diminished  by  the  same  cause.  There  is,  therefore,  a  practi- 
cal limit  to  the  increase  of  the  power,  and  that  degree  of  sharpness  only  is  to 
be  given  to  the  tool  which  is  consistent  with  the  strength  requisite  for  the 
purpose  to  which  it  is  to  be  applied.  In  tools  intended  for  cutting  wood,  the 
angle  is  generally  about  30°.  For  iron,  it  is  from  50'°  to  60°  ;  and  for  brass, 
from  80°  to  90°.  Tools  which  act  by  pressure  maybe  made  more  acute  than 
those  which  are  driven  by  a  blow  ;  and,  in  general,  the  softer  and  more  yield- 


388 


THE  INCLINED  PLANE,  WEDGE,  AND  SCREW. 


the 


le  divided  is, 
wedge  may 


and  the  less  the  power  required 


substance  1 
it,  the  more  acute  the  wedge  may  be  constructed. 

In  many  cases,  the  utility  of  the  wedge  depends  on  that  which  is  entirely 
omitted  in  its  theory,  viz.,  the  friction  which  arises  between  its  surface  and  the 
substance  which  it  divides.  This  is  the  case  when  pins,  bolts,  or  nails,  are 
used  for  binding  the  parts  of  structures  together ;  in  which  case,  were  it  not 
for  the  friction,  they  would  recoil  from  their  places,  and  fail  to  produce  the 
desired  effect.  Even  when  the  wedge  is  used  as  a  mechanical  engine,  the 
presence  of  friction  is  absolutely  indispensable  to  its  practical  utility.  The 
power,  as  has  already  been  stated,  generally  acts  by  successive  blows,  and  is 
therefore  subject  to  constant  intermission,  and,  but  for  the  friction,  the  wedge 
would  recoil  between  the  intervals  of  the  blows  with  as  much  force  as  it  had 
been  driven  forward.  Thus  the  object  of  the  labor  would  be  continually  frus- 
trated. The  friction,  in  this  case,  is  of  the  same  use  as  a  ratchet-wheel,  but 
is  much  more  necessary,  as  the  power  applied  to  the  wedge  is  more  liable  to 
intermission  than  in  the  cases  where  ratchet-wheels  are  generally  used. 

When  a  road  directly  ascends  the  side  of  a  hill,  it  is  to  be  considered  as  an 
inclined  plane  ;  but  it  will  not  lose  its  mechanical  character,  if,  instead  of  di- 
rectly ascending  toward  the  top  of  the  hill,  it  winds  successively  round  it, 
and  gradually  ascends,  so  as,  after  several  revolutions,  to  reach  the  top.  In 
the  same  manner  a  path  may  be  conceived  to  surround  a  pillar,  by  which  the 
ascent  may  be  facilitated  upon  the  principle  of  the  inclined  plane.  Winding 
stairs  constructed  in  the  interior  of  great  columns  partake  of  this  character  ; 
for  although  the  ascent  be  produced  by  successive  steps,  yet  if  a  floor  could  be 
made  sufficiently  rough  to  prevent  the  feet  from  slipping,  the  ascent  would  be 
accomplished  with  equal  facility.  In  such  a  case,  the  winding  path  would  be 
equivalent  to  an  inclined  plane,  bent  into  such  a  form  as  to  accommodate  it  to 
the  peculiar  circumstances  in  which  it  would  be  required  to  be  used.  It  will 
not  be  difficult  to  trace  the  resemblance  between  such  an  adaptation  of  the  in- 
clined plane  and  the  appearances  presented  by  the  thread  of  a  screw ;  and  it 
may  hence  be  easily  understood  that  a  screw  is  nothing  more  than  an  inclined 
plane  constructed  upon  the  surface  of  a  cylinder. 

This  will  perhaps  be  more  apparent  by  the  following  contrivance  :  Let  A  B, 
fig.  6,  be  a  common  round  ruler,  and  let  C  D  E  be  a  piece  of  white  paper  cut 

Fig.  6. 


in  the  form  of  an  inclined  plane,  whose  height  C  D  is  equal  to  the  length  of 
the  ruler  A  B,  and  let  the  edge  C  E  of  the  paper  be  marked  with  a  broad 
black  line  :  let  the  edge  C  D  be  applied  to  the  ruler  A  B,  and,  being  attached 
thereto,  let  the  paper  be  rolled  round  the  ruler ;  the  ruler  will  then  present  the 
appearance  of  a  screw,  fig.  7,  the  thread  of  the  screw  being  marked  by  the 
black  line  C  E,  winding  continually  round  the  ruler.  Let  D  F,  fig.  6,  be  equal 
to  the  circumference  of  the  ruler,  and  draw  F  G  parallel  to  D  C,  and  G  H 
parallel  to  D  E,the  part  C  G  F  D  of  the  paper  will  exactly  surround  the  ruler 
once  ;  the  part  C  G  will  form  one  spire  of  the  thread,  and  may  be  considered 
as  the  length  of  one  inclined  plane  surrounding  the  cylinder,  C  H  being  the 
corresponding  height,  and  G  H  the  base.  The  power  of  the  screw  does  not, 
as  in  the  ordinary  cases  of  the  inclined  plane,  act  parallel  to  the  plane  or 


thread,  but  at  right  angles  to  the  length  of  the  cylinder  A  B,  or,  what  is  to  the 
same  effect,  parallel  to  the  base  H  G  ;  therefore  the  proportion  of  the  power 
to  the  weight  will  be,  according  to  principles  already  explained,  the  same  as 
that  of  C  H  to  the  space  through  which  the  power  moves  parallel  to  H  G  in 
one  revolution  of  the  screw.  H  C  is  evidently  the  distance  between  the  suc- 
cessive positions  of  the  thread  as  it  winds  round  the  cylinder  ;  and  it  appears, 
from  what  has  been  just  stated,  that  the  less  this  distance  is,  or,  in  other  words, 
the  finer  the  thread  is,  the  more  powerful  the  machine  will  be. 

In  the  application  of  the  screw,  the  weight  or  resistance  is  not,  as  in  the 
inclined  plane  and  wedge,  placed  upon  the  surface  of  the  plane  or  thread.  The 
power  is  usually  transmitted  by  causing  the  screw  to  move  in  a  concave  cyl- 
inder, on  the  interior  surface  of  which  a  spiral  cavity  is  cut,  corresponding 
exactly  to  the  thread  of  the  screw,  and  in  which  the  thread  will  move  by  turn- 
ing round  the  screw  continually  in  the  same  direction.  This  hollow  cylinder 
is  usually  called  the  nut  or  concave  screw.  The  screw  surrounded  by  its  spiral 
thread  is  represented  in  fig.  8  ;  and  a  section  of  the  same  playing  in  the  nut 
is  represented  in  fig.  9. 


Fia:.  H. 


Fisr.  9 


There  are  several  ways  in  which  the  effect  of  the  power  may  be  conveyed 
to  the  resistance  by  this  apparatus.  ' 

First,  let  us  suppose  that  the  nut  A  B  is  fixed.  If  the  screw  be  continually 
turned  on  its  axis,  by  a  lever  E  F  inserted  in  one  end  of  it,  it  will  be  moved 
in  the  direction  C  D,  advancing  every  revolution  through  a  space  equal  to  the 
distance  between  two  contiguous  threads.  By  turning  the  Jevei;  in  an  oppo- 
site direction,  the  screw  will  be  moved  in  the  direction  D  C. 

If  the  screw  be  fixed,  so  as  to  be  incapable  either  of  moving  longitudinally 
or  revolving  on  its  axis,  the  nut  A  B  maybe  turned  upon  the  screw  by  a  lever, 
and  will  move  on  the  screw  toward  C  or  toward  D,  according  to  the  direction 
in  which  the  lever  is  turned. 

In  the  former  case,  we  have  supposed  the  nut  to  be  absolutely  immoveable, 
and  in  the  latter  case,  the  screw  to  be  absolutely  immoveable.  It  may  happen, 
however,  that  the  nut,  though   capable   of  revolving,  is  incapable  of  moving 

VOL,.  II.— 19 


290 


THE  INCLINED  PLANE,  WEDGE,  AND  SCREW. 


longitudinally  ;  and  that  the  screw,  though  incapable  of  revolving,  is  capable 
of  moving  longitudinally.  In  that  case,  by  turning  the  nut  A  B  upon  the  screw 
by  the  lever,  the  screw  will  be  urged  in  the  direction  C  D  or  D  C,  according 
to  the  way  in  which  the  nut  is  turned. 

The  apparatus  may,  on  the  contrary,  be  so  arranged,  that  the  nut,  though 
incapable  of  revolving,  is  capable  of  moving  longitudinally  ;  and  the  screw, 
though  capable  of  revolving,  is  incapable  of  moving  longitudinally.  In  this 
case,  by  turning  the  screw  in  the  one  direction,  or  in  the  other,  the  nut  A  B 
will  be  urged  in  the  direction  C  D  or  D  C. 

All  these  various  arrangements  may  be  observed  in  different  applications  to 
the  machine. 

A  screw  may  be  cut  upon  a  cylinder  by  placing  the  cylinder  in  a  turning- 
lathe,  and  giving  it  a  rotatory  motion  upon  its  axis.  The  cutting  point  is  then 
presented  to  the  cylinder,  and  moved  in  the  direction  of  its  length,  at  such  a 
rate  as  to  be  carried  through  the  distance  between  the  intended  thread,  while 
the  cylinder  revolves  once.  The  relative  motions  of  the  cutting  point  and  the 
cylinder  being  preserved,  with  perfect  uniformity,  the  thread  will  be  cut  from 
one  end  to  the  other.  The  shape  of  the  threads  may  be  either  square,  as  in 
fig.  8,  or  triangular,  as  in  fig.  10. 

Fig.  10. 


The  screw  is  generally  used  in  cases  where  severe  pressure  is  to  be  excited 
through  small  spaces;  it  is  therefore  the  agent  in  most  presses.     In  fig.  11, 

Fig.  11. 


the  nut  is  fixed,  and  by  turning  the  lever,  which  passes  through  the  head  of 
the  screw,  a  pressure  is  excited  upon  any  substance  placed  upon  the  plate  im- 
mediately under  the  end  of  the  screw.  In  fig.  12,  the  screw  is  incapable  of 
revolving,  but  is  capable  of  advancing  in  the  direction  of  its  length.  On  the 
other  hand,  the  nut  is  capable  of  revolving,  but  does  not  advance  in  the  direc- 
tion of  the  screw.  When  the  nut  is  turned  by  means  of  the  screw  inserted  in 
it,  the  screw  advances  in  the  direction  of  its  length,  and  urges  the  board  which 
is  attached  to  it  upward,  so  as  to  press  any  substance  placed  between  it  and 
the  fixed  board  above. 

In  cases  where  liquids  or  juices  are  to  be  expressed  from  solid  bodies,  the 
screw  is  the  agent  generally  employed.     It  is  also  used  in  coining,  where  the 


impression  of  a  die  is  to  be  made  upon  a  piece  of  metal,  and  in  the  same  way 
in  producing  the  impression  of  a  seal  upon  wax  or  other  substance  adapted  to 
receive  it.  When  soft  and  light  materials,  such  as  cotton,  are  to  be  reduced 
to  a  convenient  bulk  for  transportation,  the  screw  is  used  to  compress  them, 
and  they  are  thus  reduced  into  hard,  dense  masses.  In  printing,  formerly, 
the  paper  was  urged  by  a  severe  and  sudden  pressure  upon  the  types  by 
means  of  a  screw. 

As  the  mechanical  power  of  the  screw  depends  upon  the  relative  magnitude 
of  the  circumference  through  which  the  power  revolves,  and  the  distance  be- 
tween the  threads,  it  is  evident,  that,  to  increase  the  efficacy  of  the  machine, 
we  must  either  increase  the  length  of  the  lever  by  which  the  power  acts,  or 
diminish  the  magnitude  of  the  thread.  Although  there  is  no  limit  in  theory  to 
the  increase  of  the  mechanical  efficacy  by  these  means,  yet  practical  incon- 
venience arises  which  effectually  prevents  that  increase  being  carried  beyond 
a  certain  extent.  If  the  lever  by  which  the  power  acts  be  increased,  the  same 
difficulty  arises  as  was  already  explained  in  the  wheel  and  axle  :  the  space 
through  which  the  power  should  act  would  be  so  imwieldy,  that  its  applica- 
tion would  become  impracticable.  If,  on  the  other  hand,  the  power  of  the 
machine  he  increased  by  diminishing  the  size  of  the  thread,  the  strength  of 
the  thread  will  be  so  diminished,  that  a  slight  resistance  will  tear  it  from  the 
cylinder.  The  cases  in  which  it  is  necessary  to  increase  the  power  of  the 
machine  being  those  in  which  the  greatest  resistances  are  to  be  overcome,  the 
object  will  evidently  be  defeated  if  the  means  chosen  to  increase  that  power 
deprive  the  machine  of  the  strength  which  is  necessary  to  feustain  the  force  to 
which  it  is  to  be  submitted. 

These  inconveniences  are  removed  by  a  contrivance  of  Mr.  Hunter,  which, 
while  it  gives  to  the  machine  all  the  requisite  strength  and  compactness,  allows 
it  to  have  an  almost  unlimited  degree  of  mechanical  efficacy. 

This  contrivance  consists  in  the  use  of  two  screws,  the  threads  of  which 
may  have  any  strength  and  magnitude,  but  which  have  a  very  small  difference 
of  breadth.  While  the  working  point  is  urged  forward  by  that  which  has  the 
greater  thread,  it  is  drawn  back  by  that  which  has  the  less  ;  so  that,  during 
each  revolution  of  the  screw,  instead  of  being  advanced  through  a  space  equal 
to  the  magnitude  of  either  of  the  threads,  it  moves  through  a  space  equal  to 
their  difference.  The  mechanical  power  of  such  a  machine  will  be  the  same 
as  that  of  a  single  screw,  having  a  thread  whose  magnitude  is  equal  to  the 
difference  of  the  magnitudes  of  the  two  threads  just  mentioned. 

Thus,  without  inconveniently  increasing  the  sweep  of  the  power,  on  the  one 
hand,  or,  on  the  other,  diminishing  the  thread  until  the  necessary  strength  is 


292 


THE  INCLINED  PLANE,  WEDGE,  AND  SCREW. 


lost,  the  machine  will  acquire  an  efficacy  limited  by  nothing  but  the  smallness 
of  the  difference  between  the  two  threads. 

Fig.  13. 


This  principle  was  first  applied  in  the  manner  represented  in  fig.  13.  A  is 
the  greater  thread,  playing  in  the  fixed  nut  ;  B  is  the  lesser  thread,  cut  upon  a 
smaller  cylinder,  and  playing  in  a  concave  screw,  cut  within  the  greater  cyl- 
inder. During  every  revolution  of  the  screw,  the  cylinder  A  descends  through 
a  space  equal  to  the  distance  between  its  threads.  At  the  same  time,  the 
smaller  cylinder  B  ascends  through  a  space  equal  to  the  distance  between  the 
threads  cut  upon  it :  the  effect  is,  that  the  board  D  descends  through  a  space 
equal  to  the  difference  between  the  threads  upon  A  and  the  threads  upon  B, 
and  the  machine  has  a  power  proportionate  to  the  smallness  of  this  difference. 
Thus,  suppose  the  screw  A  has  twenty  threads  in  an  inch,  while  the  screw 
B  has  twenty-one  :  during  one  revolution,  the  screw  A  will  descend  through 
a  space  equal  to  the  twentieth  part  of  an  inch.  If,  during  this  motion,  the 
screw  B  did  not  turn  within  A,  the  board  D  would  be  advanced  through  the 
twentieth  of  an  inch  ;  but  because  the  hollow  screw  within  A  turns  upon  B, 
the  screw  B  will,  relatively  to  A,  be  raised  in  one  revolution  through  a  space 
equal  to  the  twenty-first  part  of  an  inch.  Thus,  while  the  board  D  is  depressed 
through  the  twentieth  of  an  inch  by  the  screw  A,  it  is  raised  through  the  twen- 
ty-first of  an  inch  by  the  screw  B.  It  is.  therefore,  on  the  whole,  depressed 
through  a  space  equal  to  the  excess  of  the  twentieth  of  an  inch  above  the 
twenty-first  of  an  inch — that  is,  through  the  four  hundred  and  twentieth  of  an 
inch. 

The  power  of  this  machine  will,  therefore,  be  expressed  by  the  number  of 
times  the  four  hundred  and  twentieth  of  an  inch  is  contained  in  the  circum- 
rence  through  which  the  power  moves. 

In  the  practical  application  of  this  principle  at  present,  the  arrangement  is 
somewhat  different.  The  two  threads  are  usually  cut  on  different  parts  of  the 
same  cylinder.  If  nuts  be  supposed  to  be  placed  upon  these,  which  are  ca- 
pable of  moving  in  the  direction  of  the  length,  but  not  of  revolving,  it  is  evi- 
dent that  by  turning  the  screw  once  round,  each  nut  will  be  advanced  through 
a  space  equal  to  the  breadth  of  the  ■  respective  threads.  By  this  means  the 
two  nuts  will  either  approach  each  other,  or  mutually  recede,  according  to  the 
direction  in  which  the  screw  is  turned,  through  a  space  equal  to  the  difference 
of  the  breadth  of  the  threads,  and  they  will  exert  a  force  either  in  compressing 
or  extending  any  substance  placed  between  them,  proportionate  to  the  small- 
ness of  that  difference. 

A  toothed  wheel  is  sometimes  used  instead  of  a  nut,  so  that  the  same  qual- 
ity by  which  the  revolution  of  the  screw  urges  the  nut  forward  is  applied  to 
make  the  wheel  revolve.     The  screw  is  in  this  case  called  an  endless  screw, 


THE  INCLINED  PLANE,  WEDGE,  AND  SCREW. 


293 


Fig.  14. 


because  its  action  upon  the  wheel  may  be  continued  without  limit.  This  ap- 
plication of  the  screw  is  represented  in  fig.  14.  P  is.  the  winch  to  which  the 
power  is  applied  ;  and  its  effect  at  the  circumference  of  the  wheel  is  estimated 
in  the  same  manner  as  the  effect  of  the  screw  upon  the  nut.  This  effect  is  to 
be  considered  as  a  power  acting  upon  the  circumference  of  the  wheel  ;  and  its 
proportion  to  the  weight  or  resistance  is  to  be  calculated  in  the  same  manner 
as  the  proportion  of  the  power  to  the  weight  in  the  wheel  and  axle. 

We  have  hitherto  considered  the  screw  as  an  engine  used  to  overcome  great 
resistances.  It  is  also  eminently  useful  in  several  departments  of  experimental 
science,  for  the  measurement  of  very  minute  motions  and  spaces,  the  magni- 
tude of  which  could  scarcely  be  ascertained  by  any  other  means.  The  very 
slow  motion  which  may  be  imparted  to  the  end  of  a  screw,  by  a  very  consid- 
erable motion  in  the  power,  renders  it  peculiarly  well  adapted  for  this  purpose. 
To  explain  the  manner  in  which  it  is  applied — suppose  a  screw  to  be  so  cut 
as  to  have  fifty  threads  in  an  inch,  each  revolution  of  the  screw  will  advance 
its  point  through  the  fiftieth  part  of  ^n  inch.  Now,  suppose  the  head  of  the 
screw  to  be  a  circle,  whose  diameter  is  an  inch,  the  circumference  of  the  head 
will  be  something  more  than  three  inches  ;  this  may  be  easily  divided  into  a 
hundred  equal  parts  distinctly  visible.  If  a  fixed  index  be  presented  to  this 
graduated  circumference,  the  hundredth  part  of  a  revolution  of  the  screw  may 
be  observed,  by  noting  the  passage  of  one  division  of  the  head  under  the  index. 
Since  one  entire  revolution  of  the  head  moves  the  point  through  the  fiftieth  of 
an  inch,  one  division  will  correspond  to  the  five  thousandth  of  an  inch.  In 
order  to  observe  the  motion  of  the  point  of  the  screw  in  this  case,  a  fine  wire 
is  attached  to  it,  which  is  carried  across  the  field  of  view  of  a  powerful  mi- 
croscope, by  which  the  motion  is  so  magnified  as  to  be  distinctly  perceptible. 

A  screw  used  for  such  purposes  is  called  a  micrometer  screw.  Such  an  ap-, 
paratus  is  usually  attached  to  the  limbs  of  graduated  instruments,  for  the  pur- 
poses of  astronomical  and  other  observation.  Without  the  aid  of  this  appara- 
tus, no  observation  could  be  taken  with  greater  accuracy  than  the  amount  of 
the  smallest  division  upon  the  limb.  Thus,  if  an  instrument  for  measuring 
angles  were  divided  into  small  arches  of  one  minute,  and  an  angle  were  ob- 
served which  brought  the  index  of  the  instrument  to  some  point  between  two 
divisions,  we  could  only  conclude  that  the  observed  angle  must  consist  of 
a  certain  number  of  degrees  and  minutes,  together  with  an  additional  number 
of  ssconds,  which  would  be  unknown,  inasmuch  as  there  would  be  no  means 
of  aecertaining  the  fraction  of  a  minute  between  the  index  and  the  adjacent 
division  of  the  instrument.  But  if  a  screw  be  provided,  the  point  of  which 
moves  through  a  space  equal  to  one  division  of  the  instrument,  with  sixty  revo- 
lutions of  the  head,  and  the  head  itself  be  divided  into  one  hundred  equal  parts, 
each  complete  revolution  of  the  screw  will  correspond  to  the  sixtieth  part  of 
a  minute,  or  to  one  second,  and  each  division  on  the  head  of  the  screw  will 
correspond  to  the  hundredth  part  of  a  second.  The  index  being  attached  to 
this  screw,  let  the  head  be  turned  until  the  index  be  moved  from  its  observed 


position  to  the  adjacent  division  of  the  limb.  The  number  of  complete  revo- 
lutions of  the  screw  necessary  to  accomplish  this  w^ill  be  the  number  of  sec- 
onds ;  and  the  number  of  parts  of  a  revolution  over  the  complete  number  of 
revolutions  will  be  the  hundredth  parts  of  a  second  necessary  to  be  added  to 
the  degrees  and  minutes  primarily  observed. 

It  is  not,  however,  only  to  angular  instruments  that  the  micrometer  screw  is 
applicable  ;  any  spaces  whatever  may  be  measured  by  it.  An  instance  of  its 
mechanical  application  may  be  mentioned  in  a  steel-yard,  an  instrument  for 
ascertaining  the  amount  of  weights  by  a  given  weight,  sliding  on  a  long  grad- 
uated arm  of  a  lever.  The  distance  from  the  fulcrum,  at  which  this  weight 
counterpoises  the  weight  to  be  ascertained,  serves  as  a  measure  to  the  amount 
of  that  weight.  When  the  sliding  weight  happens  to  be  placed  between  two 
divisions  of  the  arm,  a  micrometer  screw  is  used  to  ascertain  the  fraction  of 
the  division. 

Hunter's  screw,  already  described,  seems  to  be  well  adapted  to  micrometri- 
cal  purposes  ;  since  the  motion  of  the  point  may  be  rendered  indefinitely  slow, 
without  requiring  an  exquisitely  fine  thread,  such  as,  in  the  single  screw,  would 
in  this  case  be  necessary. 


,. J 


EBULLITIOI. 


Process  of  boiling. — Vaporization  and  Condensation. — Latent  Heat  of  Steam. — Experiments  of 
Black. — Effect  of  atmospheric  Pressure  on  boiling  Point. — Ebullition  under  increased  Pressure — 
under  diminished  Pressure. — Relation  between  the  Barometer  and  boiling  Point. — Effect  of  the 
Altitude  of  the  Station  of  the  boiling  Point. — Elasticity  of  Steam.— Its  Lightness. — Sum  of  the 
latent  and  sensible  Heat  always  the  sanie.^Effect  of  the  Compression  of  Steam  wthout  Loss  of 
Heat. — Steam  cannot  be  liquefied  by  mere  Pressure. — Boiling  Points  and  latent  Heat  of  other 
Liquids. — Condensation  of  Vapor. — Principle  of  the  Steam-Engine. — Nature  of  permanent  Gas- 
es.— Examples  of  the  Application  of  the  Properties  of  Steam.  _  ■ 


EBULLITION. 


297 


EBULLITION. 


It  is  known  that  the  continued  application  of  heat  to  a  solid  causes  it  ulti- 
mately to  pass  into  the  liquid  form.  We  propose,  in  the  present  discourse,  to 
examine  the  effects  which  would  be  produced  by  the  continued  application  of 
heat  to  a  liquid. 

Let  a  small  quantity  of  water  be  placed  in  a  glass  flask  of  considerable  size, 
and  then  closed  so  as  to  prevent  the  escape  of  any  vapor.-  Let  this  vessel  be 
now  placed  over  the  flame  of  a  spirit  lamp,  so  as  to  cause  the  water  it  contains 
to  boil.  For  a  considerable  time  the  water  will  be  observed  to  boil,  and  ap- 
parently to  diminish  in  quantity,  until  at  length  all  the  water  disappears,  and 
the  vessel  is  apparently  empty.  If  the  vessel  be  now  removed  from  the  lamp, 
and  suspended  in  a  cool  atmosphere,  the  whole  of  the  interior  of  its  surface 
will  presently  appear  to  be  covered  with  a  dewy  moisture  ;  and  at  length  a 
quantity  of  water  will  collect  in  the  bottom  of  it  equal  to  that  which  had  been 
in  it  at  the  commencement  of  the  process.  That  no  water  has  at  any  period 
of  the  experiment  escaped  from  it  may  be  easily  determined  by  performing  the 
experiment  with  the  glass  flask  suspended  from  the  arm  of  a  balance,  counter- 
poised by  a  sufficient  weight  suspended  from  the  other  arm.  The  equilibrium 
will  be  preserved  throughout,  and  the  vessel  will  be  found  to  have  the  same 
weight,  when  to  all  appearance  it  is  empty,  as  when  it  contains  the  liquid 
water.  It  is  evident,  therefore,  that  the  water  exists  in  the  vessel  in  every 
stage  of  the  process,  but  that  it  becomes  invisible  when  the  process  of  boiling 
has  continued  for  a  certain  length  of  time  ;  and  it  may  be  shown  that  it  will 
continue  to  be  invisible,  provided  the  flask  be  exposed  to  a  temperature  consid- 
erably elevated.  Thus,  for  example,  if  it  be  suspended  in  a  vessel  of  boiling 
water,  the  water  which  it  contains  will  continue  to  be  invisible  ;  but  the  mo- 
ment it  is  withdrawn  from  the  boiling  water,  and  exposed  to  the  cold  air,  the 
water  will  again  become  visible,  as  above  mentioned,  forming  a  dew  on  the 
inner  surface,  and  Anally  collecting  in  the  bottom  as  in  the  commencement  of 
the  experiment. 


298 


EBULLITION. 


In  fact,  the  liquid  has,  by  the  process  of  boiling,  been  converted  into  vapor 
or  steam,  which  is  a  body  similar  in  its  leading  properties  to  common  air,  and, 
like  it,  is  invisible.  It  vfill  hereafter  appear  that  it  likewise  possesses  the 
property  of  elasticity  and  other  mechanical  qualities  enjoyed  by  gases  in  gen- 
eral. 

Again,  let  an  open  vessel  be  filled  with  water  at  60°,  and  placed  in  a  mer- 
curial bath,  which  is  maintained  by  a  fire  or  lamp  applied  to  it  at  the  tempera- 
ture of  230°.  Place  a  thermometer  in  the  water,  and  it  will  be  observed  grad- 
ually to  rise  as  the  temperature  of  the  water  is  increased  by  the  heat  which  it 
receives  from  the  mercury  in  which  it  is  immersed.  The  water  will  steadily 
rise  in  this  manner  until  it  attains  the  temperature  of  212°  ;  but  here  the  ther- 
mometer immersed  in  it  will  become  stationary.  At  the  same  time  the  water 
contained  in  the  vessel  will  become  agitated,  and  its  surface  will  present  the 
same  appearance  as  if  bubbles  of  air  were  rising  from  the  bottom,  and  issuing 
at  the  top.  A  cloudy  vapor  will  be  given  off  in  large  quantities  from  its  sur- 
face. This  process  is  called  ebullition  or  boiling.  If  it  be  continued  for  any 
considerable  time,  the  quantity  of  water  in  the  vessel  will  be  sensibly  dimin- 
ished ;  and  at  length  every  particle  of  it  will  disappear,  and  the  vessel  will 
remain  empty.  During  the  whole  of  this  process,  the  thermometer  immersed 
in  the  water  will  remain  stationary  at  212°. 

Now,  it  will  be  asked,  what  has  become  of  the  water  ?  It  cannot  be  im- 
agined that  it  has  been  annihilated.  We  shall  be  able  to  answer  this  by  adopt- 
ing means  to  prevent  the  escape  of  any  particle  of  matter  from  the  vessel  con- 
taining the  water  into  the  atmosphere  or  elsewhere.  Let  us  suppose  that  the 
top  of  the  vessel  containing  the  water  is  closed,  with  the  exception  of  a  neck 
communicating  with  a  tube,  and  let  that  tube  be  carried  into  another  close  ves- 
sel removed  from  the  cistern  of  heated  mercury,  and  plunged  in  another  cistern 
of  cold  water.     Such  an  apparatus  is  represented  in  fig.  1. 

Fig.  I. 


A  is  a  cistern  of  heated  mercury,  in  which  the  glass  vessel  B,  containing 
water,  is  immersed.  From  the  top  of  the  vessel  B  proceeds  a  glass  tube  C 
inclining  downward,  and  entering  a  glass  vessel  D,  which  is  immersed  in  a  cis- 
tern E  of  cold  water.  If  the  process  already  described  be  continued  until  the 
water  by  constant  ebullition  has  disappeared,  as  already  mentioned,  from  the 
vessel  B,  it  will  be  found  that  a  quantity  of  water  will  be  collected  in  the  ves- 
sel D  ;  and  if  this  water  be  weighed,  it  will  be  found  to  have  exactly  the  same 
weight  as  the  water  had  which  was  originally  placed  in  the  vessel  B.  It  is, 
therefore,  quite  apparent  that  the  water  has  passed  by  the  process  of  boiling 
from  one  vessel  to  the  other ;  but,  in  its  passage,  it  was  not  perceptible  by  the 
sight.  The  tube  C  and  the  upper  part  of  the  vessel  B  had  the  same  appear- 
ance exactly  as  if  they  had  been  filled  with  atmospheric  air.  That  they  are 
not  merely  filled  with  atmospheric  air  in  the  vessel,  may,  however,  be  easily 
proved.  When  the  process  of  boiling  first  commences,  it  will  be  found  that 
the  tube  C  is  cold,  and  the  inner  surface  dry.     When  the  process  of  ebullition 


EBULLITION. 


299 


has  continued  a  short  time,  the  tube  C  will  become  gradually  heated,  and  the 
inner  surface  of  it  covered  with  moisture.  After  a  time,  however,  this  moist- 
ure disappears,  and  the  tube  attains  the  temperature  of  212°.  In  this  state 
it  continues  until  the  whole  of  the  water  is  discharged  from  the  vessel  B  to  the 
vessel  D. 

These  effects  are  easily  explained.  The  water  in  the  vessel  B  is  incapable 
of  receiving  any  higher  temperature  than  212°,  consistently  with  its  retaining 
the  liquid  form.  Small  portions,  therefore,  are  constantly  converted  into  steam 
by  the  heat  received  from  the  surrounding  mercury,  and  bubbles  of  steam  are 
formed  on  the  bottom  and  sides  of  the  vessel  B.  These  bubbles,  being  very  much 
lighter,  bulk  for  bulk,  than  water,  rise  rapidly  through  the  water,  just  in  the 
same  manner  as  bubbles  of  air  would,  and  produce  that  peculiar  agitation  at 
its  surface  which  has  been  taken  as  the  external  indication  of  boiling.  They 
escape  from  the  surface,  and  collect  in  the  upper  part  of  the  vessel.  The 
steam  thus  collected,  when  it  first  enters  the  tube  C,  is  cooled  below  the  tem- 
perature of  212°  by  the  surface  of  the  tube  ;  and  consequently,  being  incapa- 
ble of  remaining  in  the  state  of  vapor  at  any  lower  temperature  than  212°,  it  is 
reconverted  into  water,  and  forms  the  dewy  moisture  which  is  observed  in  the 
commencement  of  the  process  on  the  interior  of  the  tube  C.  At  length,  how- 
ever, the  whole  of  the  tube  C  is  heated  to  the  temperature  of  212°,  and  the 
moisture  which  was  previously  collected  upon  its  inner  surface  is  again  con- 
verted into  steam.  As  the  quantity  of  steam  evolved  from  the  water  in  B  in- 
creases, it  drives  before  it  the  steam  previously  collected  in  the  tube  C,  and 
forces  it  into  the  vessel  B.  Here  it  encounters  the  inner  surface  of  this  ves- 
sel, which  is  kept  constantly  cold  by  being  surrounded  with  the  cold  water  in 
which  it  is  immersed  ;  and  the  vapor,  being  thus  immediately  reduced  below 
the  temperature  of  212°,  is  reconverted  into  water.  At  first  it  collects  in  a 
dew  on  the  surface  of  the  vessel  D  ;  but  as  this  accumulates,  it  drops  into  the 
bottom  of  the  vessel,  and  forms  a  more  considerable  quantity.  As  the  quantity 
of  water  is  observed  to  be  gradually  diminished  in  the  vessel  B,  the  quantity 
will  be  found  to  be  gradually  increased  in  the  vessel  D  ;  and  if  the  operation 
be  suspended  at  any  stage  of  the  process,  and  the  water  in  the  two  vessels 
weighed,  it  will  be  found  that  the  weight  of  the  water  in  D  is  exactly  equal  to 
the  weight  which  the  water  in  B  has  lost. 

The  demonstration  is,  therefore,  perfect,  that  the  gradual  diminution  of  the 
boiling  water  in  the  vessel  B  is  produced  by  the  conversion  of  that  water  into 
steam  by  the  heat.  In  the  process  first  described,  when  the  top  of  the  vessel 
B  was  supposed  to  be  open,  this  steam  made  its  escape  into  the  air,  where  it 
was  first  dispersed,  and  subsequently  cooled  in  separate  particles,  and  was  de- 
posited in  minute  globules  of  moisture  on  the  ground  and  on  surrounding  objects. 

In  reviewing  this  process,  we  are  struck  by  the  fact,  that  the  continued  ap- 
plication of  heat  to  the  vessel  B  is  incapable  of  raising  the  temperature  of  the 
water  contained  in  it  above  212°.  This  presents  an  obvious  analogy  to  the 
process  of  liquefaction,  and  leads  to  inquiries  of  a  similar  nature  which  are 
attended  with  a  like  result.  We  must  either  infer  that  the  water,  having  ar- 
rived at  212°,  received  no  more  heat  from  the  mercury  ;  or  that  such  heat,  if 
received,  is  incapable  of  afl^ecting  the  thermometer  ;  or,  finally,  that  the  steam 
which  passes  off,  carries  this  heat  with  it.  That  the  water  receives  heat  from 
the  mercury  will  be  proved  by  the  fact,  that,  if  the  vessel  B  be  removed  from 
the  mercury,  other  things  remaining  as  before,  the  temperature  of  the  mercury 
will  rapidly  rise,  and,  if  the  fire  be  continued,  it  will  even  boil ;  but  so  long  as 
the  vessel  B  remains  immersed,  it  prevents  the  mercury  from  increasing  in 
temperature.  It  therefore  receives  that  heat  which  would  otherwise  raise  the 
temperature  of  the  quicksilver. 


300 


EBULLITION. 


If  a  thermometer  be  immersed  in  the  steam  which  collects  in  the  upper  part 
of  the  vessel  B,  it  will  show  the  same  temperature  (of  212°)  as  the  water  from 
which  it  is  raised.  The  heat,  therefore,  received  from  the  mercury  is  clearly 
not  imparled  in  a  sensible  form  to  the  steam,  which  has  the  same  temperature 
in  the  form  of  steam  as  it  had  in  the  form  of  water.  The  result  of  investiga- 
tions respecting  liquefaction  would  lead  us,  by  analogy,  to  suspect  that  the 
heat  imparted  by  the  mercury  to  the  water  has  become  latent  in  the  steam,  and 
is  instrumental  to  the  conversion  of  water  into  steam,  in  the  same  manner  as 
heat  was  formerly  found  to  be  instrumental  to  the  conversion  of  ice  into  water. 
As  the  fact  was  in  that  case  detected  by  mixing  ice  with  water,  so  we  shall,  in 
the  present  instance,  try  it  by  a  like  test,  viz.,  by  mixing  steam  with  water. 
Let  about  five  and  a  half  ounces  of  water,  at  the  temperature  of  32°,  be  placed 
in  a  vessel  A  (fig.  2),  and  let  another  vessel,  B,  in  which  water  is  kept  con- 


stantly boiling  at  the  temperature  of  212°,  communicate  with  A  by  a  pipe  C 
proceeding  from  the  top,  so  that  the  steam  may  be  conducted  from  B,  and  es- 
cape from  the  mouth  of  the  pipe  at  some  depth  below  the  surface  of  the  water 
in  A.  As  the  steam  issues  from  the  pipe,  it  will  be  immediately  reconverted 
into  water  by  the  cold  water  which  it  enters  ;  and,  by  continuing  this  process, 
the  water  in  A  will  be  gradually  heated  by  the  steam  combined  with  it  and 
received  through  the  pipe  C.  If  this  process  be  continued  until  the  water  in 
A  is  raised  to  the  temperature  of  212°,  it  will  boil.  Let  it  then  be  weighed, 
and  it  will  be  found  to  weigh  six  and  a  half  ounces  ;  whence  we  infer  that  one 
ounce  of  water  has  been  received  from  the  vessel  B  in  the  form  of  steam,  and 
has  been  reconverted  into  water  by  the  inferior  temperature  of  the  water  in  A. 
Now,  this  ounce  of  water  received  in  the  form  of  steam  into  the  vessel  A  had, 
when  in  that  form,  the  temperature  of  212°.  It  is  nowconverted  into  the  liquid 
form,  and  still  retains  the  same  temperature  of  212°  ;  but  it  has  caused  the  five 
and  a  half  ounces  of  water  with  which  it  has  been  mixed,  to  rise  from  the  tem- 
perature of  32^  to  the  temperature  of  212°,  and  this  without  losing  any  tempera- 
ture itself.  It  follows,  therefore,  that,  in  returning  to  the  liquid  state,  it  has 
parted  with  as  much  heat  as  is  capable  of  raising  five  and  a  half  times  its  own 
weight  of  water  from  32°  to  212°.  This  heat  is  combined  with  the  steam, 
though  not  sensible  to  the  thermometer  ;  and  was,  therefore,  latent.  Had  it 
been  sensible  in  the  water  in  B,  it  would  have  caused  the  water  to  have  risen 
through  a  number  of  thermometric  degrees,  amounting  to  five  and  a  half  times 
the  excess  of  212°  above  32"  :  that  is,  through  five  and  a  half  times  180'^  ;  for 
it  has  caused  five  and  a  half  times  its  own  weight  of  water  to  receive  an  equal 
increase  of  temperature.  But  five  and  a  half  times  180°  is  990°,  or,  to  use 
round  numbers  (for  minute  accuracy  is  not  here  our  object),  1,000°.  It  follows, 
therefore,  that  an  ounce  of  water,  in  passing  from  the  liquid  state  at  212°  to 
the  state  of  steam  at  212°,  receives  as  much  heat  as  would  be  sufficient  to 
raise  it  through  1,000  thermometric  degrees,  if  that  heat,  instead  of  becoming 
latent,  had  been  sensible. 


EBULLITION. 


301 


The  fact  that  the  steam  into  which  the  water  is  converted  contains  a  con-  ( 
siderable  quantity  of  latent  heat,  and  the  computation  of  the  exact  amount  of   ) 
that  quantity  will  be  more  clearly  understood  if  we  compare  the  effects  pro-  c 
duced  by  mixing  an  ounce  of  water  at  212°  and  an  ounce  of  steam  at  212°,  ) 
respectively,  with  five  and  a  half  ounces  of  water  at  32°.     We  have  seen  that  ( 
an  ounce  of  steam  at  212°,  mixed  with  five  and  a  half  ounces  of  water  at  32°,  \ 
forms  six  and  a  half  ounces  of  water  at  212°.     Now,  if  one  ounce  of  water  at  < 
212°  be  mixed  with  five  and  a  half  ounces  of  water  at  32°,  the  mixture  will  ^ 
have  a  temperature  of  about  60°.     In  fact,  the  180°,  by  which  the  temperature  < 
of  the  ounce  of  water  at  212°  exceeds  the  temperature  of  the  five  and  a  half    < 
ounces  of  water  at  32°,  are  distributed  through  the  mixture  in  the  proportion  ' 
of  the  quantity  of  water,  so  that  each  of  the  five  and  a  half  ounces  receives  the  , 
same  increment  of  temperature  ;  and  the  loss  of  temperature  which  the  ounce  ' 
of  water  at  212°  sustains  is  equally  divided  among  the  other  five  and  a  half    [ 
ounces.     Now,  the  mixture,  in  this  case,  having  a  temperature  of  only  60°, 
while,  in  the  case  where  an  ounce  of  steam  at  212°  was  mixed  with  five  and  ', 
a  half  ounces  of  water  at  32°,  the  mixture  had  the  temperature  of  212°,  it  fol- 
lows that  the  steam  from  which  the  increased  heat  is  all  derived  contains  so 
much  more  heat  than  the  ounce  of  water  at  the  same  temperature  as  would  be 
necessary  to  raise  six  and  a  half  ounces  of  water  from  the  temperature  of  60° 
to  the  temperature  of  212°,  or  six  and  a  half  times  as  much  heat  as  would  be 
requisite  to  raise  one  ounce  of  water  through  about  152°  of  temperature.    This 
quantity  of  heat  will,  therefore,  be  found  by  multiplying  152°  by  six  and  a  half, 
which  will  give  a  product  of  983°,  being  nearly  equal  to  the  quantity  of  latent 
heat  determined  by  the  former  calculation. 

On  a  subject  so  important  as  the  latent  heat  of  steam,  it  may  not  be  uninter- 
esting here  to  mention  some  of  the  means  by  which  Dr.  Black,  the  discoverer 
of  latent  heat,  computed  the  quantity  absorbed  by  water  in  its  conversion  into 
vapor. 

If  a  given  weight  of  water  be  exposed  to  a  regular  source  of  heat,  and  the 
time  required  to  raise  it  from  the  temperature  of  50°  to  its  boiling  point  be  ob- 
served, the  rate  at  which  it  receives  heat  per  minute  may  be  computed.      Let 
the  time  be  then  observed  which  elapses  from  the  commencement  of  the  ebul- 
lition to  the  total  disappearance  of  the  water  ;  and  if  it  be  assumed  that  in  each 
minute  the  same  quantity  of  heat  was  communicated  to  the  boiling  water  as 
was  communicated  before  ebullition  commenced,  the  quantity  of  heat  carried 
,  off  by  the  steam  may  easily  be  calculated.     Some  water  placed  in  a  tin  vessel 
'  on  a  red-hot  iron,  was  observed  to  rise  from  50°  to  212°  in  four  minutes,  being 
\  at  the  rate  of  forty  and  a  half  degrees  per  minute.     The  same  water  boiled  off 
I  in  twenty  minutes.     If  it  received  during  each  of  these  twenty  minutes  forty 
[  and  a  half  degrees  of  heat,  it  must  have  carried  off  as  much  heat  in  the  form 
•  of  steam  as  would  be  sufficient  to  raise  water  through  twenty  times  forty  and  a 
,  half  degrees,  or  810°  ;  a  result  corresponding  nearly  with  the  quantity  of  latent 

>  heat  already  determined. 

,       If  water  submitted  to  pressure  be  raised  to  the  temperature  of  400°,  and  the 

>  mouth  of  the  vessel  which  contains  it  be  then  suddenly  opened,  about  a  fifth 
J  of  the  whole  quantity  of  water  will  escape  in  the  form  of  steam,  and  the  tem- 
)  perature  of  the  remainder  will  immediately  fall  to  212°.  Thus  the  whole  mass 
(  of  water  has  suddenly  lost  188°  of  temperature,  which  is  all  carried  away  by 
)  one  fifth  of  the  mass  in  the  form  of  steam.  Thus,  the  heat  which  has  become 
(  latent  in  the  steam  will  be  determined  by  multiplying  188°  by  five,  which 
)  gives  a  product  of  940°.  The  steam,  therefore,  is  water  combined  with  at 
s  least  940°  of  heat,  the  presence  of  which  is  not  indicated  by  the  therinoni- 
)  eter. 


302 


EBULLITION. 


The  close  coincidence  of  these  early  observations  of  Dr.  Black  with  the  re- 
sults of  more  recent  experiments  is  worthy  of  notice.  The  following  are  the 
results  of  observations  made  by  five  distinguished  philosophers  to  ascertain  the 
quantity  of  heat  rendered  latent  by  water  in  the  process  of  vaporization  at  212°  : 
Watt,  950°  ;  Southern,  945°  ;  Lavoisier,  1,000°;  Rumford,  1,004°  8  ;  Des- 
pretz,  955°  8. 

The  average  of  all  these  is  about  980°  ;  so  that  the  round  number  of  1,000° 
may  be  taken  as  a  close  approximation  to  the  latent  heat  of  steam  raised  from 
water  at  the  temperature  of  212°. 

In  order  to  derive  all  the  knowledge  from  these  experiments  which  they  are 
capable  of  imparting,  it  will  be  necessary  to  examine  very  carefully  how  water 
comports  itself  under  a  variety  of  circumstances. 

If  water  be  boiled  in  an  open  vessel,  with  a  thermometer  immersed,  on  dif- 
ferent days,  it  will  be  observed  that  the  fixed  temperature  which  it  assumes  in 
boiling  will  be  subject  to  a  variation  within  certain  small  limits.  Thus,  at  one 
time  it  will  be  found  to  boil  at  the  temperature  of  210°  ;  while,  at  others,  the 
thermometer  immersed  in  it  will  rise  to  213°;  and,  on  different  occasions,  it 
will  fix  itself  at  different  points  within  these  limits.  It  will  also  be  found,  if 
the  same  experiment  be  performed  at  the  same  time  in  distant  places,  that  the 
boiling  points  will  be  subject  to  a  like  variation.  Now,  it  is  natural  to  inquire 
what  cause  produces  this  variation  ;  and  we  shall  be  led  to  the  discovery  of 
the  cause,  by  examining  what  other  physical  effects  undergo  a  simultaneous 
change. 

If  we  observe  the  height  of  a  barometer  at  the  time  of  making  each  experi- 
ment, we  shall  find  a  very  remarkable  correspondence  between  it  and  the  boil- 
ing temperature.  Invariably,  whenever  the  barometer  stands  at  the  same  height, 
the  boiling  temperature  will  be  the  same.  Thus,  if  the  barometer  stand  at 
thirty  inches,  the  boiling  temperature  will  be  212°.  If  the  barometer  fall  to 
twenty -nine  and  a  half  inches,  the  thermometer  stands  at  a  small  fraction  above 
211°.  If  the  barometer  rise  to  thirty  and  a  half  inches,  the  boiling  temperature 
rises  to  nearly  213°.  The  variation  in  the  boiling  temperature  is,  then,  ac- 
companied by  a  variation  in  the  pressure  of  the  atmosphere  indicated  by  the 
barometer  ;  and  it  is  constantly  found  that  the  boiling  point  will  remain  un- 
changed so  long  as  the  atmospheric  pressure  remains  unchanged,  and  that  every 
increase  in  the  one  causes  a  corresponding  increase  in  the  other. 

From  these  facts  it  must  be  inferred  that  the  pressure  excited  on  the  surface 
of  the  water  has  a  tendency  to  resist  its  ebullition,  and  to  make  it  necessary, 
before  it  can  boil,  that  it  should  receive  a  higher  temperature  ;  and,  on  the  con- 
trary, that  every  diminution  of  pressure  on  the  surface  of  the  water  will  give 
an  increased  facility  to  the  process  of  ebullition,  or  will  cause  that  process  to 
take  place  at  a  lower  temperature.  As  these  facts  are  of  the  utmost  impor- 
tance in  the  theory  of  heat,  it  may  be  useful  to  verify  them  by  direct  experi- 
ment. 

If  the  variable  pressure  excited  on  the  surface  of  the  water  by  the  atmo- 
sphere be  the  cause  of  the  change  in  the  boiling  temperature,  it  must  happen 
that  any  change  of  pressure  produced  by  artificial  means  on  the  surface  of  the 
water  must  likewise  change  the  boiling  point,  according  to  the  same  law. 
Thus,  if  a  pressure  considerably  greater  than  the  atmospheric  pressure  be  ex- 
cited on  a  liquid,  the  boiling  point  may  be  expected  to  rise  considerably  above 
212°;  and,  on  the  other  hand,  if  the  surface  of  the  water  be  relieved  from  the 
pressure  of  the  atmosphere,  and  be  submitted  to  a  considerably  diminished 
pressure,  the  water  would  boil  below  212°. 

Let  B,  fig.  3,  be  a  strong  spherical  vessel  of  brass,  supported  on  a  stand  S, 
under  which  is  placed  a  large  spirit  lamp  L,  or  other  means  of  heating  it.     In 


EBULLITION. 


303 


Fis.  3. 


the  top  of  this  vessel  are  three  apertures,  in  two  of  which  are  screwed  a  ther- 
mometer T,  the  bulb  of  which  enters  the  hollow  brass  sphere,  and  a  stopcock 
C,  which  may  be  closed  or  opened  at  pleasure,  to  confine  the  steam,  or  allow 
it  to  escape.  In  the  third  aperture,  at  the  top,  is  screwed  a  long  barometer  , 
tube,  open  at  both  ends.  The  lower  end  of  this  tube  extends  nearly  to  the  ) 
bottom  of  the  spherical  vessel  B.  In  the  bottom  of  this  vessel  is  placed  a 
quantity  of  mercury,  the  surface  of  which  rises  to  some  height  above  the  lower 
end  of  the  tube  A.  Over  the  mercury  is  poured  a  quantity  of  water,  so  as  to 
half  fill  the  vessel  B.  Matters  being  thus  arranged,  the  screws  are  made  tight 
so  as  to  confine  the  water,  and  the  lamp  is  allowed  to  act  on  the  vessel  ;  the 
temperature  of  the  water  is  raised,  and  steam  is  produced,  which,  being  con- 
fined within  the  vessel,  exerts  its  pressure  on  the  surface  of  the  water,  and 
resists  its  ebullition.  The  pressure  of  the  steam  acting  on  the  surface  of  the 
water,  is  communicated  to  the  surface  of  the  mercury,  and  it  forces  a  portion 
of  the  mercury  into  the  tube  A,  which  presently  rises  above  the  point  where 
the  tube  is  screwed  into  the  top  of  the  vessel  B.  As  the  action  of  the  lamp 
continues,  the  thermometer  T  exhibits  a  gradually  increasing  temperature  ; 
while  the  column  of  mercury  in  A  shows  the  force  with  which  the  steam 
presses  on  the  surface  of  the  water  in  B,  this  column  being  balanced  by  the 
pressure  of  the  steam.  Thus,  the  temperature  and  pressure  of  the  steam  at 
the  same  moment  may  always  be  observed  by  inspecting  the  thermometer  T 
and  the  tube  A.  When  the  column  in  the  tube  A  has  risen  to  the  height  of 
30  inches  above  the  level  of  the  mercury  in  the  vessel  B,  then  the  pressure  of 
the  steam  will  be  equivalent  to  double  the  pressure  of  the  atmosphere,  because 


304 


EBULLITION. 


the  tube  A  being  open  at  the  top,  the  atmosphere  presses  on  the  surface  of  the 
mercury  in  it.  The  thermometer  T  will  be  observed  gradually  to  rise  until  it 
attains  the  temperature  of  212°  ;  but  it  will  not  stop  there,  as  it  would  do  if 
immersed  in  water  boiled  in  an  open  vessel.  It  will,  on  the  other  hand,  con- 
tinue to  rise  ;  and  when  the  column  of  mercury  in  A  has  attained  the  height 
of  30  inches,  the  thermometer  T  will  have  risen  to  250°,  being  18°  above  the 
ordinary  boiling  point. 

During  the  whole  of  this  process,,  the  surface  of  the  water  being  submitted 
to  a  constantly  increasing  pressure,  its  ebullition  is  prevented,  and  it  continues 
to  receive  heat  without  boiling.  That  it  is  the  increased  pressure  which  re- 
sists its  ebullition,  and  causes  it  to  receive  a  temperature  above  212°,  may  be 
easily  shown.  Let  the  stopcock  C  be  opened  :  immediately  the  steam  in  B, 
having  a  pressure  considerably  greater  than  that  of  the  atmosphere,  will  rush 
out,  and  will  continue  to  issue  from  C,  until  its  pressure  is  balanced  by  the 
atmosphere.  At  the  same  time  the  column  of  mercury  in  A  will  be  observed 
rapidly  to  fall,  and  to  sink  below  the  orifice  by  which  it  is  inserted  in  the  ves- 
sel B.  The  thermometer  T  also  falls  until  it  attains  the  temperature  of  212°. 
At  that  point,  however,  it  remains  stationary  ;  and  the  water  will  now  be  dis- 
tinctly heard  to  be  in  a  state  of  rapid  ebullition.  If  the  stopcock  C  be  once 
more  closed,  the  thermometer  will  begin  to  rise,  and  the  column  of,  mercury 
ascending  in  A  will  be  again  visible. 

If,  instead  of  a  stopcock  being  at  C,  the  aperture  were  made  to  communicate 
with  a  valve,  like  the  safety-valve  of  a  steam-engine,  loaded  with  a  certain 
weight,  say  at  the  rate  of  fifteen  pounds  on  the  square  inch,  then  the  thermom- 
eter T,  and  the  mercury  in  the  tube  A,  would  not  rise  indefi«itely  as  before. 
The  thermometer  would  continue  to  rise  till  it  attained  the  temperature  of  250°, 
and  the  mercury  in  the  tube  A  would  rise  to  the  height  of  30  inches.  At  this 
limit  the  resistance  of  the  valve  wolild  be  balanced  by  the  pressure  of  the 
steam  ;  and  as  fast  as  the  water  would  have  a  tendency  to  produce  steam  of  a 
higher  pressure,  the  valve  would  be  raised,  and  the  steam  suffered  to  escape  ; 
the  thermometer  T  and  the  column  of  mercury  in  A  remaining  stationary  du- 
ring this  process.  If  the  valve  were  loaded  more  heavily,  the  phenomena 
would  be  the  same,  only  that  the  mercury  in  T  and  A  would  become  stationary 
at  certain  heights.  But,  on  the  other  hand,  if  the  valve  were  loaded  at  a  less 
pressure  than  fifteen  pounds  on  the  square,  inch,  then  the  mercury  in  the  two 
tubes  would  become  stationary  at  lower  points. 

These  experiments  show  that  every  increase  of  pressure  above  the  ordinary 
pressure  of  the  atmosphere  causes  an  increase  in  the  temperature  at  which 
water  boils.  We  shall  now  inquire  whether  a  diminution  of  pressure  will  pro- 
duce a  corresponding  effect  on  the  boiling  point. 

This  may  be  easily  accomplished  by  the  aid  of  an  air-pump.  Let  water  at 
the  temperature  of  200°  be  placed  in  a  glass  vessel  under  the  receiver  of  an 
air-pump,  and  let  the  air  be  gradually  withdrawn.  After  a  few  strokes  of  the 
pump  the  water  will  boil  ;  and  if  the  mercurial  gauge  of  the  pump  be  observed, 
it  will  be  found  that  its  altitude  will  be  about  twenty-three  and  a  half  inches. 
Thus  the  pressure  to  which  the  water  is  submitted  has  been  reduced  from  the 
ordinary  pressure  of  the  atmosphere  expressed  by  the  column  of  thirty  inches 
of  mercury  to  a  diminished  pressure  expressed  by  twenty-three  and  a  half 
inches  ;  and  we  find  that  the  temperature  at  which  the  water  boils  has  been 
lowered  from  212°  to  200°.  Let  the  same  experiment  be  repeated  with 
water  at  the  temperature  of  180°,  and  it  will  be  found  that  a  further  rarefac- 
tion of  the  air  is  necessary,  but  the  water  will  at  length  boil.  If  the  gauge  of 
the  pump  be  now  observed,  it  will  be  found  to  stand  at  about  fifteen  inches, 
I  showing  that  at  the  temperature  of  180°  water  will  boil  under  half  the  ordinary 


EBULLITION. 


pressure  of  the  atmosphere.  These  experiments  maybe  varied  and  repeated  ;  / 
and  it  will  be  always  found,  that  as  the  pressure  is  diminished  or  increased,  S 
the  temperature  at  which  the  water  will  boil  will  be  also  diminished  or  in-  ( 
creased.  ; 

The  same  effects  may  be   exhibited  in  a  striking  manner  without  an  air-  ( 
pump,  by  producing  a  vacuum  by  the  condensation  of  steam.     Let  a  small  | 
quantity  of  water  be  placed  in  a  thin  glass  flask,  and  let  it  be  boiled  by  hold-  I 
ing  it  over  a  spirit  lamp.     When  the  steam  is  observed  to  issue  abundantly 
from  the  mouth  of  the  flask,  let  it  be  quickly  corked  and  removed  from  ihe  ( 
lamp.     The   process  of  boiling  will  then  cease,  and   the  water  will  become  J 
quiescent ;   but  if  the  flask  be  plunged  in  a  vessel  of  cold  water,  the  water  it  i 
contains  will  again  pass  into  a  state  of  violent  ebullition,  thus  exhibiting  the  | 
singular  fact  of  water  being  boiled  by  cooling  it.     This  effect  is  produced  by  i 
the  cold  medium  in  which  the  flask  is  immersed  causing  the  steam  above  \ 
the  surface  of  the  water  in  it  to  be  condensed,  and  therefore  relieving  the  i 
water  from  its  pressure.     The  water,  under  these   circumstances,  boils  at  a 
lower  temperature  than  when  submitted  to  the   pressure  of  the  uncondensed 
vapor. 

There  is  no  limit  to  the  temperature  to  which  water  may  be  raised,  if  it  be 
submitted  to  a  sufficient  pressure  to  resist  its  tendency  to  take  the  vaporous 
form.  If  a  strong  metallic  vessel  be  nearly  filled  with  water,  so  as  to  prevent 
the  liquid  from  escaping  by  any  force  which  it  can  exert,  the  water  thus  en- 
closed may  be  heated  to  any  temperature  whatever  w^ithout  boiling ;  in  fact,  it 
may  be  made  red  hot,  and  the  temperature  to  which  it  may  be  raised  will  have 
no  limit,  except  the  strength  of  the  vessel  containing  it,  or  the  point  at  which 
the  metal  of  which  it  is  formed  may  begin  to  soften  or  to  be  fused. 

The  following  table  will  show  the  temperature  at  which  water  will  boil  un- 
der different  pressures  of  the  atmosphere  corresponding  to  the  altitudes  of  the 
barometer  between  26  and  31  inches  : — 

Barometer.  Boiling  point. 

26      inches 204^-91 

26-5 ^ 205°-79 

,  27    2060-67 

I  27-5 207'^-55 

'  28    2080-43 

'  28'5 2090-31 

;  29    210O- 19 

I  29-5 2110-07 

>  30     21 20 

;  30.5 2120-88 

!  31 2130-76 

>  From  this  table  it  appears  that  for  every  tenth  of  an  inch  which  the  baro- 
(  metric  column  varies  between  these  limits,  the  boiling  temperature  changes 
)  by  the  fraction  of  a  degree  expressed  by  the  decimal -176,  or  nearly  to  the 
(  vulgar  fraction  i. 

)  It  is  well  known,  that  as  we  ascend  in  the  atmosphere,  the  pressure  is  di- 
S  minished  in  consequence  of  the  quantity  of  air  left  below  it,  and  consequently 
?  the  barometer  falls  as  it  is  elevated.  It  follows,  therefore,  that  in  stations  at 
S  different  heights  in  the  atmosphere,  water  will  boil  at  different  temperatures; 
}  and  the  medium  temperature  of  ebullition  at  any  given  place  must,  therefore, 
}  depend  on  the  elevation  of  that  place  above  the  surface  of  the  sea.  Hence  the 
?  temperature  of  boiling  water,  other  things  being  the  same,  becomes  an  indica- 
S  tion  of  the  height  of  the  station  at  which  the  water  is  boiled,  or  in  other  words, 
r  becomes  an  indication  of  the  atmospheric  pressure  j  and  thus  the  thermometer 
J  serves  in  some  degree  the  purpose  of  a  barometer. 
)         VOIi.  II.— »o 


306 


EBULLITION. 


We  have  seen  that  the  vapor  into  vv^hich  water  is  converted  by  heat  posses- 
ses the  leading  qualities  of  common  atmospheric  air ;  and  if  not  submitted  to 
a  minute  examination,  might  be  mistaken  for  highly  heated  air.  It  is  perfectly 
transparent  and  invisible  ;  for,  in  the  first  experiment  described  in  this  dis- 
course, when  the  water  was  boiled  in  the  flask  until  the  whole  of  the  liquid 
had  been  converted  into  steam,  the  flask  had  the  same  appearance  as  if  it  were 
filled  with  air.  It  might  be  objected  to  this  statement,  that  the  steam  which 
issues  from  the  spout  of  a  boiling  kettle,  or  which  proceeds  from  the  surface 
of  water  boiling  in  an  open  vessel,  is  visible,  since  it  presents  the  appearance 
of  a  cloudy  smoke.  This  appearance,  however,  is  produced,  not  by  steam, 
but  by  very  minute  particles  of  water  arising  from  the  condensation  of  steam 
in  passing  through  the  cold  air.  These  minute  particles,  floating  in  the  air, 
become  in  some  degree  opaque,  and  are  visible  like  the  particles  of  smoke. 
Such  cloudy  substances,  therefore,  are  not  true  vapor  or  steam. 

But  the  most  important  property  which  steam  enjoys  in  common  with  atmo- 
spheric air  and  other  gases,  and  on  which,  like  them,  all  its  mechanical  prop- 
erties depend,  is  its  elasticity  or  pressure.  If  a  quantity  of  pure  steam  be  con- 
fined in  a  close  vessel,  it  will,  like  air,  exert  on  every  part  of  the  interior  sur- 
face of  that  vessel  a  certain  determinate  pressure,  directed  outward,  and  having 
a  tendency  to  burst  the  vessel.  A  bladder  might  thus  be  inflated  with  steam 
in  the  same  manner  as  with  atmospheric  air  ;  and,  provided  the  temperature 
of  the  bladder  be  sustained  at  that  point  necessary  to  prevent  the  steam  from 
returning  to  the  liquid  form,  its  inflation  would  continue. 

By  virtue  of  this  property  of  elasticity,  steam  or  air  is  expansible,  and, 
when  freed  from  the  limits  which  confine  it,  will  dilate  into  any  space  to 
which  it  may  have  access.  Suppose  a  piston  placed  in  a  cylinder,  in  which 
it  moves  steam-tight,  and  between  the  piston  and  the  bottom  of  the  cylinder 
let  any  quantity  of  steam  be  contained  ;  if  the  piston  be  drawn  upward,  so  as 
to  produce  a  larger  space  below  it  in  the  cylinder,  the  steam  will  expand,  and 
fill , the  increased  space  as  effectually  as  it  filled  the  more  limited  dimensions 
in  which  it  was  first  contained.  As  it  expands,  however,  its  elastic  pressure 
diminishes  in  exactly  the  same  manner,  and  in  the  same  proportion,  as  that  of 
atmospheric  air.  When  the  space  it  occupied  is  doubled,  its  temperature 
being  preserved,  its  elastic  pressure  is  halved  ;  and,  in  like  manner,  in  what- 
ever proportion  the  space  it  fills  be  increased,  its  elastic  pressure  will  be  in 
the  same  proportion  diminished. 

It  is  found  that  the  steam  which  is  raised  from  water  boiling  under  any  given 
pressure  has  an  elasticity  always  equal  to  the  pressure  under  which  the  water 
boils.  Thus,  when  water  is  boiled  under  the  ordinary  atmospheric  pressure^ 
when  the  barometer  stands  at  thirty  inches,  the  steam  which  is  dismissed  at 
the  temperature  of  212°  has  an  elastic  pressure  equal  to  that  of  the  atmo- 
•sphere.  If  water  be  boiled  under  a  diminished  pressure,  and  therefore  at  a 
lower  temperature,  the  steam  which  is  produced  from  it  will  have  a  pressure 
which  is  diminished  in  an  equal  degree.  Thus,  water  boiled  under  pressure 
corresponding  to  fifteen  inches  of  mercury,  and  at  a  temperature  of  180°,  will 
produce  steam,  the  elasticity  of  which  will  be  equivalent  to  a  column  of  fifteen 
inches  of  mercury. 

Numerous  experiments  have  been  made,  and  investigations  instituted,  with 
a  view  to  determine  some  fixed  relation  between  the  temperature  at  which 
water  boils,  and  the  elasticity  of  the  steam  which  it  produces  ;  but  hitherto 
without  success.  That  some  fixed  relation  does  exist,  there  can  be  no  doubt ; 
because  at  the  same  temperature  steam  of  the  same  elasticity  is  invariably  pro- 
duced. Tables  are  constructed  expressing  the  elasticity  or  pressure  corre- 
sponding to  different  temperatures,  and  empirical  formulae  or  rules  have  been 


EBULLITION. 


attempted  to  be  formed  from  the  results  of  these  tables,  by  which  the  elasticity 
may  in  general  be  deduced  from  the  temperature,  and  vice  versa. 

Another  remarkable  property  which  steam  enjoys,  in  common  with  the  air 
and  the  gases,  is  its  extreme  lightness  compared  with  the  ordinary  weight  of 
bodies  in  the  liquid  and  solid  forms  ;  when  water  is  boiled  under  the  medium  pres- 
sure of  the  atmosphere,  the  barometer  standing  at  thirty  inches,  the  steam  which 
is  produced  from  it  is,  bulk  for  bulk,  nearly  seventeen  hundred  times  lighter 
than  the  water  from  which  it  is  raised.  Thus,  a  cubic  inch  of  water,  when  con- 
verted into  steam  at  212°,  will  produce  about  seventeen  hundred  cubic  inches 
of  steam.  At  a  first  view  it  might  be  supposed  that  this  enormous  increase  of 
bulk  might  proceed  from  the  circumstance  of  some  other  body  being  combined 
with  the  water  in  forming  the  steam  ;  but  that  this  is  not  the  case,  or,  at  least, 
that  no  ponderable  body  is  so  combined  with  it,  may  be  determined  by  weigh- 
ing the  steam  and  the  water  respectively.  These  weights  will  always  be 
found,  as  already  stated,  to  be  equal.  This  expansion  which  water  undergoes 
in  its  transition  from  the  liquid  to  the  vaporous  state  is  subject  to  great  varia- 
tion, as  we  shall  presently  explain,  according  to  the  temperature  and  pressure 
at  which  it  is  raised. 

In  the  experiment  already  described,  by  which  the  latent  heat  of  steam  was 
determined,  the  water  was  supposed  to  be  boiled  under  the  ordinary  pressure 
of  the  atmosphere.  Having  seen,  however,  that  water  may  boil  at  different 
temperatures  under  different  pressures,  the  inquiry  presents  itself,  whether  the 
heat  absorbed  in  vaporization  at  different  temperatures  and  under  different  pres- 
sures, is  subject  to  any  variation  ?  Experiments  of  the  same  nature  as  those 
already  described,  instituted  upon  water  in  a  state  of  ebullition  at  different  tem- 
peratures as  well  below  as  above  212°,  have  led  to  the  discovery  of  a  very 
remarkable  fact  in  the  theory  of  vapor.  It  has  been  found  that  the  heat  ab- 
sorbed by  vaporization  is  always  less,  the  higher  the  temperature  at  which  the 
ebullition  takes  place  ;  and  less,  by  the  same  amount  as  the  temperature  of 
ebullition  is  increased.  Thus,  if  water  boil  at  312°,  the  heat  absorbed  in 
ebullition  will  be  less  by  100°  than  if  it  boiled  at  212° ;  and  again,  if  water 
be  boiled  under  a  diminished  pressure,  at  112°,  the  heat  absorbed  in  vaporiza- 
tion will  be  100°  more  than  the  heat  absorbed  by  water  boiled  at  212°.  It 
follows,  therefore,  that  the  actual  consumption  of  heat  in  the  process  of  vapori- 
zation must  be  the  same,  whatever  be  the  temperature  at  which  the  vaporiza- 
tion takes  place ;  for  whatever  heat  is  saved  in  the  sensible  form  is  consumed 
in  the  latent  form,  and  vice  versa. 

Let  us  suppose  a  given  weight  of  water  at  the  temperature  of  32°  to  be  ex- 
posed to  any  regular  source  by  which  heat  may  be  supplied  to  it.  If  it  be 
under  the  ordinary  atmospheric  pressure,  the  first  180°  of  heat  which  it  re- 
ceives will  raise  it  to  the  boiling  point,  and  the  next  1,000°  will  convert  it  into 
steam.  Thus,  in  addition  to  the  heat  which  it  contains  at  32°,  the  steam  at 
212°  contains  1,180°  of  heat.  But  if  the  same  water  be  submitted  to  a  pres- 
sure equal  to  half  the  atmospheric  pressure,  then  the  first  148°  of  heat  which 
it  receives  will  cause  it  to  boil,  and  the  next  1,032°  will  convert  it  into  vapor. 
Thus,  steam  at  the  temperature  of  180°  contains  a  quantity  of  heat  more  than 
the  same  quantity  of  water  at  32°,  by  1,032°  added  to  148°,  which  gives  a 
sum  of  1,180°.  Steam,  therefore,  raised  under  the  ordinary  pressure  of  the 
atmosphere  at  212°,  and  steam  raised  under  half  that  pressure  at  180°,  contain 
the  same  quantity  of  heat,  with  this  difference  only,  that  the  one  has  more 
latent  heat,  and  less  sensible  heat,  than  the  other. 

From  this  fact,  that  the  sum  of  the  latent  and  sensible  heats  of  the  vapor  of 
water  is  constant,  it  follows  that  the  same  quantity  of  heat  is  necessary  to  con- 
vert a  given  weight  of  water  into  steam,  at  whatever  temperature  or  under 


308 


EBULLITION. 


whatever  pressure  the  water  may  be  boiled.  It  follows  also  that,  in  the  steam- 
engine,  equal  weights  of  high-pressure  and  low-pressure  steam  are  produced 
by  the  same  consumption  of  fuel ;  and  that,  in  general,  the  consumption  of  fuel 
is  proportional  to  the  quantity  of  water  vaporized,  whatever  the  pressure  of 
the  steam  may  be. 

The  quantity  of  heat  consumed  thus  depending  on  the  weight  of  water  evap- 
orated, it  is  obviously  a  point  of  considerable  practical  importance  to  determine 
the  specific  gravities  or  densities  of  steam  raised  under  different  pressures, 
and  at  different  temperatures ;  yet  this  is  a  point  on  which  even  philosophical 
authorities,  in  general  entitled  to  respect,  appear  to  have  fallen  into  error.  It 
has  been  stated  that  the  specific  gravity  or  density  of  steam  is  always  propor- 
tional to  its  pressure.*  This,  however,  is  not  correct.  The  true  law  for  the 
variation  of  the  density  or  specific  gravity  of  steam  is  the  same  as  that  of  air  : 
it  is  proportional  to  the  pressure  or  elasticity,  provided  the  temperatures  are  the 
same.  If,  then,  we  have  steam  raised  from  water  under  two  different  pres- 
sures, and  at  two  different  temperatures,  let  the  temperatures  be  equalized  by 
applying  heat  to  the  steam  of  the  lesser  pressure  out  of  contact  with  water,  its 
pressure  being  meanwhile  preserved.  When  the  temperatures  are  thus  ren- 
dered equal,  then  their  densities  or  specific  gravities  will  be  in  the  same  pro- 
portion as  their  pressures. 

If  the  space  below  the  piston  P,  in  the  cylinder  A  B,  fig.  4,  be  completely 

Fig.  4. 


filled  with  water,  and  a  suflicient  force  be  exerted  on  the  piston  to  prevent  it 
from  rising  in  the  cylinder,  the  water  under  it  may  be  heated  to  any  required 
temperature  ;  because,  no  space  being  allowed  for  the  formation  of  steam,  no 
heat  can  become  latent,  and  therefore  all  the  heat  communicated  to  the  water 
will  be  effective  in  raising  its  temperature.  If  the  temperature  of  the  water 
under  these  circumstances  were  raised  until  it  attained  the  limit  of  1,212°,  it 
would  have  all  the  heat  necessary  to  give  it  the  vaporous  form,  no  part  of  that 
heat  being  in  this  case  latent.  In  fact,  the  water  would,  under  such  circum- 
stances, be  converted  into  vapor,  in  which  the  whole  of  the  heat  would  be 
sensible,  and  which  would  have  no  latent  heat  except  such  as  the  water  pos- 
sessed in  the  liquid  state.  If  the  piston,  under  these  circumstances,  be  raised, 
the  water,  or  rather  steam,  below  it,  will  expand  ;  and  as  it  expands,  its  tem- 
perature will  fall,  a  portion  of  the  sensible  heat  becoming  latent.  If  the  piston 
were  raised  until  the  space  below  it  were  increased  seventeen  hundred  times, 
the  steam  would  fall  to  the  temperature  of  212°,  and  1,000°  of  heat  would  be- 

*  Thomson  on  Heat  and  Electricity,  p.  221. 


come  latent.  In  fact,  the  steam  would  then  be  identical  in  its  constitution  and 
properties  with  steam  raised  from  water  at  the  temperature  of  212^,  and  under 
the  ordinary  atmospheric  pressure.  If  the  piston  be  raised  or  lowered  under 
these  circumstances,  the  steam  would  take  all  possible  temperatures  and  pres- 
sures, and  would,  in  each  case,  be  identical  with  the  steam  raised  from  water 
under  a  corresponding  pressure  and  temperature. 

The  sum  of  the  latent  and  sensible  heats  of  steam  being  always  the  same, 
it  follows  that,  if  we  know  the  latent  heat  of  steam  at  any  one  temperature, 
the  latent  heats  at  all  other  temperatures  is  a  subject  of  easy  calculation. 
Thus,  if  the  sum  of  the  latent  and  sensible  heats  be  1,212°,  the  latent  heat  of 
steam  at  500°  of  temperature  must  necessarily  be  712°,  and  steam  at  the  tem- 
perature of  1,000°  will  have  only  212°  of  latent  heat. 

It  follows  also  that,  in  order  to  maintain  water  in  a  state  of  vapor,  the  sum 
of  its  latent  and  sensible  heats  cannot  be  less  than  1,212°;  and  if  it  be  re- 
duced below  this,  by  being  caused  to  impart  heat  to  any  other  object,  then  a 
portion  of  the  vapor  must  return  to  the  liquid  state,  giving  its  latent  heat  to  the 
vapor  which  remains,  so  as  to  raise  the  sum  of  the  latent  and  sensible  heats 
of  that  vapor  to  1,212°.  When  so  much  steam  becomes  liquid  as  is  capable 
of  accomplishing  this,  then  the  remainder  of  the  vapor  will  continue  in  the 
aeriform  state.  If  steam  receives  no  heat  except  that  which  is  imparted  to 
the  water  during  the  process  of  vaporization,  the  sum  of  its  latent  and  sensible 
heats  cannot  be  greater  than  1,212°,  and  therefore  such  steam  cannot  lose  any 
heat  without  undergoing  partially  the  process  of  condensation  ;  but  if  steam, 
after  the  process  of  vaporization,  has  received  an  increase  of  temperature  by 
heat  supplied  from  some  external  source,  then  the  sum  of  its  latent  and  sensi- 
ble heats  will  be  greater  than  1,212°  by  the  heat  so  received,  and  the  steam 
may  lose  that  excess  of  heat  above  1,212°  without  undergoing  any  condensa- 
tion. 

In  considering  the  properties  of  steam  at  present,  we  shall,  however,  regard 
it  as  having  received  no  heat  except  that  which  it  receives  in  the  process  of 
vaporization,  unless  the  contrary  be  distinctly  expressed. 

It  is  well  known  that  air  and  the  gases  generally  admit  of  compression  and 
rarefaction  without  any  practical  limit,  and  that  their  elasticity  is  susceptible 
of  increase  and  diminution,  as  the  space  they  fill  is  contracted  or  enlarged. 
Let  a  cylinder,  in  which  a  piston  moves  air-tight,  have  the  space  below  the 
piston  filled  with  atmospheric  air  in  its  ordinary  state.  By  the  application  of 
adequate  mechanical  force,  the  piston  may  be  pressed  toward  the  bottom  of 
the  cylinder,  so  that  the  air  beneath  it  shall  be  forced  into  a  more  confined 
space.  The  effect  of  this  compression  will  be  twofold — an  increase  of  tem- 
perature and  an  increase  of  elasticity.  If  the  piston,  on  the  other  hand,  be 
raised  so  as  to  allow  the  air  to  expand  into  a  more  enlarged  space,  the  contrary 
effects  will  ensue — the  temperature  of  the  air  will  fall,  and  its  elasticity  will 
be  diminished.  Whether  air  thus  enclosed  be  compressed  into  a  more  limited 
space,  or  allowed  to  expand  into  a  more  enlarged  space,  it  never  passes  from 
the  aeriform  state,  nor  loses  its  property  of  elasticity.  No  known  degree  of 
compression  has  caused  it  to  become  a  liquid,  nor  has  any  degree  of  expansion 
caused  it  to  lose  its  elastic  property. 

Let  us  now  suppose  the  space  below  the  piston,  instead  of  air,  to  be  filled 
with  steam  raised  from  water  at  the  temperature  of  212°.  If  the  piston  be 
raised,  this  steam  will  expand,  its  temperature  will  fall,  and  its  elastic  force 
will  diminish  in  the  same  manner  as  already  described  for  common  air,  and, 
as  with  common  air,  there  is  no  known  limit  to  the  extent  of  this  expansion. 

If,  however,  the  piston  be  pressed  toward  the  bottom  of  the  cylinder,  it  has 
been  generally  stated  that  steam  will  not  comport  itself  like  common  air  under 


310 


EBULLITION. 


the  same  circumstances  ;  that  it  will  not  retain  the  vaporous  form  on  being 
compressed,  nor  increase  its  elasticity  ;  but  that,  on  the  contrary,  as  the  piston 
is  depressed,  it  will  be  partially  restored  to  the  liquid  state,  and  that  the  por- 
tion which  remains  in  the  vaporous  form  will  retain  the  same  density  and  elas- 
ticity as  it  had  before  the  piston  was  moved.  In  fact,  if  the  piston  be  de- 
pressed so  as  to  reduce  the  space  occupied  by  the  steam  to  one  half  its  origi- 
nal dimensions,  it  has  been  assumed  that  in  that  case  one  half  the  steam  under 
the  piston  would  be  restored  to  the  liquid  form,  and  would  become  water  of 
the  temperature  of  212°,  while  the  remaining  half  would  still  retain  the  vapor- 
ous form,  and  have  the  same  temperature  and  density  as  before.* 

From  this  statement,  however  universally  admitted,  I  must  most  distinctly 
dissent,  unless  it  be  assumed,  at  the  same  time,  that  a  large  quantity  of  heat 
has  been  abstracted  from  that  portion  of  the  steam  which  is  reduced  to  the 
liquid  form.  If  this  do  not  happen,  and  the  same  quantity  of  heat  remain  in 
the  vapor  under  the  piston,  no  change  to  the  liquid  form  can,  in  ray  opinion, 
take  place.  The  steam  originally  contained  in  the  cylinder  below  the  piston 
has  that  quantity  of  latent  and  sensible  heat  which  is  necessary  and  sufficient 
to  maintain  it  in  the  vaporous  form  in  all  degrees  of  density.  If  the  steam  be 
compressed  by  the  piston,  we  cannot  suppose  a  portion  of  it  to  be  condensed 
into  a  liquid,  without  at  the  same  time  supposing  that  portion  to  part  with  about 
1,000°  of  latent  heat ;  but  this  supposition  cannot  be  admitted,  unless  we  sup- 
pose the  heat  so  dismissed  to  pass  off  to  some  external  object,  the  contrary  of 
which  is  the  supposition  upon  which  I  have  here  argued. 

I  consider  that  the  effects  of  the  compression  of  steam  thus  enclosed  would 
be  the  same  as  already  described  with  respect  to  air.  The  temperature  and 
pressure  will  be  increased,  but  no  portion  of  it  will  be  condensed  into  a  liquid. 
In  every  state  of  density  to  which  it  will  be  reduced  by  compression  it  will 
take  that  temperature  and  pressure  which  steam  of  the  same  density  raised  im- 
mediately from  water  would  have.  If  the  piston  be  depressed  so  as  to  reduce 
the  steam  to  one  half  its  original  bulk,  then,  its  density  being  doubled,  it  will 
acquire  that  temperature  at  which  steam  of  double  the  degree  of  density  would 
be  raised  from  water.  The  steam  will  be  in  all  respects,  both  with  regard  to 
its  latent  and  sensible  heat,  its  density  and  its  elasticity,  the  same  as  steam 
raised  from  water  boiled  at  the  increased  temperature.  Similar  observations 
may  be  applied  to  any  degree  of  compression  whatever ;  and  it  will  follow,  not 
only  that  no  part  of  the  steam  will  be  restored  to  the  liquid  form  by  reducing 
its  bulk,  but  that  no  degree  of  compression  whatever  will  be  capable  of  redu- 
cing any  part  of  it  to  the  liquid  state.  If  the  piston  could  be  moved  toward  the 
bottom,  so  as  to  reduce  the  dimensions  of  the  steam  to  those  which  it  had 
when  it  existed  in  the  liquid  state,  which  would  be  accomplished  by  advancing 
it  within  a  distance  of  the  bottom  of  the  cylinder  equal  to  about  the  seventeen 
hundredth  part  of  its  original  distance,  it  would  continue  to  be  steam,  but  would 
have  a  prodigiously  increased  elastic  force,  and  a  temperature  of  1,212°.  The 
steam  would  in  such  case  be  reduced  to  the  state  explained  in  page  308,  and 
would  be  identical  with  water  raised  in  a  close  vessel  to  the  temperature  of 
1,212°.  It  is  obvious  that  the  practical  exhibition  of  such  effects  as  here  de- 
scribed would  be  obstructed  by  the  difficulty  of  preventing  the  escape  of  the 
sensible  heat  developed  in  the  compression  of  the  steam. 

The  true  cause  of  the  conversion  of  any  part  of  a  vapor  to  the  liquid  form,  I 
consider  to  be  the  diminution  of  that  sum  of  sensible  an,d  latent  heat  which  is  es- 
sential to  the  existence  of  vapor.  Such  a  loss  of  heat  would  equally  cause  the 
vapor  to  return  to  the  liquid  state,  whether  compressed  into  a  less  bulk  or  ex- 


See  Biot,  Traite  de  Physique,  torn,  i.,  p.  266,  and  physical  and  chemical  writers  generally. 


panded  into  a  greater  one.  If  the  piston  had  been  previously  raised,  and  a 
small  quantity  of  heat  at  the  same  time  abstracted  from  the  vapor,  a  portion  of 
the  vapor  would  immediately  be  condensed,  and  a  small  portion  would  be  con- 
densed b}^  the  same  loss  of  heat,  in  whatever  state  of  compression  or  rarefac- 
tion the  steam  may  exist.  This  condensation  is  therefore  altogether  indepen- 
dent of  any  effects  produced  on  the  density  of  the  steam  by  any  mechanical 
compression.* 

The  pressure  on  the  surface  of  water,  though  the  principal  cause  which 
affects  the  boiling  point,  is  not  the  only  one.  It  has  been  already  stated  that 
the  material  of  which  the  vessel  is  composed,  in  which  the  process  of  boiling 
takes  place,  has  also  an  effect  upon  the  boiling  temperature.  It  is  found  that 
in  a  vessel  of  glass,  water  boils  at  a  lower  temperature  than  in  a  vessel  of 
metal.  Foreign  matter  also  held  in  solution  by  the  water  produces  a  change 
in  its  boiling  point ;  but  this  should  rather  be  considered  as  a  distinct  liquid. 

If  heat  be  applied  to  other  liquids,  results  will  be  obtained  showing  that  the 
phenomena  already  explained  with  respect  to  water,  are  only  instances  of  a 
more  numerous  class,  applicable  to  all  liquids  whatever.  The  application  of 
heat  to  any  liquid  causes  its  temperature,  in  the  first  instance,  to  rise  ;  and 
this  increase  of  temperature  continues  until  the  liquid  attains  a  state  similar  to 
that  of  boiling  water,  when  a  thermometer  or  pyrometer,  immersed  in  it,  would 
become  stationary.  The  continued  application  of  heat  now  no  longer  causes 
the  liquid  to  rise  in  temperature,  but  produces  vapor  rapidly,  so  that  the  liquid 
boils  away  in  the  same  manner  as  already  described  with  respect  to  water, 
and  all  the  effects  before  explained  take  place,  differing  only  in  the  tempera- 
ture at  which  the  ebullition  commences,  and  in  the  rate  at  which  the  vapor  is 
produced.  Different  liquids  attain  the  stationary  temperature  of  ebullition  at 
different  points  ;  and  hence  the  boiling  point  becomes  a  specific  character  to 
distinguish  material  substances.  They  likewise,  in  passing  into  the  vaporous 
form,  render  different  quantities  of  heat  latent. 

Let  a  thermometer,  consisting  of  two  metallic  bars,  be  fixed  in  a  vessel  so 
as  to  extend  across  it  in  a  horizontal  position,  and  so  that  the  extremity,  bear- 
ing the  graduated  scale,  shall  pass  through  the  side  and  project  outside  the 
vessel.  Let  melted  lead  be  now  poured  into  this  vessel,  so  as  to  cover  the 
pyrometric  bars,  and  let  the  whole  be  placed  on  a  furnace.  The  divided  scale, 
during  the  continued  application  of  the  fire,  will  constantly  show  an  increasing 
temperature  until  the  lead  boils.  The  expansion  of  the  bars  will  then  cease, 
and  the  pyrometer  will  become  fixed  in  its  indication,  and  will  continue  fixed 
until  the  whole  of  the  lead  is  evaporated. 

Again,  let  a  common  thermometer  be  immersed  in  phosphorus  at  the  tem- 
perature of  300^,  and,  being  placed  in  a  vessel,  let  it  be  exposed  to  the  action 
of  heat.  It  will  continue  to  rise  until  it  attains  the  temperature  of  554°,  where 
it  will  become  stationary,  and  the  phosphorus  will  boil.  The  thermometer 
will  become  stationary  until  the  whole  of  the  phosphorus  is  evaporated. 

The  correspondence  of  these  results  with  those  obtained  in  the  experiments 
instituted  upon  water  is  obvious.  The  analogy  might  be  still  further  confirmed 
by  using  a  close  vessel,  like  that  represented  in  fig.  1,  and  carrying  over  the 
vapor  of  the  lead,  or  the  phosphorus,  into  a  vessel  exposed  to  cold,  where  it 
might  be  re-collected  in  the  liquid  form.  It  is  clear  that,  in  all  these  instances, 
during  the  process  of  ebullition,  heat  has  become  latent,  because  heat  contin- 
ues to  be  supplied  to  the  vaporizing  body,  although  the  vapor  produced  by  the 
supply  of  such  heat  is  found  to  have  no  greater  temperature  than  that  of  the 
liquid  from  which  it  is  produced.     The  same  result  would  be  obtained  by  simi- 

*  I  have  been  the  more  minute  in  these  details,  because  my  opinions  diifer  from  those  commonly 
received  respecting  the  effects  of  compression  upon  steam. 


312 


EBULLITION. 


lar  experiments  made  on  other  substances  ;  and  we  may,  therefore,  generalize 
the  facts  established  by  the  experiments  already  described  upon  water,  and 
state  that  all  bodies,  when  in  the  liquid  form,  are  capable,  by  increasing  their 
temperatures,  of  being  converted  into  vapor  ;  and  that  in  this  conversion  a  large 
quantity  of  heat  must  be  supplied,  which  becomes  latent  in  the  vapor,  because, 
notwithstanding  the  increased  supply  of  heat  given  to  it,  it  exhibits  no  corre- 
sponding increase  of  temperature. 

There  is  no  liquid  upon  which  the  effects  of  heat  have  been  so  minutely  ex- 
amined as  water.  The  latent  heats  of  a  few  other  liquids  have  been  accurately 
determined  ;  but  much  still  remains  to  be  done  in  this  department  of  physics. 
Count  Rumford  examined  the  latent  heats  of  several  vapors,  by  causing  them 
to  be  condensed  in  a  refrigeratory,  so  that  they  imparted  their  latent  heat 
to  water.  He  then  determined  the  weight  of  the  liquid  which  had  been  con- 
densed, and,  by  comparing  with  it  the  heat  imparted  to  the  water  in  the  re- 
frigeratory, he  obtained  the  latent  heat.  Dr.  Ure  and  M.  Despretz  also  made 
experiments  on  some  liquids,  the  results  of  which  were  as  follows  : — 


Latent 

Heat. 

Despretz 

956° 

Despretz 

597-4 

Despretz 

314-1 

Despretz 

299-16 

Ure 

837-28 

Ure 

531-99 

Ure 

177-87 

Latent  Heat 

referred  to 

Water. 


Steam 

Alcohol  vapor  (specific  gravity  0-793).,, 
Sulphuric  ether  (specific  gravity  0-715) 

Oil  of  turpentine   

Ammonia  (specific  gravity  -0978) 

Nitric  acid  (specific  gravity  1-494). . . . 
Naphtha 


3730-86 
163-44 
138-24 
862 
335 
73-77 


The  boiling  points  of  all  liquids  are  affected  by  pressure  in  the  same  man- 
ner as  the  boiling  point  of  water,  every  increase  of  pressure  causing  it  to  fall. 
In  comparing  the  boiling  points  of  different  liquids  one  with  the  other,  it  is, 
therefore,  necessary  to  take  them  all  under  the  same  pressure  ;  and  the  pres- 
sure usually  adopted  for  this  purpose  is  the  medium  pressure  of  the  atmo- 
sphere, or  thirty  inches  of  mercury. 

The  comparison  of  the  melting  and  boiling  points  of  bodies  does  not  present 
any  general  feature  which  could  serve  as  a  basis  for  any  obvious  inference, 
connecting  the  phenomena  of  fusion  and  ebullition  with  their  other  properties. 
Generally,  but  not  invariably,  the  higher  on  the  scale  of  temperature  the  melt- 
ing point  is,  the  higher  will  be  the  boiling  point ;  but  to  this  there  are  many 
exceptions.  Mercury  freezes  at  39°  below  0°,  and  boils  at  a  temperature  of 
about  660°;  while,  on  the  other  hand,  phosphorus  melts  at  140°  above  the 
melting  temperature  of  mercury,  and  boils  at  about  110°  below  the  boiling 
temperature  of  that  metal. 

Since,  by  continually  imparting  heat  to  it,  a  body  in  the  liquid  state  at  length 
passes  into  the  form  of  vapor  or  air,  analogy  would  lead  us  to  expect  that,  by 
continually  withdrawing  heat,  a  body  in  the  aeriform  state  would  at  length  re- 
turn to  the  liquid  state.  In  the  case  of  vapor  raised  from  liquids  by  heat,  this 
is  found  to  be  universally  true.  In  the  experiment  illustrated  by  figure  1,  the 
steam  of  water,  having  passed  from  the  heated  vessel  to  one  maintained  at  a 
lower  temperature,  was  caused  to  impart  its  heat  to  the  surrounding  medium, 
and  immediately  returned  to  the  liquid  state.  The  same  result  would  be  ob- 
tained under  the  same  circumstances  in  any  liquid  body  vaporized.  The  vapor, 
being  exposed  to  cold,  is  deprived  of  a  part  of  that  heat  which  is  necessary  to 
sustain  it  in  the  aeriform  state,  and  a  portion  of  it  is  accordingly  restored  to 
the  liquid  form,  and  this  continues  until,  by  the  constant  abstraction  of  heat, 
the  whole  of  the  vapor  becomes  liquid.     As  a  liquid,  in  passing  to  the  vapor- 


EBULLITION. 


313 


ous  form,  undergoes  an  immense  expansion  or  increase  of  bulk,  so  a  vapor,  in 
returning  to  the  liquid  form,  undergoes  a  corresponding  and  equal  diminution 
of  bulk.  A  cubic  inch  of  water  transformed  into  steam  at  212°,  enlarges  in 
magnitude  to  seventeen  hundred  cubic  inches,  as  already  observed.  The  same 
steam,  reconverted  into  water  by  abstracting  from  it  the  heat  consumed  in  its 
vaporization,  will  be  restored  to  its  former  bulk,  and  will  form  one  cubic  inch 
of  water  at  212"^.  Vapors  raised  from  other  bodies  would  undergo  a  similar 
change,  differing  only  in  the  degree  of  diminution  of  bulk  which  they  would 
suffer  respectively.  The  diminished  space  into  which  the  particles  of  a  vapor 
are  gradually  condensed  when  it  passes  into  the  liquid  state  has  caused  this 
process  to  be  called  condensation* 

No  liquid  has  been  submitted  to  so  minute  an  examination,  with  respect  to 
the  effects  produced  upon  it  by  heat,  as  water  ;  and,  with  respect  to  other  li- 
quids, we  are  compelled,  in  the  absence  of  experimental  proof,  to  reason  from 
analogy.  The  principle  that  the  sum  of  the  latent  and  sensible  heats  of  vapor 
is  the  same  for  all  temperatures,  may  be  extended,  with  a  high  degree  of  prob- 
ability, to  the  vapors  of  all  liquids  whatever ;  so  that  we  may  assume  this  sum 
to  be  constant  for  each  liquid,  though  differing  in  one  liquid  compared  with 
another.  To  maintain  the  vapor  of  any  liquid  in  the  aeriform  state,  it  is  there- 
fore necessary  that  it  should  contain  at  least  a  certain  quantity  of  heat,  what- 
ever be  its  temperature  ;  and  any  diminution  in  this  quantity  cannot  fail  to 
produce  the  condensation  of  a  corresponding  portion  of  the  vapor.  If  the  vapor 
of  a  liquid,  .therefore,  has  received  no  heat  after  having  passed  from  the  liquid 
to  the  vaporous  form,  it  cannot  lose  any  portion  of  the  heat  it  contains  without 
a  partial  condensation  ;  but  it  is  important  to  observe,  that  a  vapor,  whether  of 
water  or  any  other  liquid,  may,  after  having  attained  the  state  of  vapor,  receive 
an  additional  supply  of  heat  to  any  extent,  and  may  thus  have  its  temperature  < 
raised  to  any  point  whatever.  Independently  of  the  heat  which  it  received  in 
the  process  of  vaporization,  all  the  heat  which  it  has  thus  received  in  the  state 
of  vapor  it  may  lose,  and  yet  remain  in  that  state.  Under  such  circumstances, 
therefore,  it  must  not  be  inferred  that  a  reduction  of  temperature  in  vapor  ne- 
cessarily causes  condensation.  Condensation  cannot  commence  until  the  vapor 
loses  all  that  heat  which  it  received  after  taking  the  form  of  vapor  ;  but  when  it 
has  lost  so  much,  then  any  further  abstraction  of  heat  must  be  attended  by  con- 
densation. 

^  By  the  great  change  of  volume  which  a  vapor  undergoes  in  condensation,  it 
becomes  an  efficient  means  of  producing  a  vacuum,  without  the  exertion  of 
mechanical  force.  Let  a  glass  tube  be  provided,  having  at  one  extremity  a 
i  large  bulb,  the  other  extremity  being  open.  Let  a  small  quantity  of  liquid  be 
I  introduced  into  the  bulb  through  the  tube,  and  let  a  spirit  lamp  be  placed  under 
the  bulb,  so  as  to  cause  the  liquid  to  boil.  The  vapor  of  the  liquid  will  first 
mix  with  the  air  in  the  bulb  and  tube  ;  but,  as  its  quantity  increases,  its  elas- 
ticity will  cause  it  to  issue  through  the  tube,  which  it  will  at  length  raise  to 
its  own  temperature,  so  as  to  enable  it  to  pass  from  the  mouth  of  the  tube  in 
the  vaporous  form,  without  being  previously  condensed.  The  stream  of  vapor 
proceeding  up  the  tube  will,  after  a  time,  carry  off  with  it  the  atmospheric  air  pre- 
viously contained  in  the  bulb  and  tube  ;  and  at  length  the  space  below  the  mouth 
of  the^  tube  will  be  completely  filled  with  pure  vapor.  Let  the  tube  be  now 
inverted,  and,  its  open  end  plunged  in  a  vessel  of  water  or  other  liquid,  the 
bulb  iJeing  presented  upward.     The  space  within  the  tube  and  bulb  containing 


*  In  general,  whenever  the  dimensions  of  a  body  are  diminished,  without  any  diminution  of  its 
quantity  of  matter,  it  is  said  to  be  condensed,  and  the  process  may  without  impropi-iety  be  called 
condensation  ;  but  this  more  general  application  of  the  term  cannot  cause  any  confusion,  since  its 
meaning  is  always  easily  understood  from  the  context. 


314 


EBULLITION. 


pure  vapor  will  be  thus  cut  off  from  all  communication  with  the  air.  The  in- 
ferior temperature  of  the  surrounding  air,  taking  heat  constantly  from  the  bulb 
and  tube,  will  deprive  the  vapor  contained  in  them  of  the  quantity  of  heat  ne- 
cessary to  sustain  it  in  the  elastic  form,  and  it  will  be  condensed.  The  great 
diminution  of  bulk  which  it  will  suffer  will  cause  a  partial  vacuum  to  be  pro- 
duced in  the  bulb  and  tube,  and  the  pressure  of  the  atmosphere,  acting  on  the 
surface  of  the  water  in  the  vessel  in  which  the  tube  is  immersed,  will  force 
the  water  up  the  tube,  and  and  it  will  completely  fill  the  bulb. 

That  form  of  the  steam-engine  called  the  low-pressure  engine,  derives  its 
principal  mechanical  efficacy  from  this  property,  by  which  steam  is  instrumen- 
tal in  the  formation  of  a  vacuum.  The  moving  power  in  that  machine  is  ren- 
dered operative  by  a  piston  placed  in  a  cylinder,  in  which  it  moves  steam- 
tight.  The  atmospheric  air  and  other  gases  are  expelled  from  the  cylinder  and 
tubes  which  communicate  between  it  and  the  boiler  by  steam,  in  the  same 
manner  exactly  as  in  the  experiment  just  described.  Steam  is  allowed  to  pass 
freely  from  the  boiler  through  the  tubes  and  cylinder,  and  makes  its  escape 
finally  through  a  valve  or  cock  provided  for  that  purpose,  un  il  at  length  all  the 
atmospheric  air  is  blown  from  the  machine.  The  cock  is  then  closed,  and 
pure  steam  only  fills  every  part  of  the  engine.  A  chamber,  called  a  condenser, 
which  is  maintained  at  a  low  temperature,  by  being  immersed  in  cold  water, 
is  made  to  communicate  with  both  ends  of  the  cylinder  by  means  of  proper 
tubes  and  valves.  When  the  piston  is  required  to  descend,  the  communica- 
tion between  this  chamber  and  ihe  bottom  of  the  cylinder  is  opened,  while  a 
communication  is  at  the  same  time  opened  between  the  boiler  and  the  top  of 
the  cylinder.  The  steam  which  fills  the  cylinder  below  the  piston  rushes 
toward  the  condenser  by  its  elastic  force,  and  is  there  immediately  converted 
into  water  by  the  cold  medium  with  which  it  is  surrounded.  The  cylinder 
below  the  piston,  therefore,  remains  a  vacuum  ;  meanwhile  the  steam,  rushing 
from  the  boiler  above  the  piston,  forces  it  downward,  until  it  reaches  the  bot- 
tom of  the  cylinder.  The  communication  between  the  boiler  and  the  top  of 
the  cylinder  is  now  closed,  and  a  communication  opened  between  the  boiler 
and  the  bottom  of  the  cylinder,  and  at  the  same  time  the  communication  be- 
tween the  condenser  and  the  bottom  of  the  cylinder  is  closed,  and  a  commu- 
nication is  opened  between  the  condenser  and  the  top  of  the  cylinder.  Under 
these  circumstances,  the  steam  which  is  above  the  piston  rushes  by  its  elastic 
force  toward  the  condenser,  where  it  is  condensed,  and  the  cylinder  above  the 
piston  remains  a  vacuum.  Meanwhile  the  steam  from  the  boiler,  rushing  into 
the  cylinder  below  the  piston,  forces  it  upward,  and  the  piston  ascends  to  the 
top  of  the  cylinder  ;  and  in  the  same  way  the  alternate  motion  of  the  piston 
upward  and  downward  in  the  cylinder  is  continued. 

The  results  of  experimental  inquiry,  as  we  have  seen,  justify  us  in  assuming 
as  a  universal  law,  that  by  the  application  of  a  sufficient  quantity  of  heat  all 
solids  may  be  converted  into  liquids  ;  and,  by  the  abstraction  of  a  correspond- 
ing quantity  of  heat,  all  liquids  may  be  converted  into  solids.  We  have  like- 
wise seen,  that,  by  the  supply  of  heat  in  sufficient  quantities,  all  liquids  may 
be  converted  into  the  vaporous  or  gaseous  form  ;  and  analogy  would  lead  us  to 
infer,  that,  by  the  due  abstraction  of  heat,  the  bodies  that  exist  in  the  gaseous 
form  might  be  reduced  to  liquids.  The  practical  results  here,  however,  fall 
far  short  of  the  anticipations  to  which  analogy  leads  us.  There  is  a  numerous 
class  of  bodies  existing  in  the  gaseous  form,  among  which  atmospheric  air  may 
be  mentioned  as  the  most  obvious,  which  no  means  hitherto  known  have  con- 
verted into  liquids.  Arguments,  however,  similar  to  those  which  led  us  to  in- 
fer that  charcoal  and  alcohol  are  not  real  exceptions  to  the  liquefaction  of 
solids,  and  the  solidification  of  liquids,  but  that  they  transcend  the  power  of 


EBULLITION. 


315 


art,  without  falling  beyond  the  limits  of  the  general  law,  lead  to  similar 
conclusions  respecting  the  more  numerous  class  of  bodies  called  permanent 
gases. 

Bodies  existing  in  the  aeriform  state  are  divided  into  two  classes,  called 
vapors  and  gases.  Vapors  are  those  aeriform  substances  which  are  known  to 
have  been  raised  from  liquids  by  the  application  of  heat,  and  which  may  al- 
ways be  restored  to  the  liquid  form  by  the  due  abstraction  of  heat.  On  the 
other  hand,  gases  are  those  aeriform  bodies  which  have  never  been  known  to 
exist  in  any  other  than  the  aeriform  state,  and  which,  under  all  ordinary  de- 
grees of  cold,  preserve  their  elastic  form.  This  class  includes  common  air, 
and  a  great  number  of  substances  known  in  chemistry  under  a  variety  of  names, 
but  all  comprised  under  the  general  denomination  of  gases.  The  exact  corre- 
spondence of  the  mechanical  properties  of  these  bodies  with  those  of  vapors 
raised  from  liquids  by  heat,  naturally  leads  to  the  suspicion  that  they  are,  in 
fact,  vapors  of  bodies  which  vaporize  at  extremely  low  temperatures — at  tem- 
peratures lower  than  any  which  we  generally  attain  even  by  the  processes  of 
art.  Such  a  supposition  is  perfectly  consistent  with  all  the  effects  which  we 
observe  ;  for  such  bodies  would  then  maintain  all  the  gaseous  qualities  which 
they  are  observed  to  possess  at  present,  though  they  should  be  true  vapors  ca- 
pable of  being  condensed,  and  even  solidified,  if  we  possessed  practical  means 
of  depriving  them  of  a  sufficient  quantity  of  the  heat  which  they  contain. 

These  observations  derive  considerable  probability  and  force  from  the  results 
which  the  improved  powers  of  science  have  more  recently  furnished.  In  pro- 
portion as  more  powerful  means  of  extorting  heat  from  gases  have  been  in- 
vented, a  greater  number  of  them  have  been  forced  within  the  limits  of  the 
law  of  condensation.  The  substance  called  ammonia  was  known  only  as  a 
gas  until  a  temperature  of  — 46°  was  attained.  Exposed  to  that  temperature,  it 
became  a  liquid.  Such  a  body,  in  high  northern  latitudes,  would,  at  different 
seasons,  exist  in  the  different  forms  of  liquid  and  gas  ;  in  winter  it  would  be 
liquid,  and  at  other  seasons  gas. 

Since  it  is  certain  that  gases  may  lose  a  considerable  quantity  of  heat,  with- 
out undergoing  any  degree  of  condensation,  we  must  look  upon  them  as  vapors  ; 
which,  besides  the  sum  of  the  latent  and  sensible  heat  necessary  to  sustain 
them  in  the  elastic  form,  have,  subsequently  to  attaining  that  form,  received  a 
large  accession  of  heat ;  and  yet,  from  their  nature,  with  all  this  supply  of 
heat,  their  temperature  does  not  exceed  the  ordinary  temperature  of  the  globe.. 
It  would  be  necessary  to  abstract  from  them  all  the  heat  which  they  have  re- 
ceived subsequently  to  taking  the  vaporous  form  before  condensation  could 
begin.  As  our  power  of  producing  artificial  cold  is,  however,  very  limited, 
never  having  yet  exceeded  — 100°  (if,  indeed,  that  limit  has  been  attained),  it 
cannot  be  surprising  that  all  the  redundant  heat  contained  by  gases,  over  and 
above  the  sum  of  latent  and  sensible  heat  necessary  to  maintain  them  in  the 
elastic  form,  should  not  have  been  extracted  by  this  means. 

Some  facility  in  the  attainment  of  this  object  may  be  gained  by  a  knowledge 
of  the  fact  that  the  mechanical  compression  of  a  gas  raises  its  temperature. 
If,  therefore,  a  permanent  gas  be  submitted  to  severe  mechanical  compression, 
its  temperature  will  be  raised,  and  the  heat  which  it  contains  may  be  more 
easily  withdrawn  from  it,  and  imparted  to  freezing  mixtures,  or  extorted  by  any 
of  the  usual  means  of  exposing  it  to  extremely  low  temperatures.  By  contin- 
ually carrying  on  the  process  of  compression,  additional  quantities  of  heat  may 
be  developed  and  withdrawn,  so  that  at  length  we  may  succeed  in  reducing 
the  quantity  of  heat  contained  in  the  gas  to  that  sum  of  latent  and  sensible 
heat  which  seeuis  the  limit  of  the  quantity  necessary  to  maintain  the  elastic 
form.     Any  further  reduction  would  be  necessarily  followed  by  condensation. 


316 


EBULLITION. 


Means  similar  to  these  have  accordingly  been  applied,  and  succeeded,  in 
the  hands  of  Faraday.  By  submitting  gases  in  small  quantities,  in  strong 
glass  tubes,  to  a  severe  pressure,  produced  by  their  own  elasticity,  and  the 
force  with  which  they  were  generated  by  chemical  action,  heat  was  extracted 
in  considerable  quantities,  and  was  carried  off  by  evaporation  from  the  external 
surface  of  the  glass.  In  this  way,  nine  gases  were  condensed  into  the  liquid 
form.  , 

Faraday  attempted,  without  success,  the  condensation  of  various  other  gases 
by  the  same  means.  Oxygen,  azote,  and  hydrogen,  have,  it  is  said,  been  sub- 
mitted to  a  pressure  of  eight  hundred  atmospheres  without  passing  to  the  liquid 
state.* 

It  appears,  therefore,  that,  in  proportion  as  the  povvers  of  science  are  ad- 
vanced, the  exceptions  to  the  general  law  of  condensation  become  more  and 
more  circumscribed  ;  and  it  is  not,  perhaps,  overstepping  the  limits  of  justifia- 
ble theory  to  assume,  as  a  general  law,  that  all  bodies  whatever,  existing  in 
the  gaseous  form,  may,  by  a  sufficient  abstraction  of  heat  from  them,  be  reduced 
to  the  liquid  state. 

The  absorption  of  heat,  in  the  process  by  which  liquids  are  converted  into 
steam,  will  explain  why  a  vessel  containing  a  liquid,  though  constantly  exposed 
to  the  action  of  fire,  can  never,  while  it  contains  any  liquid,  receive  such  a 
degree  of  heat  as  might  destroy  it.  A  tin-kettle  containing  water  may  be  ex- 
posed to  the  action  of  the  most  fierce  furnace,  and  yet  the  tin,  which  is  a  very 
fusible  metal,  will  remain  uninjured  ;  but  if  the  kettle  without  containing  water 
were  placed  on  a  fire,  it  would  be  immediately  destroyed.  The  heat  which 
the  fire  imparts  to  the  kettle  is  immediately  absorbed  by  the  bubbles  of  water, 
«-hkh.  are  converted  into  steam  at  the  bottom,  and  rendered  latent  in  them. 
Ihfcse  bubbles  ascend  through  the  water,  and  escape  at  the  surface,  continu- 
ally carrying  with  them  the  heat  conveyed  from  the  fire  through  the  bottom  of 
the  kettle.  So  long  as  water  is  contained  in  the  kettle,  this  absorption  of  heat 
by  the  steam  continues  ;  and  it  is  impossible  that  the  temperature  of  the  kettle 
can  exceed  the  temperature  of  boiling  water.  But  if  any  part  of  the  kettle  not 
filled,  with  water  be  exposed  to  the  fire,  there  being  then  no  means  of  dismis- 
sing the  heat  which  it  receives  from  the  fire,  the  metal  will  presently  melt,  and 
the  vessel  be  destroyed. 

The  latent  heat  of  steam  may  be  used  with  great  convenience  for  many  do- 
.raestic  purposes.  In  cookery,  if  steam  raised  from  boiling  water  be  allowed  to 
pass  through  meat  or  vegetables,  it  will  be  condensed  upon  their  surfaces,  im- 
parting to  them  the  heat  latent  in  it  before  its  condensation,  and  they  will  thus 
be  as  effectually  boiled  as  if  they  were  immersed  in  boiling  water. 

In  dwelling-houses  where  pipes  convey  cold  water  to  diflierent  parts  of  the 
building,  steam-pipes  carried  from  the  lower  part  will  enable  hot  water  to  be 
procured  in  every  part  of  the  house  with  great  speed  and  facility.  The  cock 
of  a  steam-pipe  being  immersed  in  a  vessel  containing  cold  water,  the  steam 
which  escapes  from  it  will  be  condensed  by  the  water,  and  will  very  speedily, 
by  imparting  to  it  its  latent  heat,  cause  it  to  boil.  Warm  baths  may  thus  be 
prepared  in  a  few  minutes,  the  water  of  which  would  require  a  long  period  to 
boil. 

From  all  that  has  been  explained  in  the  present  discourse,  it  will  be  apparent 
that  the  solid,  liquid,  and  gaseous  states  are  not  necessarily  connected  with 
the  essential  properties  of  the  bodies  which  assume  these  states  respectively. 

*  An  opinion,  which  I  consider  to  be  erroneous,  has  hitherto  prevailed,  that  gases  and  vapors 
may  be  condensed  by  -mere  mechanical  compression.  I  conceive  that  mechanical  compression  con- 
tributes in  no  other  way  to  the  condensation  of  a  gas  or  a  vapor,  than  so  far  as  it  is  tlie  means  of 
raising  the  temperature  of  the  gas  compressed,  and  therefore  facilitating  the  process  by  which  it 
may  be  deprived  of  heat. 


EBULLITION. 


317 


Water,  whether  it  exist  in  the  state  of  liquid,  in  the  state  of  steam,  or  in  the 
state  of  ice,  is  evidently  the  same  substance,  composed  of  the  same  elements, 
and  possessing  properties  in  all  respects  the  same,  except  in  those  mechanical 
effects  which  are  immediately  connected  with  the  three  states  just  mentioned. 
In  fact,  the  state  in  which  water  may  be  found  is  a  mere  accident  consequent 
on  the  surrounding  temperature  ;  nor  can  one  rather  than  another  state  with 
propriety  be  called  the  natural  state  of  the  body. 

If  the  expression  natural  state  have  any  meaning,  it  must  be  that  state  in 
which  the  substance  is  most  commonly  found  ;  and  in  that  sense  the  natural 
state  of  water  in  different  parts  of  the  globe  is  different. 

The  variations  of  temperature  incident  to  any  part  of  our  globe  are  included 
within  no  very  extended  limits  ;  and  these  limits  determine  the  bodies  which 
are  found  to  exist  most  commonly  in  the  several  states  of  solid,  liquid,  and  gas. 
A  body  whose  boiling  point  is  below  the  lowest  temperature  of  the  climate, 
must  always  exist  in  the  state  of  vapor  or  gas,  and  one  whose  melting  point  is 
above  the  highest  temperature  incident  to  the  climate  must  always  exist  in  the 
solid  form.  Bodies  whose  melting  point  is  below  the  lowest  temperature  of 
the  climate,  while  their  boiling  point  is  above  the  highest  temperature  of  the 
climate,  will  permanently  exist  in  the  liquid  form.  The  permanent  gases  af- 
ford examples  of  the  first-mentioned  class.  Most  solid  bodies  are  examples 
of  the  second ;  and  such  fluids  as  mercury  are  examples  of  the  third.  A  liquid 
whose  melting  point  is  a  little  above  the  lowest  limit  of  temperature  will  gener- 
ally exist  in  the  liquid  state,  but  occasionally  in  the  solid.  Water  is  an  ex- 
ample of  this.  A  liquid,  on  the  other  hand,  whose  boiling  point  is  a  little 
below  the  highest  limit  of  temperature,  will  generally  exist  in  the  liquid  form, 
but  occasionally  in  the  gaseous.  Ether,  in  hot  climates,  is  an  example  of  this. 
Its  boiling  point  is  98°  ;  and  it  could  not  exist,  at  certain  seasons  of  the  year, 
in  the  liquid  form,  in  India  and  other  hot  countries. 

Some  bodies  are  at  present  retained  in  the  liquid  form  only  by  the  atmo- 
spheric pressure.  Ether  and  rectified  spirits  of  wine  are  examples  of  this. 
If  these  liquids  be  placed  under  a  receiver  of  an  air-pump,  and  the  pressure  of 
the  air  be  partially  removed,  they  will  be  observed  to  boil  at  the  ordinary  tem- 
perature of  the  air  ;  whence  it  appears,  that,  if  the  pressure  of  the  atmosphere 
were  considerably  less  than  it  is,  these  substances  would  have  existed  only  as 
permanent  gases. 

Great  convulsions  of  nature,  such  as  earthquakes,  volcanic  effects,  and  the 
like,  by  which  extraordinary  quantities  of  heat  are  evolved,  form  exceptions  to 
this  uniform  state  ;  and  the  effects  of  such  exceptions  are  discoverable  upon 
and  beneath  the  surface  of  the  earth :  but,  under  ordinary  circumstances,  the 
states  of  gases  or  airs,  of  liquids,  and  of  solids,  are  determined  by  the  condi- 
tion just  mentioned,  namely,  by  the  relation  which  their  boiling  and  freezing 
points  bear  to  the  extreme  limits  of  the  temperature  of  our  climate. 

These  considerations  will  lead  us  to  perceive  what  would  be  the  effect,  if 
the  earth's  distance  from  the  sun  were  to  undergo  considerable  change,  either 
by  increase  or  diminution,  other  circumstances  being  supposed  to  remain  the 
same.  If  its  proximity  to  the  sun  were  increased,  the  increased  influence  of 
solar  heat  would  render  it  impossible  for  many  substances  now  commonly  li- 
quid on  the  surface  of  the  earth  to  exist  in  any  other  state  than  that  of  air  ; 
and,  at  the  same  time,  many  solid  bodies  would  be  incapable  of  maintaining 
the  solid  form,  and  would  become  permanently  liquid.  It  would  be  possible, 
under  such  circumstances,  that  the  water  which  now  constitutes  the  ocean 
would  be  changed  into  an  atmosphere,  and  that  many  of  the  metals  which  now 
exist  in  the  solid  form,  distributed  through  the  earth,  would  become  liquid,  and 
fill  the  beds  of  the  sea.     If,  on  the  other  hand,  the  distance  from  the  sun  were 


318 


EBULLITION. 


considerably  increased,  the  solar  heat  would  undergo  a  corresponding  diminu- 
tion, and  many  of  the  substances  which  now  assume  the  liquid  form  would 
then  become  solid.  The  sea  which  surrounds  the  globe  would  take  the  form 
of  a  mass  of  solid  crystal.  Substances  now  in  the  gaseous  state  might  be  re- 
duced to  the  form  of  a  liquid  ;  nay,  that  the  atmosphere  should  be  converted 
into  a  sea  by  a  sufficient  diminution  of  temperature,  is  an  effect  not  only  within 
the  bounds  of  possibility,  but  probable  upon  the  clearest  and  best-founded 
analogy. 

In  reviewing  what  has  been  stated  in  the  present  discourse,  it  will  be  per- 
ceived that  the  following  general  facts  have  been  established,  which  form  the 
basis  of  all  investigations  concerning  the  phenomena  of  the  conversion  of 
liquids  into  vapor  by  ebullition  : — 

1.  A  liquid,  when  raised  to  a  certain  temperature,  boils,  and  is  converted 
into  vapor.  The  boiling  point  of  a  liquid  varies  with  the  pressure  to  which  it 
is  submitted :  the  greater  this  pressure,  the  greater  will  be  the  temperature  at 
which  the  liquid  boils. 

2.  During  the  process  of  ebullition  no  increase  of  temperature  takes  place, 
though  a  considerable  portion  of  heat  is  imparted  to  the  boiling  liquid. 

3.  Different  liquids  undergo  the  process  of  ebullition  under  the  same  pres- 
sure at  different  temperatures  ;  and  the  temperature  at  which  a  liquid  boils  under 
the  medium  pressure  of  the  atmosphere,  or  thirty  inches  of  mercury,  is  called 
its  boiling  point. 

4.  Different  liquids  absorb  different  quantities  of  heat  in  the  process  of 
ebullition. 

5.  The  elastic  force  of  the  vapor  into  which  a  liquid  is  converted  is  equal 
to  the  pressure  under  which  the  liquid  boils. 

6.  The  states  of  liquid  or  vapor  are  not  essentially  connected  with  the  na- 
ture of  bodies,  but  are  merely  accidental  on  the  temperature  to  which  bodies 
are  exposed,  nor  does  a  body  change  its  nature  or  essential  properties  in  passing 
from  the  one  state  to  the  other. 


COMBUSTION. 


Flame  produced  by  chemical  Combination. — Supporters  of  Combustiou  and  Combustibles. — Oxygen 
chief  Supporter. — Heat  of  Combustion. — Flame. — Its  illuminating  Powers. — Combustion  without 
Flame. — Property  of  spongy  Platinum. — Table  of  Heat  evolved  in  Combustion. —  Theory  of  La- 
voisier.— Of  Hook  and  others. — Electric  Theory. 


COMBUSTION. 


321 


COMBUSTION, 


Many  examples  have  been  presented,  in  which  the  chemical  combination 
of  two  bodies  was  accompanied  by  a  change  of  temperature.  When  sulphuric 
acid  and  pure  water  are  mixed  together  at  the  same  temperature  of  60°,  the 
mixture  will  suddenly  rise  to  the  temperature  of  boiling  water.  In  like  nmn- 
ner,  when  snow  at  the  temperature  of  32°  is  mixed  with  common  salt  at  the 
same  temperature,  the  compound  resulting  will  fall  many  degrees  below  <^"^ 
common  temperature  of  the  constituents.  It  may  be  taken,  therefore,  as  a  g. 
eral  principle,  that  chemical  combination  is  one  of  the  numerous  causes  L 
which  heat  may  be  developed  or  absorbed.  Every  part  of  chemical  science 
abounds  in  facts  illustrative  of  this  principle. 

We  have  seen  that  an  extreme  increase  of  temperature  is  attended  by  the 
presence  of  light.  Now,  if  these  two  general  laws  be  placed  in  juxtaposition, 
it  may  be  expected  that,  if  chemical  combinations  can  be  discovered  in  which 
extreme  quantities  of  heat  may  be  developed,  the  product  may  attain  that  tem- 
perature at  which  it  will  be  luminous. 

Such  are  the  principles  which  form  the  foundation  of  the  ordinary  process 
of  comhiistion  or  burning.  When  fire  is  produced,  such  a  combination  always 
takes  place  between  the  particles  of  two  bodies  as  produces  a  development  of 
heat  so  extreme  as  to  produce  light.  If  the  body  emitting  light  in  this  case 
have  the  solid  form,  the  effect  is  called  fire ;  but  if  it  be  vapor,  it  is  called 
flame. 

It  so  happens  that,  among  the  infinite  variety  of  natural  substances  by  the 
combination  of  which  this  remarkable  phenomenon  is  produced,  one  of  the  two 
combining  bodies  is,  almost  in  every  case,  the  substance  called  in  chemistry 
oxygen  gas  ;  and  that  in  the  iew  cases  where  oxygen  is  not  present  there  is  a 
very  limited  number  of  other  substances,  one  or  the  other  of  which  must  be  one  ' 
of  the  combining  substances.  i 

Among  these  other  substances,  the  principal  are  three  bodies,  called  in  chem-  ] 
istry  chlorine,  bromine,  and  iodine.  \ 

VOL.11.— 31 


322 


COMBUSTION. 


Some  one  of  these  four  bodies — oxygen,  chlorine,  bromine,  and  iodine — ^be- 
ing, almost  in  every  case,  one  of  the  two  bodies  by  the  combination  of  which 
combustion  is  produced,  and  the  other  bodies  with  which  they  severally  com- 
bine being  far  more  numerous,  the  four  just  mentioned  are  distinguished  rela- 
tively to  the  phenomena  of  combustion  by  the  name  supporters  of  comhustion  ; 
while  the  other  body  forming  the  combination  with  them,  whatever  it  may  be, 
is  called  a  combustible.  These  terms,  however,  must  be  carefully  understood 
as  not  expressing  any  distinct  or  different  mode  of  action  which  the  two  com- 
bining bodies  exert  in  the  process  of  their  combination.  Supporters  of  combus- 
tion and  combustibles,  as  far  as  has  been  discovered,  have  no  other  difference  than 
this,  that  the  former  are  very  limited  in  number,  and  the  latter  very  numerous. 

Exclusive  of  the  four  supporters  of  combustion,  every  simple  substance 
known  in  chemistry  are  combustibles,  except  azote  or  nitrogen  gas.  The  mean- 
ing of  this  is,  that  all  simple  substances  are  capable  of  entering  into  combina- 
tion with  one  or  other  of  the  four  bodies  called  oxygen,  chlorine,  bromine,  or 
iodine,  in  such  a  manner  as  to  be  attended  with  a  sudden  evolution  of  light  and 
heat. 

After  the  discovery  of  the  true  nature  of  the  process  of  combustion,  it  was 
long  supposed  that  the  only  supporter  of  combustion  was  oxygen,  and  the  phe- 
nomenon of  combustion  was  consequently  defined  to  be  the  rapid  combination 
of  oxygen  with  some  other  substance.  This  is,  indeed,  the  nature  of  the  phe- 
nomenon in  all  ordinary  cases  of  combustion  ;  and  it  is  only  in  few  instances, 
developed  by  the  researches  of  modern  chemists,  that  chlorine  and  the  other 
supporters  play  a  part. 

The  tendency  which  a  body  heated  considerably  above  the  temperature  of 
the  surrounding  medium  has  to  dismiss  its  heat,  whether  by  contact  or  radia- 
tion, renders  it  necessary  that  the  combination  which  produces  combustion 
should  be  so  rapid  as  to  be  almost  instantaneous  ;  for,  if  the  heat  developed 
were  produced  progressively,  it  would  be  progressively  dissipated,  and  could 
never  accumulate  so  as  to  produce  that  increased  temperature  which  is  neces- 
sary for  the  evolution  of  light. 

In  all  ordinary  cases  of  combustion,  one  of  the  combining  bodies  is  the  ox)^- 
gen,  which  forms  a  component  part  of  atmospheric  air  ;  and  one  of  the  circum- 
stances which  most  favor  combustion  is,  the  fact  that  the  constituent  elements 
of  atmospheric  air  are  mixed  togther,  either  mechanically,  or,  if  they  be  chem- 
ically combined,  their  affinity  is  of  the  weakest  imaginable  kind.  Thus  the 
oxygen  exists  in  the  atmosphere  almost  in  a  free  state,  and  ready  to  combine 
with  any  object  which  presents  to  it  the  slightest  affinity.  The  application  of 
heat  to  any  body,  by  weakening  the  energy  of  the  cohesive  principle,  leaves  its 
particles  more  free  to  obey  other  affinities ;  and  consequently  it  is  found  that 
bodies  which  cannot  combine  at  one  temperature  will  frequently  be  capable  of 
combining  when  the  temperature  of  one  or  both  is  raised.  A  body,  therefore, 
may  exist  at  a  certain  temperature,  when  surrounded  by  the  oxygen  of  the  at- 
mospheric air  ;  but  if  the  temperature  of  that  body  be  raised,  the  affinity  of  its 
molecules  for  those  of  oxygen  will  at  length  be  enabled  to  take  effect  by  the 
diminution  of  the  force  by  which  its  particles  are  held  together.  In  conformi- 
ty with  this  principle,  we  find  that  when  a  combustible  is  raised  to  a  certain 
temperature,  its  particles  rapidly  combine  with  those  of  the  oxygen  contained 
in  the  surrounding  air.  In  their  combination  heat  and  light  are  evolved,  and 
fire  is  produced.  When  phosphorus  is  raised  to  the  temperature  of  148°,  it 
burns  with  great  splendor.  The  particles  of  the  phosphorus,  in  this  case,  com- 
bine with  those  of  the  oxygen  in  the  atmosphere,  and  so  much  heat  is  devel- 
oped by  their  combination  that  the  light  is  evolved.  The  temperature  neces- 
sary to  each  different  substance,  to  combine  with  the  oxygen  and  produce  com- 


bustion,  is  very  different.  Hydrogen  gas  requires  a  heat  equal  to  that  of  in- 
candescence to  cause  it  to  begin  to  burn.  Wood,  coal,  and  other  combustibles, 
burn  when  raised  to  various  temperatures. 

According  to  the  experiments  of  Sir  Humphry  Davy,  the  temperature  neces- 
sary to  enable  the  followincr  substances  to  combine  with  oxygen  vary  in  the 
order  in  which  they  stand,  the  first  being  that  which  burns  at  the  lowest  tem- 
perature, and  the  succeeding  ones  at  temperatures  gradually  increasing  : — 


Phosphorus, 

Phosphviretted  hydrogen 

Hydrogen  and  chlorine, 

Sulphur, 

Hydrogen  and  oxygen, 

Olefiant  eas. 


;as, 


Sulphuretted  hydrogen, 

Alcohol, 

Wax, 

Carbonic  oxide, 

Carburetted  hydrogen. 


The  experimental  proofs  by  which  combustion  is  shown  to  arise  from  the 
combination  of  oxygen  with  other  principles  consist  of  the  whole  range  of  one 
department  of  chemical  science.  We  may,  however,  offer  an  experiment  as  an 
example  of  this  species  of  demonstration.  , 

Let  a  short  earthenware  tube  be  filled  with  a  coil  of  iron  wire,  the  weight 
of  which  has  been  previously  ascertained.  Let  one  extremity  of  this  tube  be 
connected  with  a  bladder  filled  with  oxygen  gas,  the  weight  of  which  is  known  ; 
and  let  the  other  extremity  be  connected  with  a  flaccid  bladder,  the  weight  of 
which,  including  the  air  which  it  contains,  is  also  exactly  known.  Let  the 
porcelain  tube  and  its  contents  be  raised  to  incandescence  by  the  application 
of  heat,  and  let  the  oxygen  contained  in  the  bladder  be  then  forced  through  the 
tube  in  contact  with  the  wire.  The  wire  in  this  case  will  burn,  and  be  rapidly 
oxidised,  and  the  product  will  be  the  oxide  of  iron.  When  this  product  is 
weighed,  it  will  be  found  to  be  heavier  than  the  iron  ;  and  when  the  two  blad- 
ders and  their  contents  are  weighed,  they  will  be  found  to  be  lighter  than  be- 
fore, by  exactly  the  weight  which  the  iron  has  gained  ;  the  oxygen,  therefore, 
which  has  been  lost  by  air  contained  in  the  bladders,  has  been  combined  with 
the  iron  during  the  process  of  combustion. 

Flame  is  gas  heated  to  whiteness  by  the  heat  produced  by  the  combustion 
of  volatile  matter.  When  a  candle  burns,  the  tallow  or  wax  of  which  it  is  com- 
posed is  first  liquefied,  and  then  drawn  up  through  the  interstices  of  the  wick 
by  capillary  attraction.  As  it  comes  in  contact  with  the  source  of  heat,  it  is 
boiled  and  converted  into  vapor ;  this  vapor  ascends  in  a  column  by  reason  of 
its  lightness,  and  is  now  raised  to  the  temperature  which  enables  it  to  form  a 
combination  with  the  oxygen  of  the  surrounding  air.  This  combination  in- 
stantly and  copiously  develops  heat,  which,  being  communicated  to  the  sur- 
rounding current  of  gas,  renders  it  luminous,  and  produces  the  white,  bright 
light  of  the  flame.  It  will  be  apparent,  from  this,  that  the  light  from  the  flame 
can  only  exist  on  its  exterior  surface,  which  is  in  contact  with  air.  The  flame 
of  a  candle  or  lamp  is,  therefore,  so  far  as  regards  heat,  hollow ;  or  rather  it  is 
a  column  of  gas,  the  exterior  surface  of  which  is  luminous,  while  the  interior 
is  non-luminous.  As  the  gas  in  the  interior  of  the  flame  ascends,  it  gets  into 
contact  with  a  fresh  portion  of  the  atmosphere,  from  which  it  receives  a  supply 
of  oxygen,  by  combination  with  which  heat  is  evolved,  which  produces  light. 
As  the  gas  ascends  from  the  centre  of  the  flame,  it  comes  successively  into 
contact  with  the  air,  and  in  this  manner  becomes  luminous,  until  at  length  the 
column  is  reduced  to  a  point.  Thus  the  flame  of  a  candle  or  lamp  gradually 
tapers  to  a  point,  until  all  the  gas  produced  from  the  boiling  matter  in  the 
wick  receives  its  due  complement  of  oxygen  from  the  air,  and  passes  off.  It 
speedily  loses  the  temperature  necessary  to  render  it  luminous,  and  the  flame 
terminates. 


324 


COMBUSTION. 


The  light  produced  by  lamps  or  candles  formed  of  different  substances  has 
different  illuminating  powers,  according  to  the  quantities  of  light  evolved  by  the 
combination  of  the  gas  or  vapor  with  oxygen. 

The  vapor  of  some  substances  is  capable  of  combining  with  oxygen  at  a  tem- 
perature below  that  which  is  necessary  for  the  production  of  flame.  Sir  Hum- 
phry Davy  coiled  a  piece  of  platinum  wire  round  the  wick  of  a  spirit-lamp, 
and,  having  lighted  the  lamp,  and  allowed  it  to  burn  till  the  wire  became  red 
hot,  he  then  extinguished  it ;  the  wire,  however,  with  the  heat  which  it  had 
acquired,  communicated  a  sufficient  heat  to  the  vapor  raised  from  the  alcohol 
to  enable  it  to  combine  with  the  oxygen  of  the  surrounding  air;  and  a  slow 
combustion,  without  flame,  was  thus  produced.  This  process  of  combustion 
might  be  continued  for  any  length  of  time,  or  as  long  as  the  alcohol  in  the  lamp 
could  supply  vapor. 

The  product  obtained  by  the  combination  of  oxygen  and  the  vapor  of  alco- 
hol in  this  case  was  of  a  nature  altogether  different  from  that  obtained  by  the 
ordinary  combustion  of  the  spirit-lamp.  Acetic  acid  forms  a  part,  but  not  the 
whole  of  the  product. 

There  are  other  vapors  which,  like  that  of  alcohol,  are  susceptible  of  com- 
bustion without  flame.  Among  these  are  the  vapors  of  ether,  camphor,  and 
some  of  the  volatile  oils. 

If  platinum  wire,  heated  to  redness,  be  introduced  into  a  receiver  containing 
a  mixture  of  coal  gas,  or  the  vapor  of  ether,  and  atmospheric  air,  it  will  con- 
tinue red  hot  until  the  whole  of  the  gas  is  consumed.  In  this  case  the  gas 
combines  with  the  oxygen  of  the  atmospheric  air  with  which  it  is  mixed,  and 
combustion  takes  place. 

Dr.  Thomson  accounts  for  this  process  by  the  fact  of  the  small  specific  heat 
and  bad-conducting  power  of  platinum  :  a  small  quantity  of  heat  is  sufficient  to 
rnake  it  red  hot,  and,. being  a  bad  conductor,  it  loses  little  heat  during  the  pro- 
cess. Platinum,  at  a  red  heat,  has  a  sufficiently  high  temperature  to  produce 
a  rapid  combination  of  the  vapor  of  alcohol  with  oxygen,  but  it  is  not  sufficient 
for  the  production  of  flame.* 

If  a  jet  of  hydrogen  gas  be  projected  on  a  small  mass  of  spongy  platinum, 
the  platinum  will  become  red  hot,  and  will  continue  so  as  long  as  the  jet  plays 
on  it.  This  forms  an  easy  means  of  producing  an  instantaneous  light,  and  an 
apparatus  is  constructed  in  a  convenient  form  for  this  purpose.  By  turning  a 
stopcock,  the  jet  of  gas  is  thrown  on  a  small  cup  containing  platinum,  which, 
immediately  becoming  red  hot,  is  capable  of  lighting  a  match.  The  same 
effect  may  be  produced  by  a  jet  of  the  gas  projected  on  other  substances,  such 
as  palladium,  rhodium,  and  iridium.  Some  others,  also,  such  as  osmium,  would 
be  attended  with  a  like  eflect,  if  their  temperatures  were  previously  raised. 
Platinum  foil  would  not,  under  these  circumstances,  redden  ;  but,  if  it  be 
crumpled,  like  paper,  it  will  undergo  the  same  effect  as  the  spongy  pla- 
tinum. - 

These  eff"ects  have  been  accounted  foi;;^  by  the  fact  that  spongy  platinum, 
and  other  substances  in  a  similar  state,  have  such  an  affinity  for  oxygen  gas, 
that  their  capillary  attraction  produces  the  absorption  of  that  gas  from  the  at- 
mospheric air  into  their  pores,  in  which  it  is  sometimes  collected  even  in  a 
condensed  state.  It  is  probable  that  spongy  platinum  contains  within  its  pores 
a  considerable  quantity  of  condensed  oxygen  gas.  Charcoal  is  known  to  ab- 
sorb by  its  capillary  attraction  nine  times  and  a  quarter  its  own  bulk  of  oxygen  ; 
and,  when  placed  in  contact  with  hydrogen  gas,  the  oxygen  absorbed  combines 
with  the  hydrogen,  and  forms  water.     The  jet  of  hydrogen  gas  projected  on  a 


*  Thomson  on  Heat,  p.  311. 


COMBUSTION. 


325 


spongy  platinum  probably  combines  with  the  oxygen  held  in  its  pores,  and  the 
heat  developed  by  the  combination  renders  the  platinum  red  hot.* 

The  determination  of  the  quantity  of  heat  produced  in  the  combustion  of 
dilFerent  substances  is  a  matter  not  only  of  great  scientific  interest,  but  of  con- 
siderable importance  in  the  useful  arts  and  manufactures.  The  mutual  relation 
between  the  quantity  of  the  combustible,  and  of  the  oxygen  combined  with  it, 
and  the  heat  developed,  if  accurately  ascertained  for  various  combustibles,  could 
not  fail  to  throw  light,  not  only  on  the  theory  of  combustion,  but,  probably,  on 
the  nature  of  heat  in  general.  In  the  arts  and  manufactures,  as  well  as  in  do- 
mestic economy,  the  due  selection  of  combustible  matter  depends,  in  a  great 
degree,  on  the  quantity  of  heat  or  light  developed  by  a  given  weight  of  it  in  the 
process  of  combustion. 

Nevertheless,  there  is  no  subject  in  experimental  physics  in  which  more  re- 
mains to  be  discovered,  and  in  which  the  process  of  discovery  is  more  difficult, 
than  in  the  determination  of  the  quantity  of  heat  developed  in  the  combustion 
of  various  substances.  Experiments  have  been  made  on  some  combustibles  by 
Lavoisier  and  Laplace  with  their  calorimeter.  A  few  others  have  been  made 
by  Dalton.  Crawfurd  and  Count  Rumford  have  also  made  some  experiments 
on  this  subject.  The  method  of  Lavoisier  and  Laplace  consisted  of  burning 
the  combustible  within  the  calorimeter,  and  measuring  the  quantity  of  ice  melt- 
ed by  the  heat  which  it  developed.  Dalton  placed  a  given  weight  of  water, 
at  a  known  temperature,  in  a  tinned  vessel.  Having  previously  ascertained 
the  specific  heat  of  this  vessel,  that  of  water  being  known,  he  applied  the  burn- 
ing matter  to  the  bottom  of  it,  so  as  to  cause  it  to  impart  its  heat  to  the  water. 
The  quantity  of  heat  developed  was  measured  by  the  increased  temperature  of 
the  water,  and  the  vessel  which  contained  it.  This  process  would  evidently 
give  results  considerably  below  the  truth,  because  it  is  impossible  that  all  the 
heat  developed  in  the  combustion  could  be  imparted  to  the  vessel ;  some  would 
be  necessarily  communicated  to  the  surrounding  air  without  reaching  the  ves- 
sel, and  more  would  be  dispersed  by  radiation.  Dr.  Crawfurd  contrived  to 
surround  the  burning  matter, with  water,  by  the  increased  temperature  of  which 
he  measured  the  heat  developed. 

Sir  Humphry  Davy  made  experiments  to  determine  the  heat  developed  by 
some  gases  in  the  process  of  combustion,  and  adopted  a  method  of  experiment- 
ing differing  little  from  that  of  Dalton.  He  caused  the  flame  to  act  on  the  bot- 
tom of  a  copper  vessel,  containing  a  given  weight  of  oil  raised  to  a  given 
temperature,  and  estimated  the  heat  produced  in  the  combustion  by  the  increas- 
ed temperature  received  to  the  oil.  The  following  are  the  results  obtained  by 
these  experiments  : — 


Substances  burned  in  one  pound. 


Oxygen 
consumed 
in  pounds 


Ice  melted  in  pounds. 


Lavoisier.  Crawfurd.     Dalton.      Rumford 


Hydrogen 

Carburetted  hydrogen 
Olefiant  gas  .  .  .  . 
Carbonic  oxide     .     .     . 

Olive  oil 

Rape  oil 

Wax 

Tallow- 

Oil  of  turpentine     .     . 

Alcohol 

Suipliuric  ether   .    .     . 

Naphtha 

Phosphorus  .  .  .  . 
(Charcoal  .     .     .     . 

Sulphur 

Camphor 

Caoutciiouc      .     .     .     . 


295-6 


1330 
96  0 


1000 
96-5 


480 


320 

85 


9407 
124- 10 
126-24 
111-58 

67-47 

107-03 
97-83 


Thomson  on  Heat,  p.  315. 


326 


COMBUSTION, 


The  great  discordance  which  is  apparent  between  the  results  of  these  ex- 
periments shows  how  much  still  remains  to  be  done  in  this  department  of  the 
physics  of  heat.  It  is  probable,  however,  that  the  results  of  the  experiments 
of  Lavoisier  and  Laplace  are  more  entitled  to  confidence  than  those  of  the 
other  experimenters.  Dr.  Thomson  thinks  that  it  is  probable  that  one  pound 
of  hydrogen  gas  gives  out  in  combustion  as  much  heat  as  would  melt  400  lbs. 
of  ice,  or  56,000°  of  heat. 

The  copious  development  of  heat,  in  the  process  of  combustion,  and  the  con- 
sequent luminous  effect,  were  accounted  for  by  Lavoisier  by  the  fact  that  a  con- 
densation of  matter  took  place.  Thus,  when  a  gaseous  substance,  by  the  pro- 
cess of  combination  with  oxygen,  passes  into  the  liquid  or  the  solid  state,  all  the 
latent  heat  which  maintained  it  in  the  form  of  gas  suddenly  becomes  sensible, 
and  an  immense  increase  of  temperature  necessarily  ensues.  The  same  effect 
takes  place  when  a  liquid  passes  into  the  solid  state.  Now  it  is  certain  that 
in  numerous  cases  of  combustion  these  effects  take  place  ;  and  all  such  cases 
admit  of  being  reduced  to  the  same  class  of  phenomena  as  the  solidification  of 
a  liquid,  or  the  condensation  of  a  vapor,  in  both  of  which  cases,  as  has  been 
already  explained,  heat  is  evolved.  Some  of  the  phenomena  of  combustion 
may,  perhaps,  be  reduced  to  the  case  of  ordinary  condensation  without  change 
of  form ;  but  there  are  instances  which  do  not  seem  to  fall  under  this  class  of 
effects.  On  the  contrary,  in  certain  cases,  solids  or  liquids,  in  the  process  of 
combustion,  pass  into  the  state  of  gases.  Thus,  when  gunpowder  is  exploded, 
the  oxygen,  which  is  contained  abundantly  in  the  saltpetre,  combining  with 
the  sulphur  and  carbon,  which  are  the  other  constituents  of  this  substance,  as- 
sumes the  gaseous  form.  At  the  same  time  a  highly  elastic  fluid  is  produced, 
as  well  as  a  large  quantity  of  heat  and  light. 

So  far,  therefore,  as  the  theory  of  Lavoisier  assumes  that  combustion  is  the 
consequence  of  rapid  chemical  combination,  and  that  such  combination  is  ac- 
companied by  a  copious  evolution  of  heat  and  light,  it  is  strictly  a  statement  of 
fact,  but  when  it  is  attempted  to  reduce  these  facts  to  the  general  class  of  phe- 
nomena, in  which  heat  and  light  are  developed  by  condensation,  the  theory  fails, 
because  all  the  phenomena  which  it  professes  to  explain  cannot  be  reduced  to 
this  class.  It  is  also  assumed,  in  the  theory  of  Lavoisier,  that  oxygen  is  a  com- 
pound of  heat,  light,  and  a  certain  unknown  base  ;  that  a  decomposition  takes 
place  by  which  the  heat  and  light  are  disengaged,  and  the  unknown  base  is 
combined  with  the  combustible.  Now  the  existence  of  this  unknown  base  is  a 
gratuitous  assumption,  inasmuch  as  such  a  base  has  never  been  exhibited  in  a 
separate  form  ;  besides  which,  it  is  assumed  that  light  and  heat  are  bodies,  and 
not  qualities  of  matter,  which  is  still  undecided. 

So  remarkable  a  phenomenon  as  combustion,  and  one  so  susceptible  of  such 
various  and  important  practical  applications,  could  not  fail,  at  an  early  period, 
to  attract  the  attention  of  chemists.  We  accordingly  find  many  theories  pro- 
pounded at  various  epochs  in  the  history  of  chemistry  for  its  explanation.  One 
of  the  earliest  of  these  theories  assumes  the  existence  of  a  first  principle,  or 
elementary  substance,  called  fire,  which  had  the  property  of  devouring  other 
bodies.  According  to  this  theory,  combustion  was  the  process  by  which  the 
combustible  was  converted  into  fire  ;  whatever  part  of  the  combustible  was  un- 
susceptible of  this  conversion  remained  behind  in  the  form  of  ashes. 

Dr.  Hook  traced  the  phenomena  of  combustion  to  the  solvent  power  over 
the  combustible  possessed  by  a  principle  found  in  atmospheric  air,  similar  to 
one  which  exists  still  more  copiously  in  nitre.  How  near  this  ingenious  hy- 
pothesis approached  to  the  true  principle  of  combustion  may  be  easily  perceived. 
But  the  theory  which  took  possession  of  the  scientific  world,  to  the  exclusion 
of  all  others,  for  a  long  period,  was  the  Stahlian  theory  of  Phlogiston.     In  this 


COMBUSTION. 


327 


theory,  the  phenomenon  of  combustion  Avas  explained  by  assuming  the  exist-  ] 
ence  of  a  body  called  phlogiston,  which  was  supposed  to  be  a  constituent  ele-  ( 
ment  of  all  combustibles.     The  process  of  combustion  consisted  in  the  sudden  ] 
separation  of  phlogiston  from  the  combustible  :  and  this  separation  was  accom-  i 
panied  by  the  heat  and  light  which  characterized  the  phenomenon.     Some  sue-  \ 
ceeding  philosophers  regarded  this  phlogiston  as  light  maintained  in  bodies,  as  i 
it  were,  in  the  latent  state,  and  with  its  ordinary  concomitant  heat.    Dr.  Priestley  ' 
and  others  discovered  that  the  atmospheric  air  in  which  combustion  takes  place  - 
becomes  incapable  of  permitting  the  same  phenomenon  to  be  repeated  in  it,  and 
likewise  that  such  air  was  rendered  incapable  of  supporting  animal  life.     He 
inferred  that  atmospheric  air  had  an  affinity  for  phlogiston,  and  that  its  presence 
Avas  necessary,  in  order  to  effect  the  extrication  of  phlogiston  from  the  com- 
bustible, and,  consequently,  that  the  presence  of  atmospheric  air  was  essen- 
tially necessary  to  combustion  ;  but  that  when  the  atmospheric  air  became  sat- 
urated with  the  phlogiston  which  it  received  during  the  process  of  combustion, 
the  same  air,  being  incapable  of  combining  with  any  greater  quantity  of  phlo- 
giston, was  incapable  of  sustaining  the  process  of  combustion. 

Still  the  phlogistic  theory  labored  under  the  capital  defect,  that  the  exist- 
ence of  phlogiston  as  a  separate  principle  was  never  proved  ;  and,  in  fact,  that 
the  assumption  of  its  existence  had  no  other  foundation  than  its  convenience 
for  the  solution  of  the  phenomena  of  combustion.  This  defect  in  the  theory  of 
Stahl  was  attempted  to  be  removed  by  a  bold  assumption  of  Kirwan,  viz., 
that  phlogiston  was  no  other  substance  than  hydrogen.  The  necessary  conse- 
quences of  the  adoption  of  such  an  hypothesis  were,  that  hydrogen  is  a  compo- 
nent part  of  every  combustible  body  ;  that  combustion  consists  in  the  decompo- 
sition of  the  combustible  into  the  hydrogen  and  its  base  ;  that,  after  issuing  from 
the  combustible,  the  hydrogen  combines  with  the  oxygen  of  the  atmospheric 
air.     Such  were  the  bases'  of  the  Kirwanian  theory. 

Matters  were  now  ripe  for  the  discovery  of  Lavoisier.     Hook  had  held  that 
a  principle  in  atmospheric  air,  identical  with  the  prominent  element  of  salt  wa- 
ter, was  a  solvent  for  all  combustibles  ;  that  the  solution  effected  by  it  was  ac- 
companied by  heat  and  light.     Kirwan  held,  that  a  combination  of  a  certain 
element  of  the  combustible  with  the  oxygen  of  the  atmospheric  air  was  the 
cause  of  combustion.     Lavoisier,  rejecting  what  was  superfluous  in  these  theo- 
ries, at  once  assumed  that  combustion  was  caused  by  the  combination  of  the 
oxygen  of  the  atmosphere,  not  with  hydrogen,  or  with  the  imaginary  substance 
of  phlogiston,  but  with  the  combustible  itself,  and  that  in  such  combination  heat 
and  light  were  produced.     He  accounted  for  the  phenomena  by  two  admitted 
chemical  laws  :  first,  that  the  chemical  affinity  of  bodies  for  each  other  is  awa- 
kened by  the  elevation  of  temperature  of  one  or  both  ;  and,  secondly,  that  a  body, 
in  passing  from  the  gaseous  to  the  liquid  or  solid  state,  produces  an  abundant 
evolution  of  heat.     The  combustible,  therefore,  when  raised  to  a  certain  tem- 
perature, is  brought  to  the  state  in  which  its  chemical  affinity  for  oxygen  is 
capable  of  taking  effect.    The  oxygen  in  combining  changes  its  form,  and  dis- 
engages a  large  quantity  of  latent  heat. 
\       This  theory  was  quickly  embraced  by  Berthollet,  Fourcroy,  Morveau,  and 
'  other  leading  chemists  of  the  times,  and  has  since  been  very  generally  received. 
\  There  are,  however,  as  has  been  already  stated,  some  phenomena  connected 
•  with  combustion  which  it  fails  to  explain.     These  are  the  cases  where,  in  the 
[  combustion,  the  change  of  form  is  the  reverse  of  that  which,  according  to  the 
>  theory  of  Black,  would  cause  a  development  of  heat.     When  the  combining 
[  substances  previously  exist  in  a  solid  state,  and  during  combustion  pass  into  the 
)  gaseous  state,  we  should  expect  a  large  absorption  of  heat,  instead  of  a  consid- 
I  erable  evolution  of  this  principle. 


COMBUSTION. 


This  defect  in  the  theory  has  given  rise  to  another,  which  has  been  proposed 
by  Sir  Humphry  Davy.  According  to  this  theory,  the  phenomena  of  affinity 
are  the  consequences  of  bodies  existing  in  different  states  of  electricity.  It  is 
known  that  bodies  when  oppositely  electrified  attract  each  other,  and  when 
similarly  electrified  repel  each  other.  If  the  molecules  of  two  bodies  be  oppo- 
sitely electrified,  and  be  so  placed  that  they  can  act  on  one  another,  their  ef- 
fects will  be  attraction,  the  energy  of  which  will  be  increased  in  a  rapid  pro- 
portion with  the  diminution  of  their  distance.  The  more  intensely  one  is  posi- 
tively electrified,  and  the  other  negatively,  with  so  much  the  greater  force  will 
they  combine,  and  the  phenomena  of  combustion  will  be  exhibited  in  their 
union.  Oxygen  is  in  an  intensely  negative  state  of  electricity,  and  hydrogen 
intensely  positive.     Hence  they  combine  with  a  great  evolution  of  heat. 


HOW  TO  OBSERVE  THE  HEAVENS, 


Interesting  Nature  of  the  Subject. — Diumal  Rotation. — Circumpolar  Stars. — Ursa  Major. — Forms 
of  the  Constellations. — The  Pointers. — The  Pole-Star. —  Cassiopeia. —  Capella. — The  Swan. — 
Equatorial  Constellations.— Onow. — Sirius,  or  the  Dogstar. — Aldeharan.^Procyon. — Auriga. — 
Columba. — Herschel's  Observations  on  Su-ius. — Dr.  Wollaston's  Observations. — Aspect  of  the 
Heavens  at  different  Seasons  of  the  Year. — Uses  of  the  Celestial  Globe. — To  ascertain  the  Aspect 
of  the  Heavens  on  any  Night — at  any  Hour. — Effect  of  the  Telescope  on  Fixed  Stars. — Relative 
Brightness  of  the  Stars. — Theoiy  of  refracting  and  reflecting  Telescopes,  ae  applied  to  the 
Stars. — Manner  in  which  Sir  W.  Herschel  applied  it.— Method  of  estimating  the  Brightness  of 
small  Stars. — Method  of  observing  variable  Stars. — Double  Stars. — Description  of  the  Micrometer. 


C ., 


HOW  TO  OBSERVE  THE  HEAVENS. 


331 


H0¥  TO  OBSERVE  THE  HEAVENS. 


To  all  persons  in  whose  minds  a  taste  for  the  study  of  the  universe  has 
been  awakened,  there  is  no  spectacle  which  affords  an  interest  so  intense  as 
that  which  the  firmament  offers  on  every  clear  night,  and  no  occupation  is 
more  delightful  to  those  endowed  with  the  slightest  contemplative  habits,  than 
from  time  to  time  through  the  year  to  observe  the  changes  which  take  place 
in  the  aspect  of  that  glorious  scene  ;  but  to  render  such  contemplations  still 
more  agreeable,  and  to  enable  the  spectator  to  turn  his  observations  to  profitable 
account,  it  will  be  necessary  that  he  should  be  familiar  with  the  objects  which 
there  present  themselves  in  such  countless  numbers  and  endless  variety. 

It  is  an  error  to  suppose  that  astronomical  observations  must  be  confined  to 
observatories,  or  that  no  one  can  enjoy  practical  astronomy  who  is  not  sup- 
plied with  telescopes  and  other  optical  and  astronomical  apparatus.  Our 
Maker  has  given  us,  in  the  eye,  an  optical  instrument  of  exquisite  structure, 
and  has  supplied  us  with  an  understanding  by  which  its  application  may  be 
guided  to  the  most  sublime  speculations.  We  shall  on  the  present  occasion 
endeavor  to  give  "fcuch  plain  and  elementary  rules  as  may  enable  every  one  by 
the  mere  use  of  his  eyes,  without  even  resorting  to  a  common  telescope,  to 
occupy  himself  advantageously  in  the  contemplation  of  the  heavens. 

If  a  person,  on  a  clear  starlight  night,  turn  his  face  directly  to  the  north, 
and  contemplate  the  heavens  for  an  hour  or  two,  he  will  observe  stars  con- 
tinually to  rise  from  the  horizon  on  his  right,  or  in  the  east,  and  other  stars  to 
disappear  and  set  on  his  left,  or  in  the  west.  The  stars  scattered  over  a  por- 
tion of  the  firmament  which  lies  immediately  above  the  northern  horizon  are 
observed  never  to  set,  but  alternately  to  rise  on  the  eastern  and  to  descend  on 
the  western  side  of  the  northern  point ;  the  extent  of  their  descent,  however, 
being  so  limited,  that  they  never  descend  so  low  as  the  horizon.  Frequent 
and  attentive  observation  of  these  appearances  will  suggest  strongly  the  idea 
that  these  objects  revolve  in  circles  round  some  point  as  a  centre,  which  is 
situate  in  the  northern  region  of  the  firmament.     But  this  impression  can  not 


be  verified  by  ocular  observation  merely,  and  still  less  can  the  position  of  that 
common  centre  be  thus  determined. 

Having  recourse,  however,  to  instruments,  by  which  the  exact  elevations 
of  the  stars  may  from  time  to  time  be  observed,  and  their  exact  bearings  noted, 
data  are  obtained  by  which  it  is  demonstrated  that  this  first  impressio'i  is 
rigorously  correct;  that  the  objects  which  glitter  on  the  firmament  do,  in  fnct, 
appear  to  revolve  round  a  certain  point  as  a  common  centre;  that  they  all  com- 
plete their  revolution  round  that  point  in  twenty-three  hours,  fifty-six  minutes, 
four  seconds,  and  nine  hundredths  of  a  second.  All  the  stars  complete  their 
revolution  round  this  point  in  exactly  the  same  time,  however  different  theii' 
distances  from  it  may  be,  and  as  they  so  revolve  they  preserve  their  relative 
position  with  respect  to  each  other.  It  has  been  shown  on  a  former  occasion 
that  this  appearance  is  an  optical  illusion,  caused  by  the  rotation  of  the  earth 
upon  its  axis. 

At  the  point  which  is  the  common  centre  of  this  motion,  and  which  is  called 
the  north  celestial  pole,  no  star  is  found  ;  but  there  is  a  star  sufficiently  bright 
to  be  distinctly  visible  to  the  naked  eye  very  close  to  it,  which  is  therefore 
called  the  pole-star.  The  method  of  recognising  this  star  we  shall  presently 
explain. 

Even  the  most  inattentive  observer,  when  assuming  the  position  we  have 
here  described,  will  be  immediately  struck  with  a  combination  of  seven  con- 
spicuous stars  arranged  in  the  relative  positions  exhibited  in  the  annexed 
diagram. 

•  Eig.l.       ,  . 


This  combination,  or  group  of  stars,  presents  to  us  the  first  and  most  striking 
example  of  vs^hat  is  called  a  constellation. 

The  peculiar  configurations  affected  by  this  and  other  groups  scattered  over 
the  firmament,  give  an  impression  that  some  physical  relation  connects  the 
component  stars  with  each  other.  But  a  more  exact  acquaintance  with  stellar 
astronomy  proves  that  such  impression  is  destitute  of  any  good  foundation. 
The  stars  which  compose  the  constellations  are  casually  scattered  o\ur  the 
firmament,  and  it  is  the  imagination  only  which  groups  them  and  invests  the 
collections  thus  associated  with  the  fanciful  figures  of  bears,  lions,  goats, 
dogs,  warriors,  and  mythological  personages.  Unreasonable  as  such  a  sys- 
tem must  be  allowed  to  be,  it  is  not  without  its  use  as  a  means  of  reference, 
and  an  artificial  aid  to  the  memory.  That  a  better  system  of  signs  and  sym- 
bols might  have  been  devised  for  these  purposes  may  be  admitted ;  but  when 
it  is  considered  that  the  names  and  forms  of  the  most  conspicuous  constellations 
have  had  their  origin  in  remote  antiquity — that  they  were  handed  down  from 
the  Chaldeans  to  the  Egyptians,  and  from  the  Egyptians  to  the  Greeks,  and 
from  these  to  the  moderns — that  they  are  referred  to  in  the  works  of  every 


past  astronomer,  and  engraved  on  the  memory  of  every  living  observer — it  will 
be  readily  acknowledged  that,  even  if  a  general  change  of  stellar  nomenclature 
and  symbol  were  practicable  (which  it  assuredly  is  not),  it  would  be  neither 
advantageous  nor  advisable. 

The  northern  constellation,  to  which  we  have  referred,  is  called  Ursa  Major, 
or  the  Great  Bear.  The  seven  stars  are  only  the  most  conspicuous  of  those 
which  compose  it,  the  entire  number  of  stars  in  it  being  eighty-seven,  most  of 
which,  however,  are  so  small  as  not  to  be  visible  without  a  telescope.  Of 
the  seven  principal  stars  one  only  (that  marked  ",  fig.  1),  is  of  the  first  mag- 
nitude. Three  (marked  P,  y,  and  i  ),  are  of  the  second  magnitude,  and  the  re- 
maining three  ( •[,  ^,  and  ^  )  are  of  the  third  magnitude.     As  these  stars,  being 

Fig.  2. 


33  i 


HOW  TO  OBSERVE  THE  HEAVENS. 


visible  at  all  seasons,  and  in  all  northern  latitudes,  are  necessarily  familiar  to 
every  eye,  they  may  serve  as  standards  or  moduli  by  v\?hich  the  astronomical 
amateur  may  estimate  the  difterent  orders  of  magnitudes  of  the  stars  generally. 

One  of  the  most  convenient  methods  of  ascertaining  and  identifying  the 
principal  stars  on  the  heavens  which  the  amateur  observer  can  adopt,  consists 
in  selecting  other  known  stars  as  pointers.  We  shall  explain  this  method  by 
one  or  two  examples.  In  the  constellation  of  Ursa  Major,  there  are  besides 
the  seven  stars  above  mentioned,  five  others  of  the  third  magnitude,  which  are 
marked  0,  ',  ^>  /'?  i'>  in  the  annexed  diagram. 

To  find  0  and  ',  fig.  2,  let  the  observer  imagine  a  straight  line  drawn  from  ^  to 
P,  to  be  continued  beyond  /?•  The  first  stars  of  the  same  magnitude  as  <i  which 
it  will  meet  will  be  6  and  '•  Again,  let  a  straight  line  be  imagined  to  be  drawn 
from(Stoy,and  continued  beyond  y-  It  will  carry  the  eye  to  the  star  ^  of  the 
same  magnitude  as  i-  Finally,  if  a  line  be  imagined  to  be  drawn  from  "  per- 
pendicular, the  line  passing  through  the  four  stars  &,  0,  s,  and  ',  it  will  conduct 
the  eye  to  the  two  stars  ^  and  z^- 

If  the  observer  look  at  any  good  map  of  the  stars,*  he  will  find  that  the 
stars  ",  /?.  and  y,  are  on  the  body  of  the  figure  of  the  Bear ;  ^, «,  ?,  i,  form  the 
tail,  0  and  '  are  on  the  right  fore  leg,  4'  on  the  hinder  right  thigh,  and  X  and  /' 
on  the  hinder  right  paw. 

The  practical  usefulness  of  the  imaginary  figures  which  give  names  to  the 
constellations  will  thus  be  understood.  If  we  desire  to  express  the  position 
of  the_star  v  Ursm  Majoris,  for  example,  we  say  that  it  is  at  the  tip  of  the  tail 
of  the  Great  Bear. 

The  seven  principal  stars  of  this  constellation  being  all  less  than  forty  de- 
grees from  the  north  pole,  will  be  always  above  the  horizon  in  latitudes  greater 
than  forty  degrees.  Hence  it  is  that  this  constellation  is  so  familiarly  known. 
It  is  in  the  quarter  of  the  heavens  opposite  to  that  in  which  the  sun  is  in  the 
month  of  March,  and  is  therefore  visible  at  midnight  near  the  meridian  ahove 
the  pole  at  that  season.  In  the  month  of  September  it  is  visible  at  midnight 
helow  the  pole. 

The  point  in  the  firmament  whose  position  ought  to, be  most  familiar  to  the 
observer  is  the  pole.  Its  position  is  marked  by  a  star  of  the  second  magnitude, 
which  is  so  near  to  it  that  the  interval  can  not  be  appreciated  without  the  use 
of  good  astronomical  instruments.  It  is  therefore  very  important  that  an  easy 
method,  applicable  without  instruments,  should  be  available  for  the  discovery 
of  this  star.  The  method  already  explained  may  be  used  also  in  this  case. 
In  the  annexed  figure  the  seven  stars  of  Ursa  Major  are  represented  in  the 
lower  part.  The  stars  of  the  upper  part  are  those  of  a  constellation  near  the 
pole  called  Ursa  Minor.  The  actual  position  of  the  pole  is  represented  at  ©, 
and  the  star  immediately  above  it  is  the  pole-star. 

If  a  line  be  conceived  to  be  drawn  between  the  two  stars  «  and  "'■  fig.  3,  \n 
Ursa  Major,  and  continued  beyond  «,  it  will  pass  very  nearly  through  the  pole- 
star,  and  as  there  is  no  star  of  the  same  magnitude  near  the  latter,  the  eye 
can  not  fail  to  recognize  it.  The  other  stars  of  the  constellation  of  the  lesser 
\  Bear,  are  all  of  inferior  brightness.  The  figures  annexed  to  the  several  stars 
)  in  this  diagram  express  their  respective  magnitudes.  The  two  stars  «  and  /?, 
;  of  Ursa  Major,  have  hence  been  denominated  the  pointers.  The  apparent 
)  distance  between  the  pointers  is  5°,  while  the  distance  from  «  to  the  pole-star 
\  is  29°.     Thus  the  latter  distance  is  nearly  six  times  the  former. 

*  We  should  recommend  tlie  amateur  astronomei"  to  be  provided  with  tlie  maps  of  the  stars  pub- 
lished on  extremely  cheap  terms  by  the  London  Society  for  the  Diffusion  of  Useful  Knowledge,  to- 
gether with  the  "  Companion  to  the  Maps,"  by  Professor  de  Morgan.  These  are  always  on  hand 
at  Mr.  Baldwin's  bookstore,  Broadway,  New  York. 


HOW  TO    OBSERVE    THE   HEAVENS. 


335 


Fig.  3. 


By  attentively  obssrving  for  a  few  successive  hours  the  changes  of  position 
of  these  objects  with  relation  to  the  horizon,  it  will  be  easily  perceived  that 
the  line  through  the  pointers  and  the  pole-star  revolves  round  the  latter  point 
as  a  centre,  and  in  three  hours  it  will  be  observed  to  revolve  through  an  angle 
of  45°,  which  being  half  a  right  angle,  can  be  estimated  with  some  degree  of 
approximation  by  the  eye.  We  may  thus  see  that  the  firmament  appears  to 
revolve  round  the  axis' of  the  sphere  at  the  rate  of  about  15°  per  hour. 

Another  remarkable  group  of  stars  visible  in  northern  latitudes  at  all  sea- 
sons, is  the  constellation  called  Cassiopeia,  consisting  of  five  principal  stars. 
The  configuration  of  these,  which  is  given  in  the  annexed  diagram,  fig.  4.  is 
familiar  to  every  eye  accustomed  to  contemplate  the  heavens.  The  star  1^,  is 
of  the  second,  and  the  remaining  four  of  the  third  magnitude. 

This  constellation  being  in_  the  quarter  of  the  heavens  opposite  to  that  in 
which  the  sun  is  in  the  montn  of  October,  will  be   seen  on  the  meridian,  or 


336 


HOW  TO    OBSERVE    THE   HEAVENS. 


Fig.  4. 


near  it,  at  midnight  during  that  month,  and  being  distant  from  the  pole  about 
30°,  it  will  be  seen  a  little  south  of  the  zenith  at  all  places  between  the  lati- 
tudes 40°  and  60°. 

The  two  most  conspicuous  stars  which  appear  in  the  northern  region  of 
the  heavens  are  those  called  Capclla  and  a  Cygni.  They  are  both  stars  of  the 
first  magnitude.  Capella  is  seen  in  the  meridian  above  the  pole  at  midnight 
in  the  early  part  of  January,  and  «  Cygni  in  the  early  part  of  August.  At  New 
York  these  stars  pass  within  a  few  degrees  of  the  zenith,  through  which  they 
both  exactly  pass  at  all  places  having  the  latitude  of  45°  N. 

If  v/e  turn  due  south  and  look  to  that  point  of  the  celestial  meridian  whose 
distance  from  the  zenith  is  equal  to  the  latitude  of  the  place  of  observation,  we 
shall  see  the  point  of  the  heavens  where  the  celestial  equator  intersects  the 
meridian.  ,  Those  parts  of  the  heavens  which  e3f.tend  to  about  thirty  degrees 
above  and  below  this  point,  and  which  stretch  on  either  side  of  the  meridian 
fb  the  eastern  and  western  points  of  the  horizon,  form  by  far  the  most  inter- 
esting and  remarkable  regions  of  the  firmament.  Not  only  is  this  region  more 
rich  in  constellations,  and  adorned  by  the  most  brilliant  stars,  but  it  is  the 
space  within  which  the  range  of  the  planets  is  confined.  By  the  diurnal  mo- 
tion of  the  celestial  sphere,  these  constellations,  together  with  such  of  the 
planets  as  happen  to  be  sufficiently  removed  from  the  sun  and  the  moon, 
when  that  object  is  not  too  near  the  sun,  are  passed  nightly,  like  a  moving 
diorama,  before  the  observer.  As  he  stands  facing  the  south,  the  east  will  be 
on  his  left,  and  the  west  on  his  right.  He  will  behold  the  zodiacal  constella- 
tions successively  coming  into  view  from  below  the  horizon  at  or  near  the 
eastern  point ;  they  will  gradually  rise  toward  that  part  of  the  meridian  to 
which  we  have  referred,  and  passing  it,  will  descend  toward  the  western  part 
of  the  horizon,  where  they  will  finally  disappear. 

The  most  remarkable  of  these  equatorial  constellations  is  that  called  Orion. 
The  principal  stars  composing  it  are  those  marked  «,  y,  ^,  £,  <r,  P,  k,  i,  in  the  an- 
nexed diagram,  fig.  5.  By  reference  to  a  celestial  globe,  or  still  better,  to  a 
map  of  the  stars,  it  will  be  seen  that  this  constellation  is  made  to  form  the  out- 
line of  the  figure  of  a  warrior.  The  star  a  is  on  the  right  shoulder,  y  on  the 
left,  the  stars  "5,  ^i  o-,  on  the  belt,  '  on  the  sword,  P  on  the  left  foot,  and  ^  on  the 
right  knee.  The  stars  «  and  /?  are  both  of  the  first  magnitude,  and  both  double 
stars.  The  latter  ('^)  is  better  known  by  the  name  of  Rigel.  The  stars  y,  (5, 
and  e,  are  of  the  second  magnitude,  and  <J  is  a  double  star.  The  position  of 
the  constellatian  with  reference  to  the  meridian  will  be  perceived  by  the 
arrow,  which  indicates  the  direction  of  the  north. 

In  the  month  of  December,  this  constellation  passes  the  meridian  about 
midnight,  and  is  therefore  visible  on  the  eastern  side  of  the  heavens  during  the 


Fig.  5. 


VOL.  II.— aa 


338 


HOW  TO  OBSERVE  THE  HEAVENS. 


early  part  of  the  nirrht.  The  stars  ^,  f,  '^,  which  form  the  belt,  being  upon  the 
celestial  equator,  rise  each  evening  at  six  o'clock  precisely  at  the  point  of  the 
horizon  which  is  due  east,  and  at  nine  o'clock  the  constellation  is  elevated 
midway  between  that  point  and  the  meridian. 

If  a  line  be  imagined  to  be  drawn  in  the  direction  of  the  belt,  and  continued 
both  ways,  east  and  west,  it  will  pass  a  little  north  of  the  splendid  star  Sirius 
(S),  in  the  constellation  of  Cants  Major,  commonly  called  the  Dog-star,  on 
the  east,  and  a  little  south  of  another  brilliant  star,  Aldebaran  (A),  on  the 
west.  This  latter  star  forms  the  ei/e  of  the  zodiacal  constellation  Taurus. 
Other  stars  in  the  constellation  of  Canis  Major  are  represented  at  5  c  cZ. 

If  a  line  be  imagined  to  be  carried  from  Sirius  (S),  in  a  direction  perpen- 
dicular to  that  which  passes  through  the  belt  of  Orion,  it  will  conduct  the  eye 
to  the  bright  star  Procyon  (P),  in  the  constellation  of  Canis  Minor,  which 
is  a  star  of  the  iirst  magnitude,  with  one  of  the  third  magnitude  near  it. 

If  a  line  be  conceived  to  be  drawn  from  Rigel  (fl),  through  y,  and  carried 
upward,  it  will  pass  a  little  east  of  a  bright  star  of  the  second  magnitude  (E), 
in  the  foot  of  the  constellation  Auriga. 

If  a  line  be  imagined  to  be  drawn  from  «  through  k,  and  carried  downward, 
it  will  pass  through  another  bright  star  (C),  of  the  second  magnitude  in  the 
constellation  Columba. 

The  stars  Sirius  (S),  Procyon  (P),  and  Aldebaran  (A),  are  all  of  the 
first  magnitude,  and  very  splendid  objects.  Sirius,  however,  is  not  only  the 
most  magnificent  of  these,  but  is  the  brightest  star  in  the  firmament.  This 
star  was  frequently  submitted  to  telescopic  examination  by  the  late  Sir  William 
Herschel,  with  the  extraordinary  optical  powers  which  that  observer  com- 
manded, and  he  relates  that  when  it  approached  the  field  of  view  of  the  forty- 
feet  telescope,  the  effect  resembled  the  approach  of  sunrise,  and  when  the 
glorious  object  entered  the  field  of  view  the  splendor  was  so  overpowering  that 
he  was  obliged  to  protect  the  eye  by  a  colored  glass  ! 

Dr.  Wollaston  calculated  that  this  star  must  be  equal  to  fourteen  suns  like 
that  of  our  system.  This  calculation,  however,  was  founded  on  the  supposi- 
tion that  the  star  is  much  nearer  to  us  than  it  is  now  known  to  be,  and  the 
splendor  of  the  object  has  therefore  been  greatly  underrated  ! 

In  the  constellation  of  Orion  are  a  considerable  number  of  stars  under  the 
third  magnitude,  not  represented  in  the  diagrain,  (p.  357),  many  of  which,  when 
examined  by  powerful  telescopes,  prove  to  be  double  stars  ;  besides  which  is 
found  in  the  sword  the  most  remarkable  nebulae  in  the  firmament,  which  we 
shall  hereafter  notice  more  fully. 

If  a  line  drawn  from  Procyon  (P),  to  Rigel,  and  continued  westward,  it  will 
conduct  the  eye  to  the  star  (second  magnitude)  D,  in  the  constellation  Draco, 
known  in  astronomy  as  y  Draconis.  This  star  is  celebrated  in  astronomy  as 
that  by  observing  which  Dr.  Bradley  discovered  the  aberration  of  light. 

The  midnight  sky,  in  the  months  of  March  and  April  exhibits  the  zodiacal 
constellations  of  Leo  and  Vergo,  the  constellation  of  Bootes  and  Coma 
Berenices.  These  constellations  contain  three  of  the  most  splendid  stars  in 
the  firmament:  Regulus  in  Leo,  Spica  in  Vekgo,  and  Arcturus  in  Bootes. 

Rrgulus  is  seen  at  midnight  on  the  meridinn,  on  the  20th  February.  In 
March  it  passes  the  meridian  between  nine  and  eleven,  and  in  April  between 
seven  and  nine.  The  point  where  it  crosses  the  meridian  at  New  York,  and 
other  places  in  the  same  parallel,  is  about  30°  south  of  the  zenith.  At  places 
south  of  New  York  it  will  be  nearer  to,  and  north  of  New  York  more  distant 
froai  the  zenith. 

Spica  is  seen  at  miduiglit  on  the  meridian  on  the  lOlh  April.  In  March  it 
passes  the  meridian  between  midnight  and  two  in  the  morning,  and  in  the  end 


HOW  TO   OBSERVE    THE   HEAVENS. 


339 


of  April  and  the  beginning  of  May  it  passes  the  meridian  between  midnight 
and  nine  o'clock.  It  crosses  the  meridian  at  New  York  at  50°  south  of  the 
zenith,  and  will  therefore  be  seen  to  more  advantage  in  places  further  south. 
At  Charleston  it  passes  the  meridian  43°,  and  at  New  Orleans  40°  south  of 
the  zenith. 

Arcturus  is  upon  the  meridian  at  midnight  on  the  22d  April.  During  the 
month  of  May  it  passes  the  meridian  between  nine  and  eleven  at  night,  and  in 
June  between  seven  and  nine.  At  New  York  it  crosses  the  meridian  20° 
south  of  the  zenith,  at  Charleston  13°,  and  at  New  Orleans  10°  south  of  it. 
In  all  parts  of  America  this  star  is  therefore  seen  to  great  advantage. 

In  the  annexed  diagram,  fig.  6,  the  collocation  of  the  principal  stars  in  these 
three  constellations  is  exhibited.  The  star  Arcturus  is  placed  at  A,  with 
the  principal  stars  of  the  constellation  Bootes  around  it.  All  excerpt  Arcturus 
are  stars  of  the  third  magnitude,  and  it  is  worthy  of  note  that  they  are  all 
double  stars. 

The  star  Spica  is  at  S  ;  the  other  chief  stars  of  Virgo,  y,  v,  S,  and  Y,  being 
of  the  third  magnitude.  The  star  y  is  in  fact  two  staus  close  together,  one  of 
which  (that  to  the  west)  being  a  double  star. 

The  star  Rcgulus,  which  is  a  double  star,  is  at  R.  In  the  constellation 
of  Leo  are  also  two  stars,  P,  and  y,  of  the  second  magnitude.  These  three 
principal  stars  form  a  right-angled  triangle,  the  right  angle  of  which  is  at  y. 
This  last  star,  y,  is  a  double  star.  The  three  other  chief  stars  of  this  constel- 
lation, '/,  9,  and  <5,  form  an  isosceles  or  equal-sided  triangle,  the  base  of  which 
is  the  line  joining  ^  and  &• 

It  Avill  be  also  observed  that  Regulus,  Spica,  and  Arcturus,  form  a  right- 
angled  triangle,  the  right  angle  of  which  is  at  Spica. 

In  the  months  of  May,  June,  and  July,  the  heavens,  during  the  night,  ex- 
hibit the  constellations  Lyra,  Aquila,  Hercules,  Ophiuchus,  and  the 
zodiacal  constellations  Scorpius,  Sagittarius,  and  Capricornus.  These  in- 
clude but  three  stars  of  the  first  magnitude ;  the  star  a,  in  the  constellation  of 
Lyka,  Atair  in  xAquila,  and  Antares  in  Scorpius. 

Antares  is  on  the  meridian  at  midnight  on  the  27th  May.  During  the 
month  of  June  it  passes  the  meridian  between  ten  o'clock  and  midnight.  1  his 
star,  however,  being  about  26°  south  of  the  celestial  equator,  is  not  seen  with 
advantage  in  the  northern  hemisphere.  At  New  York  this  star  passes  the 
meridian  at  66°  from  the  zenith,  and  therefore  never  rises  to  a  greater  altitude 
than  34°.  At  New  Orleans  its  meridional  altitude  is  44°,  and  it  may  accord- 
ingly at  the  proper  season  be  seen  there  more  advantageously. 

The  star  «  Lyra  passes  nearly  through  the  zenith  of  New  York  at  midnight 
on  the  29th  June,  and  during  the  months  of  July  and  August  may  be  seen 
during  the  night,  crossing  the  meridian  between  eight  o'clock  and  midnight. 
It  is  a  splendid  star  of  the  first  magnitude,  and  having  no  other  bright  star  in 
its  neighborhood,  is  at  once  recognised.  This  is  a  double  star.  This  star 
passes  the  meridian  in  all  parts  of  the  United  States  within  less  than  ten  de- 
grees of  the  zenith. 

The  star  Atair,  in  the  constellation  Aquila,  passes  the  meridian  about  one 
hour  later  than  a  Lyra,  and  at  New  York  crosses  it  at  the  distance  of  30°  from 
the  zenith.  This  star  has  in  its  immediate  neighborhood,  forming  part  of  the 
same  constellation,  seven  stars  of  the  third  magnitude. 

In  the  annexed  diagram,  fig.  7,  L  represents  «  Lyra,  and  A  Atair.  A  line 
joining  these  two  stars  of  the  first  magnitude  passes  through  four  of  the  third 
magnitude,  y  Lyra  and  y»  /^j  and  9  Aquila.  The  four  stars  A,  ^,  i,  and  f,  are  of 
the  third  magnitude,  and  also  form  part  of  the  constellation  Aquila.  The  star 
Atair  is  a  double  star. 


HOW   TO   OBSERVE    THE   HEAVENS. 


Fig.  7 


On  the  2d  September,  the  star  Fomalhaut  (first  magnitude),  passes  the  me- 
ridian at  midnight.  This  star  being  situate  30^  south  of  the  celestial  equator, 
is  unfavorably  situate  for  observation  in  northern  latitudes.  At  New^  York  its 
greatest  altitude  is  20°.  There  are  three  conspicuous  stars  of  the  second  mag- 
nitude in  the  constellation  of  Pegasus,  two  of  which,  Markab  and  Scheat,  are 
on  the  meridian  at  the  same  time  with  Fomalhaut,  and  the  third  £,  about  an 
hour  and  a  quarter  before. 

In  the  annexed  diagram,  fig.  8,  the  most  conspicuous  stars  which  appears 
during  the  night  in  August,  September,  and  October,  are  represented.  The 
stars  marked  1,  are  in  the  constellation  Pegasus.  The  stars  a  1,  /?  1,  and  tl, 
are  of  the  second  magnitude,  the  last  being  a  double  star.  The  star  f  1,  is  of 
the  third  magnitude,  as  also  is  the  star  a  2,  which  is  on  the  right  shoulder  of 
Aquarius.  The  star  «  1,  and  0  1,  are  also  called  Markab,  and  Scheat.  These 
are  on  the  meridian  together,  and  are  separated  by  about  12°. 


The  star  y  i,  (second  magnitude),  is  on  the  wing  of  Pegasus,  and  is  on  the 
meridian  at  the  same  time  with  the  bright  star  a  3  (first  magnitude),  which  is 
on  the  head  of  Andromeda.  These  two  stars  are  on  the  meridian  at  midnight 
on  the  20th  September.  The  star  a  3  passes  the  meridian  at  New  York  about 
12°  south  of  the  zenith.     This  is  a  double  star. 

The  four  conspicuous  stars  n3,  y  1,  u  1,  and  /?  1,  are  easily  recognised  on  the 
firmament,  the  lines  which  join  them  forming  nearly  a  square.  The  star  ^3 
(second  magnitude),  is  a  double  star  on  the  girdle  of  Andromeda,  and  lying 
very  nearly  in  the  direction  of  the  diagonal  of  the  quadrilateral  formed  by  the 
stars  a3,  y  ],  a  1,  and  0  !•  The  star  <5  3,  of  the  third  magnitude,  is  on  the  breast 
of  Andromeda. 

The  two  stars  "  5,  and  /?5,  which  form  the  base  of  an  isosceles-triangle,  hav- 
ing, its  vertex  at  the  star  /?3,  are  in  the  head  of  Aries,  and  are  stars  of  the 
third  magnitude.  The  star  0  5,  is  double.  These  stars  are  on  the  meridian 
at  midnight  on  the  20th  October. 

The  a  6  is  a  conspicuous  star  of  the  second  magnitude  called  Menkar,  in 
the  constellation  of  the  Whale  (Cetus),  and  76.,  near  it,  is  a  double  s.tar  of  the 
third  magnitude,  in  the  same  constellation. 

The  amateur  observer  may,  from  the  examples  which  have  here  been  given, 
easily  extend  his  acquaintance  with  the  fixed  stars.  In  the  maps  of  the  stars 
published  by  the  Society  for  the  Diffusion  of  Useful  Knowledge,  already  referred 
to,  he  will  find  marked  the  days  on  which  each  star  is  seen  on  the  meridian 
at  midnight.  Its  place  on  the  meridian  may  be  found  by  the  following  simple 
rule,  in  which  it  is  assumed  that  the  place  of  the  observer  has  north  latitude  : — 

"Observe  on  the  map  the  distance  of  the  star  from  the  celestial  equator. 
If  the  star  be  north  of  the  equator,  the  difference  between  this  distance  and 
the  latitude  of  the  place  will  give  the  distance  of  the  star  when  on  the  meri- 
dian from  the  zenith.  It  will  be  south  of  the  zenith  if  the  latitude  of  the  place 
be  greater  than  the  distance  of  the  star  from  the  equator — north  if  less.  If 
the  latitude  of  the  place  be  equal  to  the  distance  of  the  star  from  the  equator, 
the  star  will  pass  through  the  zenith.  If  the  star  be  south  of  the  celestial 
equator,  add  its  distance  from  the  equator  to  the  latitude  of  the  place,  and  the 
result  will  be  the  distance  south  of  the  zenith  at  which  the  star  will  pass  the 
meridian."     The  following  examples  will  illustrate  this  rule  : — 

Example  1. — It  is  required  to  determine  the  point  at  which  the  star  Castor 
crosses  the  meridian  of  New  York. 

By  reference  to  the  maps  or  a  celestial  globe,  it  will   appear  that  the   star 

Castor  is  57°  45'  north  of  the  celestial  equator.     The  latitude   of  New  York 

being  assumed  to  be  40°  43'  N.,  we  shall 

From         -       57". 45' 

'         Subtract   -        40°.43'        '      . 


17°.  2' 

and  the  remainder,  17°. 2',  will  be  the  distance  north  of  the   zenith  at  which 
Castor  passes  the  meridian. 

Example  2. — To  find  the  point  at  which  Fomalhaut  passes  the  meridian  of 
New  York. 

The  distance  of  Fomalhaut  south  of  the  celestial  equator  being  taken  from 
the  map  to  be  30°. 30',  we  shall 

To         -        300-30' 
Add       -        400.43' 


710.13' 

and  the  sum  71°.  13'  will  be  the  distance  south  of  the  zenith  at  which  Fomal- 
haut passes  the  meridian. 


HOW  TO  OBSERVE  THE  HEAVENS. 


343 


Fig.  8. 


14  t 


HOW  TO  OBSERVE  THE  HEAVENS. 


Example  3. — To  find  the  point  at  which  «  Aquila  passes  the  meridian  of 
New  York. 

The  distance  of  «  Aqnila  north  of  the  celestial  equator  being  taken  fr»m  the 
map  U)  be  28^^.45',  we  shall 

From        -        40^.43' 
Subtract   -         28°. 45' 


? 


110.58' 


and  the  remainder,  11°. 58',  will  be  the  distance  south  of  the  zenith  at  which 
the  star  passes  the  meridian.* 

A  celestial  globe,  where  it  can  be  had,  will  prove  a  ready  and  convenient 
aid  to  the  amateur  in  astronomy,  superseding  the  necessity  of  many  calculations 
which  are  often  discouraging  and  repulsive,  however  simple  and  easy  they 
tiiay  be  to  those  who  are  accustomed  to  such  inquiries.  Most  of  the  almanacs 
contain  tables  of  the  principal  astronomical  phenomena,  of  the  places  of  the 
sun  and  moon,  and  of  the  principal  planets  as  well  as  the  times  when  the 
most  conspicuous  stars  are  on  the  meridian  of  Washington  after  sunset.  These 
data,  together  with  a  judicious  use  of  the  globe  and  a  tolerable  telescope,  will 
enable  any  person  to  extend  his  acquaintance  with  astronomy,  and  may  even 
enable  him  to  become  a  useful  contributor  to  the  common  stock  of  information 
which  is  now  so  fast  increasing  by  the  zeal  and  ability  of  private  observers  in 
so  many  quarters  of  the  globe. 

To  prepare  the  globe  for  use,  let  small  marks  (bits  of  paper  gummed  on  will 
answer  the  purpose)  be  placed  upon  it,  to  indicate  the  positions  of  the  sun, 
moon,  and  planets,  at  the  time  of  observing  the  heavens.  The  place  of  the 
sun  on  the  ecliptic  is  usually  marked  on  the  globe  itself.  If  not,  its  right  as- 
cension (that  is,  its  distance  from  the  vernal  equinoctial  point,  measured  on  the 
celestial  equator),  and  its  declination  (that  is,  its  distance  north  or  south  of  ( 
the  equator),  are  given  in  the  almanac  for  every  day.  The  moon's  right  ^, 
ffscension  and  declination  are  likewise  given. f 

To  find  the  place  of  an  object  on  the  globe  when  its  right  ascension  and  dec- 
lination are  known. 

Find  the  point  on  the  equator  where  the  given  right  ascension  is  marked. 
Turn  the  globe  on  its  axis  till  this  point  be  brought  under  the  meridian. 
Then  count  off  an  arc  of  the  meridian  (north  or  south  of  the  equator,  according 
as  the  declination  is  given),  of  a  length  equal  to  the  given  deplination.  and  the 
point  of  the  globe  immediately  under  the  point  of  the  meridian  thus  found,  will 
be  the  place  of  the  object.  By  this  rule,  the  position  on  the  globe  of  any 
object  of  which  the  right  ascension  and  declination  are  known,  may  be  im- 
mediately found,  and  a  corresponding  mark  put  upon  it. 

To  adjust  the  globe  so  as  to  use  it  as  a  guide  to  the  position  of  objects  on 
the  heavens,  and  as  a  means  of  identifying  the  stars  and  learning  their  names, 
let  the  lower  clamping-screw  of  the  meridian  be  loosened,  and  let  the  north 
pole  of  the  globe  be  elevated  by  moving  the  brass  meridian  until  the  arc  of  this 
meridian  between  the  pole  and  the  horizon  be  equal  to  the  latitude  of  the  place  ) 
of  observation.  Let  the  clamping-screw  be  then  tightened  so  as  to  maintain  the 
meridian  in  this  position.  Let  the  globe  be  then  so  placed  that  the  brass  me- 
ridian shall  be  directed  due  north  and  south,  the  pole  being  turned  to  the 
north.  This  being  done,  the  globe  will  correspond  with  the  heavens  so  far  as 
rrUites  to  the  poles,  the  meridian,  and  the  points  of  the  horizon. 

"  In  t'lesc  examples  I  have  taken  the  declinations  roughly  from  the  map  rather  than  from  the 
tallies,  as  tlitit  would  be  the  method  which  an  amateur  would  probably  use. 

t  Jn  iho  United  Statks  Almanac  a  sufficient  collection  of  tables  and  astronomical  data  for  all 
the  piirjioses  of  the  amatcnr  astronomer  are  given.  It  will  he  necessary  that  he  should  first  render 
himself  familiar  with  the  abbreviations  and  symbols,  after  which  he  will  find  the  greatest  advantage 
from  that  work. 


HOW  TO   OBSERVE    THE   HEAVENS. 


To  ascertain  the  aspect  of  the  firmament  at  any  hour  of  the  night,  it  is  now 
only  necessary  to  turn  the  globe  upon  its  axis  until  the  mark  indicating  the 
place  of  the  sun  shall  be  under  the  horizon  in  the  same  position  as  the  sun 
itself  actually  is  at  the  hour  in  question.  To  effect  this,  let  the  globe  be  turned 
until  the  mark  indicating  the  position  of  the  sun  is  brought  under  the  meridian. 
Observe  the  hour  marked  on  the  point  of  the  equator  which  is  then  under  the 
■meridian.  Add  to  this  hour  the  hour  at  which  the  observation  is  about  to  be 
taken,  and  turn  the  globe  until  the  point  of  the  equator  on  which  is  marked  the 
hour  resulting  from  this  addition  is  brought  under  the  meridian.  The  position 
of  the  globe  will  then  correspond  with  that  of  the  firmament.  Every  object 
on  the  one  will  correspond  in  its  position  with  its  representative  mark  or 
symbol  on  the  other.  If  we  imagine  a  line  drawn  from  the  centre  of  the  globe 
through  the  mark  upon  its  surface  indicating  any  star,  such  a  line  if  continued 
outside  the  surface  toward  the  heavens  would  be  directed  to  the  star  itself. 

For  example,  suppose  that  when  the  mark  of  the  sun  is  brought  under  the 
meridian,  the  hour  5h.  40m.  is  found  to  be  on  the  equator  at  the  meridian,  and 
it  is  required  to  find  the  aspect  of  the  heavens  at  half  past  ten  o'clock  in  the 
evening. 

H.  M. 


To 
Add 


5 
10 


40 
60 


16 


00 


Let  the  globe  be  turned  until  16h.  Om.  is  brought  under  the  meridian,  and  the 
aspect  given  by  it  will  be  that  of  the  heavens. 

We  have  frequently  spoken  of  stars  of  the  first,  second,  and  inferior  magni- 
tudes. It  is  necessary  that  the  just  application  of  this  term  magnitude  be 
clearly  understood.  The  Creator  of  the  universe  has  not  made  the  visible  stars 
in  six  moulds,  so  as  to  give  them  as  many  exact  and  distinguishable  magnitudes. 
Among  these  objects  there  is  every  gradation  of  brightness  from  the  splendor 
of  Sirius  down  to  that  of  those  stars  which  are  barely  perceptible  without  a 
telescope.  Between  those  stars  which  astronomers  have  consigned  to  the 
first  and  second  classes,  respectively,  there  is  no  distinct  and  decisive  line  of 
separation.  The  stars  of  the  first  magnitude  are  not  equally  bright;  nay,  it  is 
probable  that  no  two  of  them  could  be  selected,  which,  if  submitted  to  pho- 
tometric tests,  would  prove  to  be  of  exactly  equal  splendor.  The  least  splen- 
did of  them  is  not  distinguishable  from  the  most  splendid  of  the  stars  of  the 
second  magnitude,  and  in  general  it  may  be  said  that  the  least  bright  stars  of 
any  magnitude  are  not  distinguishable  from  the  largest  and  brightest  of  the 
class  next  below  them.  The  classification  of  stars  into  magnitudes  is  there- 
fore arbitrary,  and  not  founded  on  any  distinction  really  existing  in  nature. 
Still,  when  properly  understood,  the  classification  of  stars  by  magnitudes  is 
not  without  considerable  utility  as  means  of  record  and  of  reference^  and.  it  is 
accordingly  adopted  by  astronomers  of  every  country. 

The  term  magnitude,  however,  is  applied  to  stars  in  a  very  different  sense 
from  that  in  which  it  is  used  as  applied  to  planets.  The  latter  present,  when 
seen  in  a  telescope,  a  perfectly-defined  disk,  the  diameter  of  which  is  capable 
of  pretty  exact  measurement.  Before  the  invention  of  the  telescope,  stars 
were  supposed  also  to  have  sensible  magnitudes,  and  it  was  an  unanswerable 
argument  against  the  probability  of  the  Copernican  system  that,  admitting 
(which  was  necessarily  supposed),  that  the  fixed  stars  must  be  so  distant  that 
the  entire  orbit  of  the  earth  seen  from  them  would  seem  but  a  point,  their  ap- 
parent magnitude  rendered  it  necessary  to  admit  that  the  largest  of  them  at 
least  must  be  many  limes  larger  than  a  globe  which  would  fill  the   orbit  of 


346 


HOW  TO    OBSERVE    THE   HEAVENS. 


the  earth,  that  is,  than  a  globe  whose  bulk  would  be  above  ten  million  times 
greater  than  that  of  the  sun.  The  telescope  showed,  however,  that  the  ap- 
pearance of  magnitude  was  altogether  illusory  and  dependent  on  atmospheric 
phenomena ;  for,  though  upon  hazy  or  troubled  nights  stars  may  appear 
large,  their  magnitude  is  not  permanent,  but  accompanied  with  a  boiling, 
tremulous,  or  bubbling  outline.  And  in  good  climates  and  still  nights  no 
micrometer  will  give  a  sensible  outline  or  apparent  diameter  to  any  but  large 
stars.  That  such,  when  viewed  through  large  and  powerful  telescopes,  may 
exhibit  some  slight  sensible  magnitude,  may  be  true,  but  it  is  demonstrable  by 
the  admitted  principles  of  optics,  that  even  a  lucid  point,  if  such  could  exist, 
could  never  appear  as  a  mere  -point  through  any  telescope  constlructed  with 
spherical  refracting  or  reflecting  surfaces.  The  term  magnitude,  therefore, 
must  be  understood  as  expressing  merely  apparent  brightness,  which,  not 
being  capable  of  being  exactly  measured  as  to  degree,  must  have  an  indefinite 
application.* 

The  stars  which  have  been  placed  in  the  first  class  of  magnitude  amount  to 
about  twenty  in  number,  and  these  difl'er  from  each  other  considerably  in  ap- 
parent splendor.  Let  any  one  look  at  the  Dog-star,  and  then  immediately  turn 
his  eye  to  «  JJrs<B  Majoris,  or  to  -'  Orionis,  and  he  will  be  immediately  con- 
scious of  this.  If  the  brightness  of  the  latter  be  expressed  by  100,  Sir  John 
Herschel  estimates  that  of  the  Dog-star  at  324. 

If  the  liitightness  of  a  star  of  the  sixth  magnitude  (the  smallest  distinctly 
visible  to  ordinary  eyes  without  a  telescope),  be  supposed  to  be  expressed  by 
1,  the  brightness  of  those  of  superior  magnitudes  will,  according  to  Sir  William 
Herschel,  be  expressed  as  follows  : — 

Brightness  of  a  star  of  the  average  6th  magnitude 1 

Ditto  ditto  5th 2 

Ditto  ditto  4th 6 

Ditto  ditto  3d 12  (  ?  ) 

Ditto  ditto  2d 25 

Ditto  ditto  1st 100 

Of  these  estimates,  that  astronomer  considered  the  third  as  doubtful,  the  others 
more  exact. 

We  have  already  observed  that  the  telescope  augments  our  range  of  vision 
by  rendering  perceptible  stars  which  are  lost  to  the  eye  by  reason  of  their  dis- 
tance. It  also  multiplies  the  objects  visible  to  us,  even  in  the  radius  which  cir- 
cumscribes stars  of  the  sixth  magnitude.  It  may  however  be  asked,  how  it 
is  that  the  telescope  can  effect  this,  seeing  that  it  is  incapable  of  presenting 
any  star,  even  the  largest,  to  the  eye,  as  anything  but  a  lucid  point,  without 
definite  outline  or  appreciable  magnitude.     Let  us  therefore  explain  this. 

In  the  front  of  the  eyeball  is  a  colored  annular  membrane  called  the  iris, 
in  the  centre  of  which  appears  a  circular  black  spot.  This  spot  is  not  a  black 
substance,  but  is  an  aperture  which  seems  black  only  because  the  chamber 
within  it  is  dark.  This  aperture  is  the  window  of  the  eye  provided  for  the 
admission  of  light.  On  the  inside  of  the  eye,  lining  the  inner  surface  opposite 
this  window,  is  the  membrane  called  the  retina,  endowed  with  a  specific  sen- 
sibility, in  virtue  of  which,  when  light  strikes  upon  it,  an  effect. is  conveyed 
by  the  nerves  therewith  connected  to  the  seat  of  sensation,  by  which  vision  is 
effected. 

The  sensibility  of  the  retina  is  limited.     Light  may  act  on  it  so  slightly  as 

to  produce  no  perception.     To  produce  vision,  therefore,  it  is  not  enough  that 

light  be  admitted  through  the  pupil ;  it  must  enter  in  sufficient  quantity.     Let 

us  then   suppose  the  eye   directed   toward  a  luminous  object  such  as  a  star. 

*  See  De  Morgan  on  the  Maps  of  the  Stars,  p.  79. 


HOW  TO    OBSERVE   THE   HEAVENS. 


347 


Th 


e  quantity  of  light  which  will  enter  the  eye  will  depend  conjointly  on  the 
inagnitiide  of  the  pupil  and  the  density  of  the  light.  If  sufficient  light  to  pro- 
duce vision  do  not  enter  the  pupil,  there  are  two  and  only  two  ways  to  make 
it  sufficient.  W,e  must  either  enlarge  the  pupil,  or  augment  the  density  of  the 
light  so  as  to  send  in  through  the  unenlarged  aperture  an  increased  quantity. 

Since  the  density  of  the  light  which  diverges  from  a  visible  object  dimin- 
ishes in  a  very  high  proportion  as  the  distance  from  the  object  is  increased, 
we  can  increase  the  density,  and  thereby  render  an  invisible  object  visible  by 
diminishing  our  distance  from  it ;  that  is,  by  approaching  nearer  to  it.  This 
expedient  every  one  is  familiar  with,  but  it  is  an  expedient  not  practicable  by 
a  creature  whose  movements  are  limited  to  the  surface  of  the  earth. 

Since  we  can  not  then  approach  the  object,  we  must  see  whether  we  may 
not  enlarge  the  window  by  which  light  is  admitted  to  the  pupil.  This  the 
telescope  has  happily  accomplished.  In  fig.  9,  a  star  is  represented  with  a 
diverging  cone  of  light  proceeding  from  it  toward  the  eye.     The  number  of 

Fig.  9.  Fig.  10.  Fig.  11. 


348 


HOW  TO  OBSERVE  THE  HEAVENS. 


these  diverging  ravs  which  will  enter  the  eye  is  limited  by  the  magnitude  of 
the  pupil.  But  before  they  reach  the  eye  they  may  be  received  upon  a  glass 
lens  of  a  convex  form  (fig.  10),  which  will  have  the  effect  of  collecting  them 
into  a  space  less  in  magnitude  than  the  pupil  of  the  eye.  If  the  eye  be  placed 
w^here  the  rays  are  thus  collected,  all  the  light  transmitted  by  the  lens  will 
enter  the  pupil. 

Now  let  us  see  what  will  thus  be  effected.  The  object-lenses  of  some 
telescopes  are  above  ten  inches  diameter.  Most  common  telescopes,  however, 
are  much  smaller.  vSuppose,  for  example,  the  lens  has  five  inches  diameter, 
and  taking  the  diameter  of  the  pupil  at  a  quarter  of  an  inch,  the  ratio  of  these 
diameters  would  be  20  to  l,and  consequently  the  superfices  of  the  lens  would 
be  four  hundred  times  greater  than  the  opening  of  the  pupil,  and  would  there- 
fore admit  four  hundred  times  more  light.  A  lens  of  ten  inches  diameter, 
having  a  surface  four  times  greater  than  one  of  five  inches,  would  therefore 
admit  sixteen  hundred  times  more  light  than  the  pupil. 

What  is  effected  by  a  convex  lens,  as  represented  in  fig.  10,  may  also  be 
accomplished  by  a  concave  reflector,  as  represented  in  fig.  11.  In  the  one 
case  the  light  is  transmitted  through  the  surface  which  receives  it — in  the 
other  it  is  reflected  from  it.  In  the  one  case  the  eve  which  receives  the  light 
is  placed  behind  the  lens  and  directed  toioard  the  object — in  the  other  it  is 
placed  before  the  reflector,  and  looking  in  a  direction  opposite-  to  that  of  the 
object.  The  observer  turns  his  face  to  the  object  in  the  one  case,  his  back  in 
the  other. 

But  in  the  practical  realization  of  this,  there  are  two  circumstances  to  be 
taken  into  account.  First — There  is  no  body  which  is  capable  of  perfectly 
transmitting  or  reflecting  light ;  that  is  to  say,  there  is  none  which  will  either 
transmit  or  reflect  all  the  light  which  strik6s  upon  it.  Light  is  then  lost  in  a 
greater  or  less  proportion  whenever  refraction  or  reflection  takes  place.  If 
this  loss  of  light  were  in  the  same  proportion  as  that  of  the  mag;nitude  of,  the 
lens  or  reflector  to  that  of  the  pupil,  then  nothing  would  be  gnined  by  the  op- 
tical expedient  above  explained.  But  such  is  not  the  case.  Although  a  cer- 
tain proportion  of  the  rays  incident  on  a  lens  fail  to  pass  through  it,  and  a  much 
greater  proportion  of  those  incident  on  a  reflector  fail  to  be  regularly  reflected 
from  it,  yet  even  the  highest  of  these  proportions  of  loss  is  incomparably  less 
than  the  proportion  in  which  the  light  is  condensed. 

Secondhj — The  eye  in  general  has  been  so  constituted  by  its  Maker,  as  to 
be  capable  of  producing  distinct  vision  only  when  the  rays  which  enter  the 
pupil  from  any  point  of  a  visible  object,  are  parallel,  or  nearly  so.  Now,  when 
the  rays  are  collected  by  either  of  the  expedients  above  explained,  they  will 
first  converge  to  a  focus,  and  afterward  diverge  from  it.  If  the  eye  be  placed 
within  the  focus,  it  will  receive  converging  rays,  if  without  it,  diverging  rays  ; 
and  in  either  case  vision  would  be  indistinct.  This  difficulty  is  surmounted 
by  placing  between  the  eye  and  the  rays  collected  into  a  focus  a  small  lens, 
which  may  be  either  convex  or  concave,  and  which  is  so  adapted  that  it  will 
render  the  rays  parallel,  without  affecting  them  in  any  other  way. 

Such  is  the  combination  which  forms  an  astronomical  telescope. 

By  an  instrument  of  this  kind,  then,  we  accomplish  what  is  equivalent  to  an  ' 
enlargement  of  the  pupil,  and  objects  which  transmit  light  so  attenuated  as  to  \ 
be  incapable  of  affecting  the  retina  with  sufficient  energy  to  produce  vision,  ' 
may  by  such  means  be  rendered  visible.  If,  for  example,  the  quantity  of  light  i 
received  by  the  pupil  from  any  distant  star  be  ten  times  less  than  that  which  ' 
would  be  necessary  to  produce  vision,  such  a  star  will  become  visible  in  a  ( 
telescope  whose  object-glass  is  capable  of  condensing  the  light  so  as  lo  render  | 
it  ten  times  more  intense.  < 


HOW  TO    OBSERVE    THE   HEAVENS, 


From  what  has  been  explained,  it  will  be  apparent  that  the  telescope,  while 
it  is  incapable  of  exhibiting  to  us  even  the  nearest  of  the  stars  with  any  sen- 
sible magnitude,  may  however  be  applied  with  success  to  obtain  an  approxi- 
mate estimate  of  the  relative  distances  of  those  stars  which,  by  reason  of  their 
remoteness,  are  invisible  without  its  aid.  By  applying  proper  principles  of 
calculation,  it  is  easy  to  determine  the  magnitude  of  the  telescope  which  will 
double  or  treble,  or,  in  short,  which  will  augment  the  range  of  the  natural  eye 
in  any  required  proportion.  Thus,  if  we  assume  that  the  smallest  star  visible 
to  the  naked  eye,  is  at  a  distance  over  which  light  would  take  ten  years  to 
pass,  we  can  find  the  magnitude  of  the  lenses  or  reflectors  which  would  enable 
us  barely  to  perceive  similar  stars  at  the  distance  which  light  would  take 
twenty  years  to  move  over  ;  and  then,  by  constantly  enlarging  the  opening  of 
the  instrument,  or  what  is  the  same,  by  using  successively  telescopes  of  in- 
creased powers,  we  may  bring  into  view  objects  whose  distances  (supposing 
their  real  magnitude  and  brightness  to  be  the  same  in  the  main),  are  greater 
and  greater  in  known  or  calculable  proportions. 

Sir  William  Herschel  actually  practised  this  method  of  sounding  the  heavens. 
He  classed  the  stars  visible  to  the  naked  eye  in  twelve  orders  of  distance,  those 
of  the  twelfth  order  (or  smallest),  being  twelve  times  more  distant  than  those 
of  the  first  order.  A  telescope  which  would  just  render  visible  a  star  twice  as 
distant  as  one  of  the  twelfth  order,  and  which  therefore  would  double  the  range 
of  the  eye,  he  denominated  as  a  telescope  of  the  second  degree  of  space-penetra- 
ting power.  One  which  would  bring  into  view  stars  three  times  more  distant 
t^ian  those  of  the  twelfth  order,  he  called  a  telescope  of  the  third  power,  and 
so  on.  Calculating  in  this  way,  he  found  that  his  great  forty-feet  telescope  had 
a  space-penetrating  power  of  192.  To  reduce  this  power  to  a  still  more  definite 
expression,  let  us  call  the  distance  of  the  brightest  and  nearest  stars  1  ;  that 
of  the  smallest  stars  visible  to  the  naked  eye  will  then  be  12  ;  and  that  of  the 
smallest  stars  which  could  be  distinctly  seen  with  the  forty-feet  telescope 
would  be  192  times  12,  or  2,304.  If  the  distance  of  the  nearest  star  be  such 
as  light  would  take  ten  years  to  move  over,  the  distance  of  the  smallest  stars 
visible  with  this  instrument  would  then  be  such  as  lightVould  take  23,040 
years  to  move  over !  The  mind  is  overwhelmed  by  the  contemplation  of  such 
spaces. 

The  results  of  the  application  of  these  wonderful  instruments  of  stellar  re- 
search in  the  hands  of  Sir  William  Herschel,  will  be  stated  on  another  occa- 
sion. Meanwhile,  every  private  observer,  supplied  with  a  moderately-good 
astronomical  telescope  may,  following  the  example  thus  placed  before  him, 
render  his  labors  profitable  to  science,  by  contributing  to  the  multiplication  of 
those  facts  on  the  comparison  and  classification  of  which  the  extension  of  our 
knowledge  of  the  universe  must  depend. 

Among  the  objects  to  which  the  amateur  can  direct  his  attention  with  most 
advantage,  may  be  mentioned  the  observation  of  periodical  and  double  stars. 
Although  there  is  no  certain  or  accurate  means  of  estimating  the  brightness  of 
stars,  still,  even  such  approximation  as  an  attentive  observer  can  supply  re- 
specting the  changes  of  variable  stars,  is  not  without  its  value.  A  circumstance 
incidental  to  the  astronomical  telescope  has  supplied  a  method  of  determining 
the  relative  quantity  of  light  transmitted  by  different  stars,  which  is  somewhat 
more  accurate  than  naked  estimation.  The  instrument  used  for  measuring 
small  angular  distances  is,  as  we  have  explained  on  another  occasion,  a  system 

<  of  fine  wires  or  threads,  which  are  fixed  or  moveable,  according  to  the  observa- 
l  tions  lo  be  made,  and  which  are  placed  in  or,  near  the  focus  of  the  object-glass 

<  where  the  iina;ie  of  the  star  is  formed.     The  eye-glass  is  in  fact  a  microscope, 
by  which  this  image  is  magnified,  and  by  which,  therefore,  the  threads  or  wires 


are  also  magnified,  which  latter  circumstance  is  a  serious  inconvenience,  but 
one  which  has  been  partially  surmounted  by  using  threads  of  extreme  tenuity. 
If  no  special  means  were  provided  in  the  telescope,  these  threads  would  not 
be  visible  at  night,  for  the  light  of  a  star  would  be  insufficient  to  illuminate 
them.  To  remedy  this,  there  is  an  orifice  near  the  middle  of  the  tube, 
close  to  which  a  lamp  is  placed,  the  light  of  which  is  reflected  on  the  wires 
and  produces  a  general  illumination  of  the  field  of  view.  This  orifice  can  be 
expanded  or  contracted  to  any  desired  extent,  and  may  even  be  altogether 
closed,  so  that  the  illumination  of  the  field  may  be  varied  at  the  discretion  of 
the  observer.  Now  when  two  stars  are  of  such  a  degree  of  brightness  that  an 
opening  may  be  given  to  the  orifice  which  will  produce  such  an  illumination 
of  the  field  as  will  extinguish  them,  we  may  compare  their  brightness  by  com- 
paring those  degrees  of  light,  exposed  to  which  they  become  invisible.  This 
is  still,  however,  but  an  approximation.  It  is  not  only  difficult  to  get  a  lamp 
which  will  always  yield  light  of  the  same  intensity,  and  to  know  whether  any 
given  lamp  be  such  or  not,  but  as  various  stars  are  of  various  colors  and  tints 
of  color,  the  same  lamp  will  extinguish  a  star  of  its  own  color  with  an  opening 
of  the  orifice  by  which  it  will  not  extinguish  an  equally  bright  star  of  a  differ- 
ent color.  Thus  a  red  light  would  extinguish  a  star  of  the  same  red  tint,  while 
a  bluish  star,  even  of  inferior  lustre,  would  continue  to  be  visible  when  ex- 
posed to  it.  It  is  however  by  no  means  impossible  that  a  diligent  and  judicious 
employment  of  lights  of  different  colors  might  be  made  to  add  to  our  knowledge 
of  this  part  of  astronomy;  and  it  is  more  especially  in  such  fields  that  the  pri- 
vate observer  may  become  a  useful  assistant  to  the  public  one.  Let  us  con- 
sider more  fully,  then,  one  or  two  more  of  the  various  ways  in  which  a  person 
fond  of  looking  at  the  heavens,  provided  only  with  a  moderately-good  telescope 
and  micrometer  may  make  himself  useful  even  without  mathematical  knowl- 
edge. 

First,  then,  with  regard  to  the  variation  of  the  fixed  stars  in  magnitude  and 
color.  It  is  evident  that  the  question  whether  a  fixed  star  revolve  on  an  axis 
or  not,  as  our  sun  does,  can  never  be  settled  except  by  some  variations  of  ap- 
pearance presented  by  its  different  parts  as  they  come  one  after  another  under 
the  eye  of  the  observer,  and  also  that  a  regular  succession  of  repeated  appear- 
ances in  a  star,  is  a  very  strong  presumption  of  a  rotation  round  an  axis.  For 
instance,  the  star  0  Persei  will  at  eight  o'clock  on  Monday  evening  appear  as 
a  star  of  the  second  magnitude.  On  Tuesday,  at  midnight,  it  will  be  decidedly 
smaller,  and  on  Wednesday  night  it  will  resume  its  original  magnitude.  If  it 
be  watched  again  on  the  nights  of  Thursday,  Friday,  and  Saturday,  the  same 
succession  of  changes  will  be  observed.  By  repeated  observations  of  this 
kind  properly  compared  together,  it  has  been  calculated  that  the  exact  period 
of  this  succession  of  changes  in  0  Persei  is  two  days,  twenty  hours,  and  forty- 
eight  minutes. 

It  will  probably  be  asked  how  such  accuracy  can  be  attained  when  the 
changes  observed  are  so  gradual  that  it  is  evidently  impossible  to  determine 
even  the  five  minutes  when  the  star  resumes  the  same  degree  of  apparent 
lustre  ?  As  the  answer  to  this  very  pertinent  and  natural  question  involves  a 
point  of  universal  importance  in  almost  every  class  of  astronomical  observa- 
tions, we  shall  explain  it  pretty  fully. 

In  all  cases  of  natural  phenomena  submitted  to  experimental  inquiry,  or  to 
observation,  rough  approximations  are  first  made,  and  these  imperfect  estimates 
afterward  become  the  means  of  obtaining  others  of  greater  accuracy,  and  so  on 
until  the  highest  degree  of  precision  has  been  attained.     As  an   example   of 


3gree  ot  prec 
the  application  and  use  of  this  principle,  let   us  suppose 
year  is  to  be  determined,  that  is,  the  exact  interval 


ipk 

that  the  length  of  the 

me  which  elapses  be- 


HOW  TO    OBSERVE    THE   HEAVENS. 


3.31 


tween  two  successive  returns  of  the  centre  of  the  sun's  disk  to  the  summer 
solstitial  point.  A  single  observation,  however  accurate  it  be,  will  only  give 
a  rough  estimate  of  this,  such  a  one,  for  example,  as  will  be  liable  to  an  error, 
say  of  ten  minutes  of  time,  five  minutes  of  the  error  being  ascribed  to  each  of 
the  two  observations.  Instead  of  observing  two  successive  solstices,  let  us 
now  observe  two  solstices  having  an  interval  of  ten  years  between  them.  It 
might  be  objected  that  to  do  this  we  must  be  supposed  to  know  the  length  of 
the  year,  which  is  the  thing  we  are  in  quest  of.  But  to  this  it  is  answered, 
that  we  only  require  to  be  sure  that  the  interval  between  the  two  solstices 
which  we  observe  is  not  either  a  greater  or  a  less  number  of  years  than  ten. 
Now  although  we  do  not  yet  know  the  exact  length  of  the  year,  yet  we  do 
know  that  it  is  certainly  greater  that  an  eleventh  of  the  interval  between  the 
phenomena  we  observe,  and  consequently  that  the  interval  can  not  be  so  much 
as  eleven  years,  and  that  it  is  less  than  a  ninth,  and  that  therefore  it  must  be 
more  than  nine  years.  But  since,  from  the  nature  of  the  phenomena  observed, 
we  are  sure  that  there  must  be  a  whole  number  of  years  intervening  (subject 
only  to  ten  minutes  error,  five  minutes  at  each  observation),  we  know  that  this 
number  must  be  ten.  We  then  take  the  entire  interval  of  time  between  the 
two  observed  solstices,  and  we  divide  it  by  ten.  The  quotient  will  give  the 
length  of  the  year,  subject  to  an  error  of  a  tenth  part  of  ten  minutes,  that  is, 
subject  to  an  error  only  of  one  minute.  By  this  expedient,  in  fact,  the  sum  of 
the  errors  of  the  two  solstitial  observations  is  divided  among  ten  years,  and 
the  quantity  which  falls  on  a  single  year  is  only  the  tenth  part  of  the  whole 
error. 

This  process  may  be  carried  further.  The  error  being  thus  reduced  to  a 
single  minute,  may  again  be  spread  over  a  still  greater  interval  until  the  length 
of  the  years  be  obtained,  even  in  fractions  of  a  second  of  time. 

The  same  method  is  applicable  to  all  periodical  phenomena  and  among 
others,  to  the  periodical  variations  of  the  stars.  By  the  first  rough  observation 
of  a  single  period,  we  are  enabled  with  certainty  to  recognise  the  number  of 
complete  periods  which  intervene  between  two  similar  phases  of  the  star  ob- 
served with  a  known  interval  of  time  between  them.  We  divide  that  interval 
by  the  number  of  periods,  and  thereby  obtain  a  second  approximation,  which 
enables  us  to  say  with  certainty  how  many  complete  periods  there  are  between 
two  similar  phases  separated  by  a  much  longer  interval  than  the  former  one. 
Dividing  this  as  before  by  the  number  of  periods,  we  obtain  a  still  closer  ap- 
proximation, and  so  on. 

The  double  stars,  which  will  be  fully  noticed  on  another  occasion,  supply 
a  fruitful  and  interesting  field  of  employment  for  the  amateur.  Nor  need  he 
be  discouraged  from  devoting  himself  to  this  labor  by  the  consciousness  of  his 
inability  to  submit  his  raw  observations  to  those  processes  of  calculation  called 
reductions,  which  are  indispensable  to  render  them  ultimately  available  for  the 
high  purposes  of  science.  He  will  not  find  his  labors  neglected  or  con- 
temned. Others,  with  better  means  and  opportunities,  will  take  the  materials 
and  data  which  he  supplies,  and  apply  all  those  calculations  to  them  which 
are  requisite  to  render  them  valuable.  Nor  will  he  lose  a  particle  of  the 
credit  justly  due  to  him ;  for  to  omit  the  record  of  the  name  of  the  observer, 
the  nature  of  the  instrument,  and  the  place  where  the  original  record  of  the 
observations  is  to  be  found,  would  be  to  insure  the  rejection  of  the  results  of 
such  observations  both  abroad  and  at  home.  The  fundamental  observations 
of  double  stars  are  peculiarly  pointed  out  as  the  most  certain  field  for  the  pri- 
vate observer,  because  they  do  not  require  any  astronomical  clock.  The  day 
of  the  observation  is  all  that  is  necessary  to  be  known,  and  accordingly  a 
J  timepiece,  with   its    necessary   accompaniment,  a  transit  instrument,   is   not 


352 


HOW  TO  OBSERVE  THE  HEAVENS. 


wanted.  A  telescope  of  sufficient  power  to  separate  the  two  stars,  and  a 
wire  micrometer,  are  the  necessary  apparatus  :  of  the  principle  of  the  latter 
we  shall  ^ive  a  general  description,  not  entering  into  any  of  the  niceties 
of  its  construction,  and  supposing  throughout  that  the  instrument  is  perfect.* 

The  wire  micrometer  is  an  apparatus  to  be  annexed  to  a  telescope,  such 
that  when  inserted  in  the  tube  the  field  presents  the  usual  appearance  of  a 
himinous  circle  cut  by  four  very  fine  wires  parallel  two  and  two,  the  first  pair 
being  at  right  angles  to  the  second. 

Fig.  12. 


It  is  found  that  the  apparatus  can  be  turned  round  so  as  to  give  any  de- 
sired direction  to  the  wires.  One  pair  of  wires  is  placed  at  a  fixed  distance 
frorri  each  other.  Of  the  other  pair,  one  is  moveable,  so  as  alternately  to  ap- 
proach to  or  recede  from  that  to  which  it  is  parallel,  preserving,  however,  its 
parallelism,  during  the  motion.  In  fact,  the  interval  between  one  of  the  pairs 
of  parallel  wires  can  be  increased  and  diminished  at  pleasure.  This  motion 
is  given  by  a  screw  which  has  a  small  circular  head,  the  edge  of  which  is 
divided  into  a  certain  number  of  divisions,  say  100.  The  threads  of  this 
micrometer-screw  are  so  small  that  a  whole  revolution  of  the  graduated 
carries  the  moveable  wire  toward  or  from  that  to  which  it  is  parallel,  through 
a  very  small  space,  and  if  there  be  one  hundred  divisions  on  the  circumfer- 
ence of  the  head  which  are  sufficiently  distinct  to  be  read  to  a  quarter  of  a 
division,  we  can  ascertain  a  .motion  of  the  wire  which  corresponds  to  the 
four  hundredth  part  of  the  effect  of  one  entire  revolution.  If  we  desire  to 
measure  the  interval  between  two  stars  which  are  near  each  other,  as  is 
always  the  case  with  the  individuals  of  a  double  star,  we  have  now  only  to 
adjust  the  instrument  until  one  of  the  two  stars  moves  (by  the  diurnal  motion 
of  the  heavens  along  the  fixed  wire),  and  then  by  turning  the  screw  adjust 
the  moveable  wire,  so  that  the  other  star  shall  move  along  it.  It  is  then  only 
necessary  to  ascertain  how  many  revolutions  and  parts  of  a  revolution  of  the 
screw  are  necessary  to  bring  the  moveable  wire  to  coincidence  with  the 
fixed  wire.  The  distance  between  the  stars  will  then  be  known,  provided  we 
have  previously  ascertained  what  space  of  the  field  of  view  corresponds  to  one 
revolution  of  the  screw. 

It  might  perhaps   be   imagined,   that  in    the    original    construction    of  the 

■  micrometric  apparatus,  the  screw  would  be  cut  so  that  each  revolution  might 

correspond  to  a  certain  space,  such  as  one  second.     Mechanical  art,  however, 

has  not,  and  probably  never  will  attain  to  ^he  degree   of  perfection   necessary 

to  accomplish  this.     It  is  very  possible  to  cut  a  fine  screw  with  threads  which 

I  in  a  practical  sense   may  be   said  to  be    equal   to  each  other,  but  he  can  not 

'  do  this  and  also  insure  a  result  which  will    make  a  certain  number  of  these 

I  *  De  Morgan,  pp.  81-'i2,  pp.  84-'5,  pp.  94-5 


HOW  TO   OBSERVE    THE   HEAVENS. 


353 


threads  exactly  equal  to  an  inch.  In  short,  he  can  ensure  equality  and  fine- 
ness, but  can  not  confer  upon  the  threads  particular,  definite,  and  exact  di- 
mensions. Nor  is  it  necessary  that  this  object  should  be  attained,  even  were 
it  practicable.  The  observer  being  furnished  with  the  instrument,  each  divis- 
ion of  which  means  something,  can  find  out  from  the  heavens  what  that  some- 
thing is. 

This  is  very  easily  accomplished.  Supposing  the  observer  to  be  provided 
with  a  clock  or  watch  which  beats  seconds  (the  extreme  accuracy  of  a  chro- 
nometer is  not  here  required),  let  him  direct  the  telescope  as  nearly  as 
he  can  to  that  point  of  the  southern  meridian  where  the  equator  intersects 
it.  Very  extreme  accuracy  is  not  required  in  this  adjustment.  Let  him 
then  place  the  fixed  and  moveable  wire  in  a  vertical  position,  and  bring- 
ing them  to  coincidence,  let  him  separate  them  by  giving  ten  complete 
revolutions  to  the  screw.  Let  him  then  watch  the  moment  when  any  par- 
ticular star  is  seen  on  the  first  wire.  This  he  can  determine  (by  listening 
to  the  ticking  of  the  clock),  to  nearly  half  a  second.  Let  him  then  wait 
until,  by  the  diurnal  motion  of  the  heavens,  the  star  comes  upon  the  second 
wire,  and  observe  the  time  it  arrives  there.  He  will  then  know  the  time  the 
star  took  to  pass  from  one  wire  to  the  other.  But  since  the  firmament  makes 
a  complete  revolution  (360°)  in  twenty-four  hours,  it  moves  at  the  rate  of 
15°  per  hour,  or  15"  per  second  of  time.  Let  us  then  suppose  that  the 
time  which  the  star  took  to  pass  from  the  one  wire  to  the  other  was  22^ 
seconds.  The  space  corresponding  to  this  would  be  22.5  x  15  =  337". 5  ; 
which  would  therefore  be  the  space  between  the  wires  corresponding  to  the 
revolutions  of  the  screw.  The  space  corresponding  to  one  revolution  would 
then  be  33". 75,  and  the  space  corresponding  to  one  division  of  the  head  of 
the  screw  would  be  0.33",  or  one  third  part  of  the  second  of  a  degree. 

If  the  observer  be  not  provided  with  a  clock,  or  can  not  conveniently  use 
one,  if  he  has  it,  he  may  still  accomplish  the  object.  Let  him  in  that  case 
direct  the  instrument  to  the  sun  at  or  near  noon,  and  let  him  adjust  the  moveable 
and  fixed  wires  so  that  they  shall  just  touch  the  upper  and  lower  limb  of  the 
sun,  the  position  of  the  wires  being  horizontal.  The  space  between  the  wires 
will  then  correspond  to  the  apparent  diameter  of  the  sun.  By  reference  to 
the  nautical  almanac  (with  which  he  ought  always  to  be  provided),  he  can 
ascertain  the  apparent  diameter  of  the  sun  at  the  time  of  his  observation. 
Suppose  that  this  is  found  to  be  31',  56",  or  1916".  Then  the  interval  be- 
tween the  wires  will  be  1916".  Let  the  screw  be  turned  until -the  moveable 
wire  coincides  with  the  fixed  wire,  and  suppose  the  number  of  turns  and  parts 
of  a  turn  necessary  to  effect  this  is  found  to  be  60  complete  revolutions  and 
12  divisions,  or  6,012  divisions.;  then  1,916  divided  by  6,012  gives  0".3186 
as  the  value  of  each  division,  or  31. "86  of  each  complete  revolution. 

We  shall  now  conclude.  Enough  has  probably  been  said  to  encourage  the 
amateur  observer,  and  to  set  him  on  the  track,  by  the  pursuit  of  which  he  may 
obtain  much  personal  gratification,  some  reputation  in  the  community  of  sci- 
ence and  render  himself  useful  in  the  promotion  of  knowledge.  If  he  begin 
he  will  not  rest  content  with  these  hints,  but  will  call  to  his  aid  other  more 
ample  and  detailed  instructions,  to  be  found  in  the  works  already  referred  to, 
and  in  the  memoirs  published  by  the  diff'erent  scientific  bodies  of  Europe. 


VOTj.  it.- 33 


THE  STELLAR  UNIYERSE. 


FIRST    LECTURE. 


Range  of  Vision. — Augmented  by  the  Telescope. — Periodic  Stars. — Examples  of  this  Class. — 
Various  Hypotheses  to  explain  these  Appearances. — Their  Insufficiency. —  Temporary/  Stars- 
Remarkable  Examples  of  this  Class.^-These  may  possibly  be  periodic  Stars. — Double  Siars.- 
Their  vast  Number. — They  are  phy.sically  connected. — Telescopic  Views  of  them. — How  they 
may  indicate  the  annual  Parallax. — Researches  of  Sir  W.  Herschel. — Discovery  of  the  orbitual 
Motions  of  double  Stars. — Binary  Stars. — Extension  of  Gravitation  to  the  Stars. — Their  elliptic 
Orbits  discovered. — Effects  of  double  and  coloured  Suns. — Proper  Motions  of  the  Stars. — Prob- 
able Motion  of  tlie  Solar  System. — Analysis  of  its  Effects. — Suggestion  of  Mr.  Pond.^ — Indepen- 
dent Motions  of  the  Stars. — Proper  Motions  of  double  Stars. — Probable  Amount  of  the  real  Mo- 
tions of  the  Stars. 


THE    STELLAR  UNIVERSE. 


357 


THE  STELLAR  UNIVERSE. 


(FIRST    LECTURE.) 


The  distances,  probable  magnitudes,  splendor,  and  physical  character  of 
such  of  the  fixed  stars  as  are  visible  without  telescopic  aid,  have  been  already 
explained.*  The  range  of  this  survey  was  shown  to  be  circumscribed  by  a 
sphere,  of  which  the  solar  system  is  the  centre,  and  of  which  the  radius  is  a 
length  which  light,  moving  at  the  rate  of  two  hundred  thousand  miles  per  sec- 
ond, would  take  ten  years  to  traverse.  Such  is  the  limit  which  has  been  im- 
posed on  the  natural  power  of  the  eye.  Beyond  this  distance  the  creation 
was  concealed  from  human  vision  until  the  invention  and  improvement  of  the 
telescope.  That  instrument  has  augmented  the  range  of  observation  and  dis- 
covery in  a  very  high  proportion,  and  has  opened  to  our  examination  realms 
of  space  occupied  by  innumerable  systems,  stretching  to  distances  which  may 
be  pronounced  to  be  infinite,  in  the  only  sense  in  which  that  negative  term 
can  properly  be  used. 

But  besides  bringing  within  the  range  of  the  senses  objects  placed  beyond 
the  limits  of  that  vast  sphere,  the  telescope  has  also  greatly  multiplied  the 
number  of  visible  objects  within  it,  by  enabling  us  to  see  those  whose  minute- 
ness would  have  otherwise  rendered  them  invisible.  Among  those  stars 
which  are  visible  to  the  naked  eye,  there  are  many  thousands  respecting  which 
the  telescope  has  detected  circumstances  of  the  highest  physical  interest,  by 
which  they  have  become  more  closely  allied  with  our  own  system,  and  by 
which  it  is  demonstrated  that  the  same  material  laws  which  coerce  the  planets, 
and  give  stability,  uniformity,  and  harmony,  to  their  motions,  are  also  in  opera- 
tion in  those  remote  regions  of  the  universe.  We  shall  first  notice  some  of 
the  most  remarkable  discoveries  respecting  individuals  among  the  visible  stars, 
and  shall  afterward  explain  those  which  relate  to  the  arrangement  of  the  col- 
lective mass  of  stars  which  compose  the  visible  firmament,  and  the  result  of 
those  researches  which  the  telescope  has  enabled  astronomers  to  make  in  more 
remote  regions  of  the  universe. 

*  See  discourse  on  "  The  Visible  Stars,"  Vol.  I.,  p.  583. 


PERIODIC    STARS. 

The  stars  in  general,  as  tbey  are  stationary  in  their  apparent  positions,  are 
equally  invariable  in  their  apparent  magnitudes  and  brightness.  To  this, 
however,  there  are  several  remarkable  exceptions.  Stars  have  been  observed, 
sufficiently  numerous  to  be  regarded  as  a  distinct  class,  which  exhibit  periodi- 
cal changes  of  appearance.  Some  undergo  gradual  and  alternate  increase  and 
diminution  of  magnitude,  varying  between  determinate  limits,  and  presenting 
these  variations  in  equal  intervals  of  time.  Some  are  observed  to  attain  a  cer- 
tain maximum  magnitude,  from  which  they  gradually  and  regularly  decline 
until  they  altogether  disappear.  After  remaining  for  a  certain  time  invisible, 
they  reappear  and  gradually  increase  till  they  attain  their  maximum  splendor, 
and  this  succession  of  changes  is  regularly  and  periodically  repeated. 

Such  objects  have  been  denominated  periodic  stars.  The  most  remarkable 
of  this  class  is  the  star  called  Omihron,  in  the  neck  of  the  Whale,  which  was 
first  observed  by  David  Fabricius,  on  the  13th  August,  1596.  This  star  re- 
tains its  greatest  brightness  for  about  fourteen  days  ;  being  then  equal  to  a  large 
star  of  the  second  magnitude.  It  then  decreases  continually  for  three  months 
until  it  becomes  invisible.  It  remains  invisible  for  five  months,  when  it  again 
reappears,  and  increases  gradually  for  three  months  until  it  recovers  its  maxi- 
mum splendor.  This  is  the  general  succession  of  its  phases.  Its  entire  pe- 
riod is  about  334  days.  This  period  is  always  the  same,  but  the  gradations 
of  brightness  through  which  it  passes  are  said  to  be  subject  to  variation. 
Hevelius  states  that  in  the  interval  between  1672  and  1676  it  did  not  appear 
at  all. 

The  star  called  Algol,  in  the  head  of  Medusa,  in  the  constellation  of  Perseus, 
aflbrds  a  striking  example  of  the  rapidity  with  which  these  periodical  changes 
sometimes  succeed  each  other.  This  star  generally  appears  as  one  of  the 
second  magnitude  ;  but  an  interval  of  seven  hours  occurs  at  the  expiration  of 
every  sixty-two,  during  the  first  three  and  a  half  hours  of  which  it  gradually 
diminishes  in  brightness  till  it  is  reduced  to  a  star  of  the  fourth  magnitude, 
and  during  the  remainder  of  the  interval  it  again  gradually  increases  until  it 
recovers  its  original  magnitude.  Thus,  if  we  suppose  it  to  have  attained  its 
maximum  splendor  at  midnight  on  the  first  day  of  the  month,  its  changes  would 
be  as  follows  : — 


0 

0 

0 

to 

2 

14 

0 

2 

14 

0 

to 

2 

17 

24 

2 

17 

24 

to 

2 

20 

48 

2 

20 

48 

to 

5 

10 

48 

5 

10 

48 

to 

5 

14 

12 

5 

14 

12 

to 

5 

17 

36 

&c. 


&c. 


It  appears  of  second  magnitude. 
It  decreases  gradually  to  fourth  magnitude. 
It  increases  gradually  to  second  magnitude. 
It  appears  of  second  magnitude. 
It  decreases  to  fourth  magnitude. 
It  increases  to  second  magnitude. 
&c. 


This  star  presents  an  interesting  example  of  its  class,  as  it  is  constantly 
visible,  and  its  period  is  so  short  that  its  succession  of  phases  may  be  fre- 
quently and  conveniently  observed.  It  is  situate  near  the  foot  of  the  constel- 
lation Andromeda  and  lies  a  few  degrees  northeast  of  three  stars  of  the  fourth 
magnitude  which  form  a  triangle.  It  passes  the  meridian  of  New  York  in 
December,  at  about  four  o'clock  in  the  afternoon,  and  may  therefore  be  seen 
toward  the  west  during  the  early  hours  of  the  night. 

Goodricke,  who  discovered  the  periodic  phenomena  of  Alfrol  in  1782,  ex- 
plained these  appearances  by  the  supposition  that  some  opaque  body  revolves 
round  it,  being  thus  periodically  interposed  between  the  earth  and  the  star,  so 
as  to  intercept  a  large  portion  of  its  light.  Whatever  be  their  cause,  these 
phenomena  indicate  an  extraordinary  system  of  rapid  motions  and  changes  in 


THE  STELLAR  UNIVERSE. 


359 


distant  regions  of  the  universe  where,  as  Sir  John  Herschel  observes,  but  for 
such  evidences  we  might  conclude  all  to  be  lifeless.  Our  own  sun  requires 
nine  times  the  period  of  this  star  to  make  a  single  revolution  on  its  axis,  and 
an  opaque  body  sufficiently  large  to  produce  a  similar  temporary  obscuration 
of  it  to  a  distant  observer,  would  require  to  revolve  round  it  in  less  than  four- 
teen hours. 

The  star  called  x  Cygni,  situate  in  the  neck  of  the  Swan,  nearly  equidistant 
from  /S  and  y  Cygni,  affords  another  interesting  example  of  this  class.  The 
period  of  this  star  was  discovered  by  Kirch  in  1687.  When  it  has  its  great- 
est brightness,  it  appears  to  be  of  the  sixth  magnitude,  and  when  least,  it  be- 
comes a  telescopic  star  of  the  eleventh  magnitude.  Its  total  period  is  396 
days  and  21  hours.  It  retains  its  maximum  magnitude  for  a  fortnight.  It 
then  decreases  gradually  for  three  months  and  a  half,  and  afterward  increases 
gradually  during  an  equal  time.  It  does  not  always  attain  the  same  maximum 
brightness,  the  greatest  magnitude  varying  between  the  fifth  and  the  seventh. 

The  following  table  of  periodic  stars,  exhibiting  specimens  of  every  variety 
of  period,  has  been  given  by  Sir  John  Herschel : — 


stars'  Names. 


0  Persei 

6  Cephei 
0  Lyrae 
<T  Antinoi 
a  Herculis 
Serpentis 
RA.  i5'>-  41"- 
PD.  743  15' 
0  Ceti 
y  Cygni 
367  B.*  Hydrse 
34  Fl.  Cygni 
420  M.  Leonis 
K  Sagittarii 
\p  Leonis 


Period. 


D.  H.  M. 

2  20  48 

5  8  37 

6  9  0 

7  4  15 
60  6  0 


180 


334    

396     21       0 

494     

18  years 
Many  years 

Ditto 

Ditto 


Variation  of  Magni- 
tude. 


to 


3.4 
3 

3.4 
3 

7? 

2 
6 
4 
6 
7 
3 
6 


5 
4.5 

4.5 
4 

0 

0 
11 
10 

0 

0 


Discoverers. 


—     0 


5  Goodricke,  1782. 
I  Palitzch,  1783. 
Goodricke,  1784. 
Goodricke,  1784. 
Pigott,  1784. 
Herschel,  1796. 

Harding,  1826. 

Fabricius,  1596. 
Kirch,  1687. 
Maraldi,  1704. 
Janson,  1600. 
Koch,  1782. 
Halley,  1676. 
Montanari,  1667. 


What,  then,  it  will  be  asked,  are  the  probable  or  possible  causes  of  these 
singular  phenomena  ]  Several  explanations,  more  or  less  plausible,  have  been 
proposed. 

1 .  The  phenomena  of  the  spots  on  the  sun  have  afforded  ground  for  the 
supposition  that  the  stars,  being  distant  suns,  may  have  patches  more  or  less 
opaque  on  their  surfaces,  which  being  successively  presented  toward  the  earth 
by  the  rotation  of  the  star  upon  an  axis,  produce  the  effect  of  periodical  varia- 
tion in  brightness  or  apparent  magnitude.  "  Such  a  motion  of  a  star,"  says 
Sir  William  Herschel,  "  may  be  as  evidently  proved,  as  the  diurnal  motion  of 
the  earth.  Dark  spots,  or  large  portions  of  the  surface  less  luminous  than 
the  rest,  turned  alternately  in  certain  directions,  either  toward  or  from  us,  will 
account  for  all  the  phenomena  of  periodical  changes  in  the  lustre  of  the  stars 
so  satisfactorily,  that  we  certainly  need  not  look  for  any  other  cause." 

The  analogy  of  the  spots  on  the  sun,  however,  is  subject  to  an  objection.. 
They  certainly  would  not  render  the  sun  a  periodic  star  to  the  observers  of  a 
distant  system  ;  for,  to  say  nothing  of  their  inconsiderable  magnitude,  com- 
pared with  the  entire  solar  disk,  their  want  of  permanence  and  the  irregu- 
larity of  their  appearance  and  disappearance  would  entirely  preclude  such  an 

*  These  letters,  B.,  Fl.,  and  M.,  refer  to  the  catalogues  of  Bode,  Plamsteed,  and  Mayer. 


360 


THE  STELLAR  UNIVERSE. 


effect.     A  periodic  star  could  be  caused  only  by  considerable  and  permanent 
spots. 

2.  Newton  conjectured  that  the  variation  of  brightness  might  be  produced 
by  comets  falling  into  distant  suns  and  causing  temporary  conflagrations. 
Waiving  any  other  objection  to  this  conjecture,  it  is  put  aside  by  its  insuf- 
ficiency to  explain  the  periodicity  of  the  phenomena. 

3.  Maupertius  has  suggested  that  some  stars  may  have  the  form  of  thin  flat 
disks  ;  acquired  either  by  extremely  rapid  rotation  on  an  axis,  or  other  physical 
cause.  The  ring  of  Saturn  affords  an  example  of  this  within  the  limits  of 
our  own  system,  and  the  modern  discoveries  in  nebular  astronomy  offer  other 
examples  of  a  like  form.  The  axis  of  rotation  of  such  a  body  might  be  subject 
to  periodical  change  like  the  nutation  of  the  earth's  axis,  so  that  the  flat  side 
of  the  luminous  disk  might  be  present  more  or  less  toward  the  earth  at  differ- 
ent times,  and  when  the  edge  is  so  presented  it  might  be  too  thin  to  be  visible. 
Such  a  succession  of  phenomena  are  actually  exhibited  in  the  case  of  the  rings 
of  Saturn,  though  proceeding  from  different  causes. 

4.  Mr.  Dunn*  has  conjectured  that  a  dense  atmosphere  surrounding  the  stars, 
in  different  parts  more  or  less  pervious  to  light,  may  explain  the  phenomena. 
This  conjecture,  otherwise  vague,  indefinite,  and  improbable,  totally  fails  to  ex- 
plain the  periodicity  of  the  phenomena. 

5.  It  has  been  suggested  that  the  periodical  obscuration  or  total  disappear- 
ance of  the  star,  may  arise  from  transits  of  the  star  by  its  attendant  planets. 
The  transits  of  Venus  and  Mercury  are  the  basis  of  this  conjecture. 

The  transits  of  none  of  the  planets  of  the  solar  system,  seen  from  the  stars, 
could  render  the  sun  a  periodic  star.  The  magnitudes  even  of  the  largest  of 
them,  are  altogether  insufficient  for  such  an  effect.  To  this  objection  it  has 
been  answered  that  planets  of  vastly  greater  comparative  magnitude  may  re- 
volve round  other  suns.  But  if  the  magnitude  of  a  planet  were  sufficient  to 
produce  by  its  transit  these  considerable  obscurations,  it  must  be  very  little  in- 
ferior to  the  magnitude  of  the  sun  itself,  or,  at  all  events,  it  must  bear  a  very 
considerable  proportion  to  the  magnitude  of  the  sun  ;  in  which  case  it  may  be 
objected  that  the  predominance  of  attraction  necessary  to  maintain  the  sun  in 
the  centre  of  its  system  could  not  be  secured.  To  this  objection  it  is  an- 
swered, that  although  the  planet  may  have  a  great  comparative  magnitude,  it 
may  have  a  very  small  comparative  density,  and  the  gravitating  attraction  de- 
pending on  the  actual  mass  of  matter,  the  predominance  of  the  solar  mass  may 
be  rendered  consistent  with  the  great  relative  magnitude  of  the  planet  by  sup- 
posing the  density  of  the  one  vastly  greater  than  that  of  the  other.  The  den- 
sity of  the  sun  is  much  greater  than  the  density  of  Saturn. 

6.  It  has  been  suggested  that  there  may  be  systems  in  which  the  central 
body  is  a  planet  attended  by  a  lesser  sun  revolving  round  it  as  the  moon  rer 
volves  round  the  earth,  and  in  that  case  the  periodical  obscuration  of  the  sun 
may  be  produced  by  its  passage  once  in  each  revolution  behind  the  central 
planet. 

Such  are  the  various  conjectures  which  have  been  proposed  to  explain  the 
periodic  stars  and  as  they  are  merely  conjectures,  scarcely  deserving  the  name  of 
hypotheses  or  theories,  we  shall  leave  them  to  be  taken  for  what  they  are  worth. 


TEMPORARY    STARS. 


Phenomena  in  most  respects  similar  to  those  just  described,  but  exhibiting 
no  recurrence,  repetition,  or  periodicity,  have  been  observed  in  many  stars. 
Thus,  stars  have  from  time  to  time  appeared  in  various  parts  of  the  firmament ; 

*  See  PhU.  Trans.,  vol.  52. 


THE  STELLAR  UNIVERSE. 


361 


have  shone  with  extraordinary  histre  for  a  limited  time,  and  have  disappeared 
finally,  never  having  been  again  observed.     Such  are  called  temporary  stars. 

The  first  star  of  this  class  which  has  been  recorded,  is  one  observed  by 
Hipparchus,  125  B.  C,  the  disappearance  of  which  is  said  to  have  led  that 
astronomer  to  make  his  celebrated  catalogue  of  the  fixed  stars,  a  work  which 
has  proved  in  modern  times  of  great  value  and  interest.  In  the  389th  year 
of  our  era,  a  star  blazed  forth  near  a  Aquilm,  which  shone  for  three  weeks,  ap- 
pearing as  splendid  as  the  planet  Venus,  after  which  it  disappeared  and  has 
never  since  been  seen.  In  the  years  945,  1264,  and  1572,  brilliant  stars  ap- 
peared in  the  region  of  the  heavens  between  the  constellations  of  Cejyheus  and 
Cassiopeia.  The  accounts  of  the  position  of  these  objects  are  obscure  and  un- 
certain, but  the  intervals  between  the  epochs  of  their  appearances  being  nearly 
equal,  it  has  been  conjectured  that  they  were  successive  returns  of  the  same 
periodic  star,  the  period  of  which  is  about  300  years,  or  possibly  half  that 
interval. 

The  appearance  of  the  star  of  1572  was  very  remarkable,  and  having  been 
witnessed  by  the  most  eminent  astronomers  of  that  day,  the  account  of  it  may 
be  considered  to  be  well  entitled  to  confidence.  Tycho  Brake,  happening  to  be 
on  his  return  on  the  evening  of  the  1 1th  November  from  his  laboratory  to  his 
dwelling-house,  was  astonished  to  find  a  crowd  of  peasants  gazing  at  a  star 
which  he  was  sure  did  not  exist  half  an  hour  before.  This  was  the  temporary 
star  of  1572.  It  was  then  as  bright  as  the  Dog-star,  and  it  continued  to  in- 
crease in  splendor  until  it  surpassed  Jupiter  when  that  planet  is  most  brilliant, 
and  finally  it  attained  such  a  lustre,  that  it  was  visible  at  mid-day.  It  began 
to  diminish  in  December,  and  altogether  disappeared  in  March,  1574. 

On  the  10th  October,  1604,  a  splendid  star  suddenly  burst  out  in  the  con- 
stellation of  Serpentarius,  which  was  as  bright  as  that  of  1572.  It  continued 
visible  till  October,  1605,  when  it  vanished. 

To  this  class  may  be  referred  the  cases  of  numerous  stars  which  have  dis- 
appeared from  the  firmament.  On  a  careful  examination  of  the  heavens,  and 
a  comparison  of  the  objects  observed  with  former  catalogues,  and  of  catalogues 
ancient  and  modern  with  each  other,  many  stars  formerly  known  are  now  as- 
certained to  be  missing ;  and  although,  as  Sir  John  Herschel  observes,  there 
is  no  doubt  that  in  many  instances  these  apparent  losses  have  proceeded  from 
mistaken  entries,  yet  it  is  equally  certain  that  in  numerous  cases  there  can 
have  been  no  mistake  in  the  observation  or  the  entry,  and  that  the  star  has 
really  existed  at  a  former  epoch,  and  as  certainly  has  since  disappeared. 

When  we  consider  the  vast  length  of  many  of  the  periods  of  astronomical 
phenomena,  it  is  far  from  being  improbable  that  these  phenomena  which  seem 
to  be  occasional,  accidental,  and  springing  from  the  operation  of  no  regular 
physical  causes,  such  as  those  indicated  by  the  class  of  variable  stars  first  con- 
sidered, may  after  all  be  periodic  stars  of  the  same  kind,  whose  appearances 
and  disappearances  are  brought  about  by  similar  causes.  All  that  can  be  cer- 
tainly known  respecting  them  is,  that  they  have  appeared  or  disappeared  once 
in  that  brief  period  of  time  within  which  astronomical  observations  have  been 
made  and  recorded.  If  they  be  periodic  stars,  the  length  of  whose  period  ex- 
ceeds that  interval,  their  changes  could  only  have  been  once  exhibited  to  us, 
and  after  ages  have  rolled  away,  and  time  has  converted  the  future  into  the 
past,  future  astronomers  may  witness  the  next  occurrence  of  their  phases,  and 
discover  that  to  be  regular,  harmonious,  and  periodic,  which  appears  to  us  ac- 
cidental, occasional,  and  anomalous. 


362 


THE   STELLAR  UNIVERSE. 


DOUBLE  STARS. 

When  the  stars  are  examined  individually  by  telescopes  of  a  certain  power, 
it  is  found  that  many  which  to  the  naked  eye  appear  to  be  single  stars,  are  in 
reality  two  stars  placed  so  close  together  that  they  appear  as  one.  These  are 
called  double  stars. 

A  very  limited  number  of  these  objects  had  been  discovered  before  the 
telescope  had  received  the  vast  accession  of  power  which  was  given  to  it  by 
the  labor  and  genius  of  Sir  William  Herschel.  That  astronomer  observed  and 
catalogued  five  hundred  double  stars,  and  subsequent  observers,  among  whom 
his  son.  Sir  John  Herschel,  holds  the  foremost  place,  have  augmented  the 
number  to  six  thousand. 

The  close  apparent  juxtaposition  of  two  stars  on  the  firmament  is  a  phe- 
nomenon which  might  be  easily  explained,  and  which  could  create  no  surprise. 
Such  an  appearance  would  be  produced  by  the  accidental  circumstance  of  the 
lines  of  direction  of  the  two  stars  as  seen  from  the  earth,  forming  a  very  small 
angle,  in  which  case,  although  the  two  stars  might  in  reality  be  as  far  removed 
from  each  other  as  any  stars  in  the  heavens,  they  would  nevertheless  appear 
close  together.  The  annexed  diagram,  fig.  1,  will  render  this  easily  under- 
stood. Let  a  and  b  be  the  two  stars  seen  from  c.  The  star  a  will  be  seen 
relatively  to  b,  as  if  it  were  at  d,  and  the  two  objects  will  seem  to  be  in  close 
juxtaposition,  and  if  the  angle  under  the  lines  c  a  and  c  i  be  less  than  the 
sum  of  the  apparent  semi-diameters  of  the  stars,  they  would  actually  appear 
to  touch. 


If  such  objects  were  few  in  number,  this  mode  of  explaining  them  might  be 
admitted  ;  and  such  may  in  fact  be  the  cause  of  the  phenomenon  in  some  in- 
stances. The  chances  against  such  proximity  of  the  lines  of  direction  are  so 
great  as  to  be  utterly  incompatible  with  the  vast  number  of  double  stars  that 
have  been  discovered,  even  were  there  not,  as  there  is,  other  conclusive  proof 
that  this  proximity  and  companionship  is  neither  accidental  nor  merely  ap- 
parent, but  that  the  connexion  is  real,  and  that  the  objects  are  united  by  a 
physical  bond  analogous  to  that  which  attaches  the  planets  to  the  sun. 

Among  the  most  striking  examples  of  double  stars,  may  be  mentioned  the 
bright  star  Castor,  which,  when  sufficiently  magnified,  is  proved  to  consist  af 
two  stars  between  the  third  and  fourth  magnitudes,  within  five  seconds  of  each 
other.  There  are  many,  however,  which  are  separated  by  intervals  less  than 
one  second,  such  as  c  Arietis,  Atlas  Pleiadum,  y  Corona,  n  and  ^  Herculis,  and 
T  and  ^  Ophiuchi. 

The  telescopic  appearance  of  double  stars  may  be  conceived  from  diagrams 
of  some  of  the  more  remarkable  of  the  class,  which  have  been  given  by  Dr. 
Dick,  in  his  work  on  the  heavens. 

Fig.  2  represents  a  telescopic  view  of  «  Bootis,  with  a  magnifying  power 
of  200.  This  is  considered  to  be  a  very  beautiful  double  star,  consisting  of  a 
small  and  large  one,  the  former  blue,  and  the  latter  red.  The  smaller  star  ap- 
pears about  one  third  of  the  size  of  the  larger,  and  separated  from  it  by  a  space 
equal  to  the  diameter  of  the  larger  star. 


THE   STELLAR  UNIVERSE. 


363 


Fig.  3  is  1  Herculis.  It  consists  of  a  large  and  small  star  separated  by  a 
space  equal  to  twice  the  diameter  of  the  larger.  The  smaller  star  is  blue,  and 
the  larger  white.  This  object  is  situated  in  the  head  of  the  constellation  Her- 
cules, about  thirty  degrees  southwest  of  the  conspicuous  star  a  Lyr(B,  and  six 
degrees  northwest  of  Ras  Alhague,  a  star  of  nearly  equal  magnitude. 


Fig.  4  is  a  view  of  y  Andromedcs :  the  small  star  is  of  a  fine  greenish-blue 
color,  separate  from  the  large  star  about  nine  seconds,  or  four  diameters  of  that 
star ;  the  larger  star  is  of  a  reddish-white.  It  is  situated  in  the  left  foot  of 
Andromeda,  and  is  distinguished  by  the  name  o[  Almaach.  It  is  a  star  of  the 
second  magnitude,  about  forty-two  degrees  of  north  declination.  It  is  about 
twelve  degrees  nearly  due  west  from  the  variable  star  Algol. 

Fig  5  is  ?  Cygni :  the  smaller  star  is  blue,  and  they  are  separated  about  ten 
diameters.  This  star  is  situated  in  the  eastern  wing  of  the  Swan  ;  right  as- 
cension, 21h.4m,north  declination  twenty-eight  degrees,  and  is  about  twenty 
degrees  southeast  of  Denib,  the  principal  star  of  this  constellation. 

Fig.  6  represents  i  Aquarii.  The  two  stars  are  nearly  equal  in  apparent 
magnitude,  and  one  diameter  and  a  half  separate  from  each  other ;  both  stars 
are  of  a  whitish  color.  It  is  in  the  middle  of  other  three  stars,  which  together 
form  a  figure  resembling  the  letter  Y.  Its  right  ascension  is  22h.20m,and  its 
south  declination  about  two  degrees.     It  is  a  star  of  about  the  third  magnitude. 

Fig.  7  represents  the  Pole-star.  The  accompanying  star  is  a  very  faint 
point,  and  requires  an   accurate  telescope  with  considerable  power  to  distin- 


364 


THE  STELLAR  UNIVERSE. 


giiish  it.  The  large  star  is  white,  and  the  small  star  somewhat  of  a  ruddy  ap- 
pearance, and  is  distant  from  the  larger  seventeen  seconds,  or  about  three  or 
four  of  its  diameters. 

Fig.  8  is  the  double  star  Castor.  The  smaller  star  is  nearly  half  the  size 
of  the  larger,  and  they  are  distant  about  five  seconds,  or  two  diameters  of  the 
principal  star.  They  are  both  of  a  whitish  color.  Castor  and  Pollux  lie  to 
the  northwest  of  Orion,  at  a  considerable  distance  from  it.  They  are  very 
conspicuous,  are  within  five  degrees  of  each  other,  and  rise  to  a  very  high 
elevation  when  passing  the  meridian,  and  may  be  seen  throughout  the  whole 
winter  and  spring  months.     Castor  is  the  more  elevated  of  the  two. 

Fig.  9  represents  Rigel,  a  splendid  star  in  the  left  foot  of  Orion.  The  small 
star  is  a  mere  point,  and  very  difficult  to  be  distinguished,  and  is  three  or  four 
diameters  of  the  large  star  from  it.  The  large  star  is  white,  the  small  one  of 
a  reddish  hue. 

Fig.  10  shows  the  double  star  Castor,  with  a  magnifying  power  of  300.  It 
likewise  shows  the  angular  position  of  the  small  star  at  the  present  time  in 
respect  to  Pollux  (fig.  11),  by  which  it  appears  that  it  is  nearly  at  a  right  angle 
to  a  line  joining  Castor  and  Pollux,  whereas  in  the  time  of  Dr.  Bradley  it  was 
parallel  with  a  line  joining  these  two  stars. 

Figs.  12,  13,  14,  and  15,  exhibit  views  of  the  double  star  «  Bootis,  with  four 
magnifying  powers.  Fig.  12  is  its  appearance  with  a  power  of  227;  fig.  13, 
with  a  power  of  460  ;  fig.  14,  with  a  power  of  900  ;  and  fig.  15  with  a  power 
of  1,100. 

Figs.  16,  17,  and  18,  represent  telescopic  views  of  the  triple  star  in  the  left 
fore  foot  of  the  constellation  Monoceros,  or  the  Unicorn,  which  forms  a  very 
beautiful  object  in  this  class  of  stars.  This  star  appears  at  first  double,  but 
with  some  attention  one  of  the  two  is  discovered  to  be  also  double ;  the  first 
of  them  is  the  largest.  The  color  of  all  these  stars  is  white.  With  a  small 
power  they  appear  as  in  fig.  16 :  with  a  power  of  220,  as  in  fig.  17 ;  and  with 
a  power  of  450,  as  in  fig.  18.  There  is  a  beautiful  object  of  this  description, 
but  somewhat  different  in  the  configuration  of  the  three  stars  of  which  it  is 
composed,  to  be  seen  in  the  tail  of  the  Great  Bear ;  it  is  the  star  ^  TJrsm,  called 
also  Mizar,  and  is  the  middle  star  in  the  tail. 

All  the  objects  here  enumerated  may,  be  seen  with  a  good  three  and  a  half 
feet  achromatic  telescope,  with  an  object-glass  of  2f  inch  diameter.  The 
double  star  Castor  may  be  seen  with  so  low  a  power  as  80^  but  more  distinctly 
with  higher  magnifiers. 

Fig.  19. 


,;i':::::==ii  A 


=#B 


When  the  attention  of  astronomers  was  first  attracted  to  double  stars,  it  was 
thought  they  would  afford  a  most  promising  means  of  determining  the  annual 
parallax,  and  thereby  discovering  the  distance  of  the  stars.  If  we  suppose  the 
two  individuals  composing  a  double  star,  being  situate  very  nearly  in  the  same 
direction  as  seen  from  the  earth,  to  be  at  very  different  distances,  it  might  be 
expected  that  their  apparent  relative  position  would  vary  at  different  seasons 
of  the  year,  by  reason  of  the  change  of  position  of  the  earth. 

Let  A  and  B,  fig.  19,  represent  the  two  individuals  composing  a  double  star. 
Let  C  and  D  represent  two  positions  of  the  earth  in  its  annual  orbit,  separated 
by  an  interval  of  half  a  year,  and  placed  therefore  on  opposite  sides  of  the 


THE  STELLAR  UNIVERSE. 


!  sun  S.     When  viewed  from  C,  the  star  B  will  be  above  the  star  A,  and  when 
'  viewed  from  D,  it  will   be  below  it.     During  the  intermediate  six  months  the 
!  relative  change  of  position  would  gradually  be  effected,  and  the  one  star  would 
thus  appear  either  to  revolve  annually  round  the  other,  or  would  oscillate  semi- 
!  annually  from  side  to  side  of  the  other.     The  extent  of  its  play  compared  with 
'  the  diameter  C  D  of  the  earth's  orbit  would  supply  the  data  necessary  to  de- 
termine the  proportion  which  the  distance  of  the  stars  would  bear  to  that  di- 
ameter. 

The  great  problem  of  the  stellar  parallax  seemed  thus  to  be  reduced  to  the 
measurement  of  the  small  interval  between  the  individuals  of  double  stars  ;  and 
it  happened,  fortunately,  that  the  micrometers  used  in  astronomical  instruments 
were  capable  of  measuring  these  minute  angles  with  much  greater  relative  ac- 
curacy than  could  be  attained  in  the  observations  on  greater  angular  distances. 
To  these  advantages  were  added  the  absence  of  all  possible  errors  arising 
from  refraction ;  errors  incidental  to  the  graduation  of  instruments ;  from  un- 
certainty of  levels  and  plumb-lines  ;  from  all  estimations  of  aberration  and  pre- 
cession ;  in  a  word,  from  all  effects  which,  equally  affecting  both  the  individual 
stars  observed,  could  not  interfere  with  the  results  of  the  observations,  what- 
ever they  might  be. 

These  considerations  raised  great  hopes  among  astronomers  that  the  means 
were  in  their  hands  to  resolve  finally  the  great  problem  of  the  stellar  parallax, 
and  Sir  William  Herschel  accordingly  engaged  with  all  his  characteristic 
ardor  and  sagacity  in  an  extensive  series  of  observations  on  the  numerous 
double  stars,  to  the  original  discovery  of  which  science  was  already  so  deeply 
indebted  to  his  labors.  He  had  not,  however,  proceeded  far  in  his  researches, 
when  phenomena  unfolded  themselves  before  him  indicating  a  discovery  of 
a  much  higher  order  and  interest  than  that  of  the  parallax  which  he  sought. 
He  found  that  the  relative  position  of  the  individuals  of  many  of  the  double 
stars  which  he  examined  were  subject  to  a  change,  but  that  the  period  of  this 
change  had  no  relation  to  the  period  of  the  earth's  motion.  It  is  evident  that 
whatever  appearances  can  proceed  from  the  earth's  annual  motion,  must  be  not 
only  periodic  and  regular,  but  must  pass  annually  through  the  same  series  of 
phases,  always  showing  the  same  phase  on  each  return  of  the  same  epoch  of 
the  sidereal  year.  In  the  changes  of  position  which  Sir  William  Herschel  ob- 
served in  the  double  stars,  no  such  series  of  phases  presented  themselves. 
Periods,  it  is  true,  were  soon  developed,  but  these  periods  were  regulated  by 
intervals  which  neither  agreed  with  each  other  nor  with  the  earth's  annual 
motion. 

Some  other  explanation  of  the  phenomena  must  therefore  be  sought  for,  and 
the  illustrious  observer  soon  arrived  at  the  conclusion  that  these  apparent 
changes  of  position  were  due  to  real  motions  in  the  stars  themselves ;  that 
these  stars  in  fact  moved  in  proper  orbits  in  the  same  manner  as  the  planets 
moved  around  the  sun.  The  slowness  of  the  succession  of  changes  which 
were  observed  rendered  it  necessary  to  watch  their  progress  for  a  long  period 
of  time  before  the  motions  of  these  bodies  could  be  certainly  or  accurately 
known  ;  and  accordingly,  although  these  researches  were  commenced  in  1778, 
it  was  not  until  the  year  1803  that  the  observer  had  collected  data  sufficient 
to  justify  any  positive  conclusion  respecting  their  orbitual  motion.  In  that  and 
the  following  year  Sir  William  Herschel  announced  to  the  Royal  Society,  in 
two  memorable  papers  read  before  that  body,  that  there  exist  sidereal  systems 
consisting  of  two  stars  revolving  about  each  other  in  regular  orbits,  and  consti-  \ 
tuting  what  he  called  binary  stars,  to  distinguish  them  from  double  stars,  gene-  ' 
rally  so  called,  in  which  no  such  periodic  change  of  position  is  discoverable.  ( 
Such  stars  may  be  only  accidentally  double,  and,  as  we  have  already  explained,  * 


THE   STELLAR  UNIVERSE. 


may  be  as  distant  from  each  other  as  any  other  stars  in  the  firmament,  notwith- 
standing their  apparent  juxtaposition.  But  the  individuals  of  a  binary  star  are 
at  the  same  distance  from  the  eye  in  the  same  sense  in  which  the  planet 
Uranus  and  its  attendant  satellites  are  said  to  be  at  the  same  distance. 

More  recent  observation  has  fully  confirmed  these  remarkable  discoveries. 
The  catalogue  of  binary  stars  first  given  by  Sir  William  Herschel,  consisting 
of  from  fifty  to  sixty,  comprises  nearly  all  the  most  considerable  objects  of  that 
class  that  have  yet  been  detected.  These  stars  require  the  best  telescopes  for 
their  observation,  being  generally  so  close  as  to  render  the  use  of  very  high 
magnifying  powers  indispensable. 

The  moment  the  revolution  of  one  star  round  another  was  ascertained,  the 
idea  of  the  possible  extension  of  the  great  principle  of  gravitation  to  these  re- 
mote regions  of  the  universe  naturally  suggested  itself.  Newton  has  proved 
in  his  Principia,  that  if  a  body  revolve  in  an  ellipse  by  an  attractive  force 
directed  to  the  focus,  that  force  will  vary  according  to  the  law  which  charac- 
terizes gravitation.  Thus  an  elliptical  orbit  became  a  test  of  the  presence  and 
sway  of  the  law  of  gravitation.  If,  then,  it  could  be  ascertained  that  the 
orbits  of  the  double  stars  were  ellipses,  we  should  at  once  arrive  at  the  fact 
that  the  law  of  which  the  discovery  conferred  such  celebrity  on  the  name  of 
Newton  is  not  confined  to  the  solar  system,  but  prevails  throughout  the  uni- 
verse. 

The  first  distinct  system  of  calculation  by  which  the  true  elliptic  elements 
of  the  orbit  of  a  binary  star  were  ascertained,  was  supplied  in  1830,  by  M. 
Savary,  who  showed  that  the  motion  of  one  of  the  most  remarkable  of  these 
stars  (I  UrscB  Majoris),  indicated  an  elliptic  orbit  described  in  58-1-  years.  Pro- 
fessor Encke,  by  another  process,  arrived  at  the  fact  that  the  star  70  Ophiuchi, 
moved  in  an  ellipse  with  a  period  of  74  years.  Several  other  orbits  were  as- 
certained and  computed  by  Sir  John  Herschel.  In  the  following  table,  given 
by  that  astronomer,  are  exhibited  the  principal  discoveries  in  this  branch  of 
astronomy : — 


Names  of  Stars. 

Period  of  Revolu- 
tion. 

Major  semi-cixis  of 
Ellipse. 

Eccentricity. 

Years. 

y  Leonis                   - 

1200 

y  Virginis                 - 

628-9000 

12"-090 

0-83350 

61  Cygni 

452- 

15-430 

0-  Coronffi                  - 

286-6000 

3-679 

0-61125 

Castor                      - 

252-6600 

8-086 

8' 75820 

70  Ophiuchi 

80-3400 

4-392 

0-46670 

1  Ursae                      - 

58-2625 

3-857 

0-41640 

^  Cancri                   — 

55? 

XI  Coronae                  — 

43-4000 

The  most  remarkable  of  these,  says  Sir  John  Herschel,  is  y  Virginis;  not 
only  on  account  of  the  length  of  its  period,  but  by  reason  also  of  the  great 
diminution  of  apparent  distance,  and  rapid  increase  of  angular  motion  about 
each  other,  of  the  individuals  composing  it.  It  is  a  bright  star  of  the  fourth 
magnitude,  and  its  component  stars  are  almost  exactly  equal.  It  has  been 
known  to  consist  of  two  stars  since  the  beginning  of  the  eighteenth  century, 
their  distance  being  then  between  six  and  seven  seconds ;  so  that  any  tolera- 
bly good  telescope  would  resolve  it.  Since  that  time  they  have  been  con- 
stantly approaching,  and  are  at  present  hardly  more  than  a  single  second  asun- 
der ;  so  that  no  telescope  that  is  not  of  very  superior  quality,  is  competent  to 
show  them  otherwise  than  as  a  single  star  somewhat  lengthened  in  one  direc- 
tion.    It  fortunately  happens,  that  Bradley,  in  1718,  noticed  and  recorded  in 


THE  STELLAR  UNIVERSE. 


the  margin  of  one  of  his  observation-books,  the  apparent  direction  of  their  line 
of  junction,  as   being  parallel  to  that  of  two  remarkable  stars  a  and  ^  of  the 
same  constellation,  as  seen  by  the  naked  eye ;  and  this  note,  which  has  been 
rescued  from  oblivion   by  the  diligence  of   Professor  Rigaud,  has  proved  of 
signal   service  in  the  investigation  of  their  orbit.     They  are  entered  also  as 
distinct  stars  in  Mayer's  catalogue ;  and  this  affords  also  another  means  of  re- 
,  covering  their  relative  situation  at  the  date   of  his  observations,  which  were 
made  about  the  year  1756.     Without  particularizing  individual  measurements, 
which  will  be  found  in  their  proper  repositories,  it  will  suffice  to  remark,  that 
their  whole  series  (w^hich  since  the  beginning  of  the  present  century  has  been 
very  numerous  and  carefully  made,  and  which  embraces  an  angular  motion  of 
100°,  and  a  diminution  of  distance  to  one  sixth  of  its  former  amount)  is  repre- 
sented with  a  degree  of  exactness  fully  equal  to  that  of  ohservation  itself,  by  an 
ellipse  of  the  dimensions  and  period  stated  in  the  following  little  table,  and  of 
which  the  further  requisite  particulars  are  as  follows : — 
Perihelion  passage.  August  38,  1834. 

Inclination  of  orbit  to  the  visual  ray,  ......  22"  58' 

Angle  of  position  of  the  perihelion  projected  on  the  heavens,         -  -         36°  24' 

Angle  of  position  of  the  line  of  nodes,  or  intersection  of  the  plane  of  the  )  q„o  „„, 

orbit  with  the  surface  of  the  heavens,  \  y/    /o 

The  manner  in  which  the  periodic  motion  of  a  double  star  is  observed,  will 
be  readily  apprehended  by  the  aid  of  the  annexed  diagram,  fig.  20,  by  which 
Dr.  Dick  has  represented  the  observations  of  Sir  William  Herschel  on  the 
double  star  Castor.  In  the  year  1759  Dr.  Bradley  had  observed  the  position 
of  the  two  individuals  of  this  star,  and  communicated  it  to  Dr.  Markelyne.  At 
that  time,  therefore,  it  is  known  that  the  line  joining  them  was  parallel  to  the 
line  joining  the  stars  Castor  and  Pollux,  as  seen  by  the  naked  eye.  The  fol- 
lowing table  exhibits  the  angles  which  the  same  joining  line  made  with  the 
meridian  of  Sir  William  Herschel's  observatory : — 

Times  of  the  Observations.  Angles  of  Position. 

November  1,  1759 56°  32' 

November  5,  1779 35    £9 

February  23,  1791 \ ,23    36 

December  15,  1795 jg    32 

March  26,  1800 14      3 

December  31,  1801 12    12 

February  28,  1802 12      1 

March  27,  1803 10    53 

It  appears,  therefore,  that  in  the  interval  between  November,  1759,  and 
March,  1803,  a  portion  of  an  orbit  amounting  to  45  degrees  and  39  minutes 
has  been  described  by  the  smaller  star  round  the  greater,  or  more  strictly  round 
their  common  centre  of  gravity.  This  would  be  at  the  rate  of  one  degree  and 
three  minutes  per  annum,  at  which  rate  a  complete  revolution  would  be  per- 
formed in  about  343  years. 

Let  the  small  central  circle  C  represent  the  larger  star  Castor,  and  D  the 
smaller  star,  and  let  the  line  E  F  represent  the  direction  of  the  two  stars  in  a 
line  with  the  sta.v  Pollux  at  E,  as  observed  by  Dr.  Bradley  in  1759.  In 
November,  1779,  they  were  found  in  the  position  C  H,  twenty-one  degrees 
from  the  position  they  occupied  twenty  years  before;  in  February,  1791,  they 
were  thirty-three  degrees  from  the  same  position,  &c. ;  and  in  March,  1803, 
forty-six  and  a  half  degrees,  giving  evident  indication  of  a  regular  progressive 
motion  in  a  circle.  Since  1803  its  motion  has  been  regularly  traced  by  Struve, 
Sir  John  Herschel,  and  Sir  J.  South  ;  and  in  1816  it  was  found  about  fifty- 
seven  degrees  from  its  first  position,  and  in  1830  about  sixty-eight  degrees, 
still  regularly  progressing.     In  1819  the  distance  of  the  small  star  from  Castor 


THE   STELLAR  UNIVERSE. 


Fig.  20. 


was  five  seconds  and  a  half,  and  in  1830  it  was  a  little  more  than  four  seconds 
and  a  half.  Although  Sir  William  Herschel,  as  above  stated,  conjectured  the 
period  of  revolution  to  be  about  343  years,  yet  later  astronomers,  from  a  com- 
parison of  all  the  observations  recently  made,  are  disposed  to  conclude  that  its 
period  is  little  more  than  250  years. 

Thus  in  each  succeeding  age  has  the  sagacity  and  perseverance  of  astrono- 
mers unfolded  laws  prevailing  in  the  material  universe,  whose  range  appears 
to  have  no  other  limit  than  those  of  that  universe  itself.  When  Galileo,  soon 
after  the  invention  of  the  telescope,  ascertained  the  existence  of  the  system  of 
Jupiter  and  his  moons,  exhibiting  on  a  small  scale  that  of  the  sun  and  the 
planets,  and  offered  it  to  the  world  as  an  analogy  strikingly  corroborative  of 
the  Copernican  hypothesis,  the  announcement  of  the  Florentine  observer  was 
received  with  incredulity,  and  philosophers  themselves  rejected  it,  some  de- 
claring that  they  could  not  give  credence  to  it,  even  though  attested  by  the 
evidence  of  their  senses.  What  would  have  been  said  if  the  inspiration  of 
Galileo  had  prompted  the  anticipation  of  sun  revolving  round  sun — of  system 
revolving  round  system — united  by  the  same  ruling  principle — bound  by  the 
same  tie,  and  exhibiting  a  regular  subordination  to  the  same  laws,  which  con- 
fer such  stability,  harmony,  and  regularity,  on  the  movements  of  the  solar  sys- 
tem !  Such  are  the  results  which  these  stellar  discoveries  bring  before  us. 
A  stupendous  luminous  globe,  surrounded  by  a  system  of  planets  with  their 
attendant  satellites,  presides  in  the  centre.  Around  it,  at  a  distance  incompara- 
bly greater  than  the  distances  of  its  planets,  circulates  another  sun,  attended 
by  another  system  of  planets  and  satellites  similar  to  the  first,  but  on  a  reduced 
scale !  The  lights  of  these  associated  suns  are  of  difTerent  hues,  but  their 
tints  are  so  related,  that  when  blended  together  they  will  produce  a  daylight 
like  that  of  the  solar  system.  The  distances  of  the  planets  composing  each 
of  these  systems,  from  their  respective  suns,  bear  a  proportion  to  the  distance 
which  intervenes  between  these  suns  similar,  doubtless,  to  that  which  the 
distances  of  the  satellites  of  Jupiter  or  Saturn  bear  to  the  distances  of  these 
planets  from  the  sun.  "A  less  distinctly  characterized  subordination  would," 
as  Sir  John  Herschel  observes,  "  be  incompatible  with  the  stability  of  their 
system,  and  with  the  planetary  nature  of  their  orbits.     Unless  closely  nestled 


THE   STELLAR  UNIVERSE. 


under  the  protecting  wing  of  their  immediate  superior,  the  sweep  of  their  other 
sun  in  its  perihelion  passage  round  their  own,  might  carry  them  off,  or  whirl 
them  into  orbits  utterly  incompatible  with  the  conditions  necessary  for  the  ex- 
istence of  their  inhabitants.  It  must  be  acknowledged  that  such  a  spectacle 
presents  a  strangely  wild  and  novel  field  for  speculative  excursions,  and  one 
which  it  is  difficult  to  avoid  luxuriating  in." 

Those  who  are  unaccustomed  to  the  consideration  of  geometrical  questions 
will  be  enabled  to  acquire  a  tolerably  clear  idea  of  such  a  system  as  has  been 
just  described  by  means  of  the  annexed  diagram. 

Fig.  21. 


/ 


./ 


The  larger  sun  with  its  planets  is  represented  at  S,  in  the  focus  of  an  ellipse, 
in  which  the  lesser  sun  accompanied  by  its  planets  moves.  At  A,  this  latter 
sun  is  in  its  perihelion,  and  nearest  to  the  greater  sun  S.  Moving  in  its  peri- 
odical course  to  B,  it  is  at  its  mean  distance  from  the  sun  S.  At  C  it  is  at 
aphelion,  or  its  most  distant  point,  and  finally  returns  through  D  to  its  perihelion 
A.  The  sun  S,  because  of  its  vast  distance  from  the  system  A,  would  appear 
to  the  inhabitants  of  the  planets  of  the  system  A  much  smaller  than  their  proper 
sun,  but  on  the  other  hand  this  effect  of  distance  would  be  to  a  certain  extent 
compensated  by  its  greatly  superior  magnitude  ;  for  analogy  justifies  the  infer- 
ence that  the  sun  S  is  greater  than  the  sun  A  in  a  proportion  equal  to  that  of 
the  magnitude  of  our  sun  to  one  of  the  planets.  The  inhabitants  of  the  planets 
of  the  system  A  will  then  behold  the  spectacle  of  tiuo  suns  in  their  firmament. 
The  annual  motion  of  one  of  these  suns  will  be  determined  by  the  motion  of 
the  planet  itself  in  its  orbit,  but  that  of  the  other  and  more  distant  sun  will  be 
determined  by  the  period  of  the  lesser  sun  around  the  greater  in  the  orbit 
A  B  C  D.  The  rotation  of  the  planets  on  their  axes  will  produce  two  days 
of  equal  length,  but  not  commencing  or  ending  simultaneously.  There  will  be 
m  general  two  sunrises  and  two  sunsets !  When  a  planet  is  situate  in  the  part 
of  its  orbit  between  the  two  suns,  there  will  be  no  night.  The  two  suns  will 
then  be  placed  exactly  as  our  sun  and  moon  are  placed  when  the  moon  is  full. 
When  the  one  sun  sets  the  other  will  rise,  and  when  the  one  rises  the  other 
will  set.     There  will  be,  therefore,  continual  day.     On  the  other  hand,  when 

VOL,-  II.— 34 


370 


THE  STELLAR  UNIVERSE. 


a  planet  is  at  such  a  part  of  its  orbit  that  both  suns  lie  in  nearly  the  sanne  di- 
rection as  seen  from  it,  both  suns  will  rise  and  both  will  set  together.  There 
will  then  be  the  ordinary  alternation  of  day  and  night  as  on  the  earth,  but  the 
day  will  have  more  than  the  usual  splendor,  being  enlightened  by  two  suns. 

In  all  intermediate  seasons  the  two  suns  will  rise  and  set  at  different  times. 
During  a  part  of  the  day  both  will  be  seen  at  once  in  the  heavens,  occupying 
different  places,  and  reaching  the  meridian  at  different  times.  There  will  be 
two  noons.  In  the  morning  for  some  time,  more  or  less,  according  to  the 
season  of  the  year,  one  sun  only  will  be  apparent,  and  in  like  manner,  in  the 
evening  the  sun  which  first  rose  will  be  the  first  to  set,  leaving  the  dominion 
of  the  heavens  to  its  splendid  companion. 

The  diurnal  and  annual  phenomena  incidental  to  the  planets  attending  the 
central  sun  S,  will  not  be  materially  different,  except  that  to  them  the  two  suns 
will  have  extremely  different  magnitudes,  and  will  afford  proportionally  differ- 
ent degrees  of  light.  The  lesser  sun  will  appear  much  smaller,  both  on  ac- 
count of  its  really  inferior  magnitude  and  its  vastly  greater  distance.  The  two 
days,  therefore,  when  they  occur,  will  be  of  very  different  splendor,  one 
being  probably  as  much  brighter  than  the  other  as  the  light  of  noonday  is  to 
that  of  full  moonlight,  or  to  that  of  the  morning  or  evening  twilight. 

But  these  singular  vicissitudes  of  light  will  become  still  more  striking  when 
it  is  remembered  that  the  two  suns  diffuse  light  of  different  colors.  Let  us 
examine  the  very  common  case  of  the  combination  of  a  crimson  with  a  blue  suii. 
In  general  they  will  rise  at  different  times.  When  the  blue  rises,  it  will  for  a 
time  preside  alone  in  the  heavens,  diffusing  a  blue  morning.  Its  crimson  com- 
panion, however,  soon  appearing,  the  lights  of  both  being  blended,  a  white  day 
will  follow.  As  evening  approaches,  and  the  two  orbs  descend  toward  the 
western  horizon,  the  blue  sun  will  first  set,  leaving  the  crimson  one  alone  in 
the  heavens.  Thus  a  ruddy  evening  closes  this  curious  succession  of  varying 
lights.  As  the  year  rolls  on,  these  changes  will  be  varied  in  every  conceivable 
manner.  At  those  seasons  when  the  suns  are  on  opposite  sides  of  a  planet, 
crimson  and  blue  days  will  be  alternate,  without  any  intervening  night ;  and  at 
the  intermediate  epochs  all  the  various  intervals  of  rising  and  setting  of  the 
two  suns  will  be  exhibited. 

"  Other  suns,  perhaps, 
With  their  attendant  moons,  thou  wilt  descry, 
Communicating  male  and  female  light 
(Which  two  great  sexes  animate  the  world), 
Stored  in  each  orb,  perhaps,  with  some  that  live." 

Paradise  Lost,  viii.,  148. 

PROPER    MOTION    OF    THE    STARS. 


In  common  parlance  the  stars  are  said  to  he  fixed.  They  have  received  this 
epithet  to  distinguish  them  from  the  planets,  the  sun,  and  the  moon,  all  of 
wjiich  constantly  undergo  changes  of  apparent  position  on  the  surface  of  the 
heavens.  The  stars,  on  the  contrary,  so  far  as  the  powers  of  the  eye  unaided 
by  art  can  discover,  never  change  their  relative  position  in  the  firmament,  which 
seems  to  be  carried  round  us  by  the  diurnal  motion  of  the  sphere,  just  as  if 
the  stars  were  attached  to  it,  and  merely  shared  in  its  apparent  motion. 

But  the  stars,  though  subject  to  no  motion  perceptible  to  the  naked  eye,  are 
not  absolutely  fixed.  When  the  place  of  a  star  on  the  heavens  is  exactly  ob- 
served by  means  of  good  astronomical  instruments,  it  is  found  to  be  subject  to 
a  change  from  month  to  month  and  from  year  to  year,  small  indeed,  but  still 
easily  observed  and  certainly  ascertained. 

It  has  been  demonstrated  by  Laplace  that  a  system  of  bodies,  such  as  the 


THE  STELLAR  UNIVERSE. 


solar  system,  placed  in  space,  and  submitted  to  no  other  continued  force  except 
the  reciprocal  attractions  of  the  bodies  which  compose  it,  must  either  have 
the  common  centre  of  gravity  stationary  or  in  a  state  of  uniform  rectilinear 
motion. 

The  chances  against  the  conditions  w^hich  w^ould  render  the  sun  stationary, 
compared  with  those  which  would  give  it  a  motion  in  some  direction,  with  some 
velocity,  are  so  numerous  that  we  may  pronounce  it.  to  be  morally  certain  that 
our  system  is  in  motion  in  some  determinate  direction  through  the  universe. 
Now  if  we  suppose  the  sun  attended  by  the  planets  to  be  thus  moved  through 
the  universe  in  any  direction,  an  observer  placed  on  the  earth  would  observe 
the  effects  of  such  a  motion,  as  a  spectator  in  a  steamboat  moving  on  a  river 
would  perceive  his  progressive  motion  on  the  stream  by  an  apparent  motion 
of  the  banks  in  a  contrary  direction.  The  observer  on  the  earth  would  there- 
fore detect  such  a  motion  of  the  solar  system  through  space  by  the  apparent 
motion  in  the  contrary  direction  with  which  the  stars  would  be  affected. 

Such  a  motion  of  the  solar  system  would  affect  different  stars  differently. 
All  would,  it  is  true,  appear  to  be  affected  by  a  contrary  motion,  but  all  would 
not  be  equally  affected.  The  nearest  would  appear  to  have  the  most  per- 
ceptible motion,  the  more  remote  would  be  affected  in  a  less  degree,  and 
some  might,  from  their  extreme  distance,  be  so  slightly  affected  as  not  to 
exhibit  any  apparent  change  of  place,  even  when  examined  with  the  most 
delicate  instruments.  To  whatever  degree  each  star  might  be  affected, 
all  the  changes  of  position  would,  however,  apparently  take  place  in  the  same 
direction. 

The  apparent  effects  would  also  be  exhibited  in  another  manner.  The  stars 
in  that  region  of  the  universe  toward  which  the  motion  of  the  system  is  di- 
rected would  appear  to  recede  from  each  other.  The  spaces  which  separate 
them  would  seem  to  be  gradually  augmented,  while,  on  the  contrary,  the  stars 
in  the  opposite  quarter  would  seem  to  be  crowded  more  closely  together,  the 
distances  between  star  and  star  being  gradually  diminished.  This  will  be 
more  clearly  comprehended  by  the  annexed  diagram. 


Fig.  22. 


BJi^=^=^--.:;--,-- 


7^  e 


Let  the  line  S  S'  represent  the  direction  of  the  motion  of  the  system,  and 
let  S  and  S'  represent  its  positions  at  any  two  epochs.  At  S,  the  stars  ABC 
would  be  separated  by  intervals  measured  by  the  angles  A  S  B,  and  B  S  C, 
while  at  S'  they  would  appear  separated  by  the  lesser  angles  A  S'  B,  and 
B  S'  C.  Seen  from  S',  the  stars  ABC  would  seem  to  be  closer  together 
than  they  were  when  seen  from  S.  For  like  reasons  the  stars  a  b  c,  toward 
which  the  system  is  here  supposed  to  move,  would  seem  to  be  closer  together 
when  seen  from  S,  than  when  seen  from  S^.  Thus,  in  the  quarter  of  the 
heavens  toward  which  the  system  is  moving,  the  stars  might  be  expected  to 
separate  gradually,  while  in  the  opposite  quarter  they  would  become  more 
■condensed.  In  all  the  intermediate  parts  of  the  heavens  they  would  be  affected 
by  a  motion  contrary  to  that  of  the  solar  system.  Such  in  general  would  be 
the  effects  of  a  progressive  motion  of  our  system. 


372 


THE   STELLAR  UNIVERSE. 


If  phenomena  like  these  were  clearly  ascertained  among  the  stars,  the  mo- 
tion of  the  solar  system  would  be  proved  ;  but  on  the  other  hand,  such  appear- 
ances not  being  discovered,  we  must  infer,  not  the  quiescence  of  the  system, 
but  the  absence  of  any  motion  sufficiently  rapid  to  produce  an  observable 
effect  on  the  apparent  positions  of  bodies  so  distant  as  the  fixed  stars.  In 
a  word,  it  must  be  concluded,  that  within  the  limited  period  of  time  over 
which  astronomical  observation  has  extended,  the  space  through  which  the 
solar  system  has  moved  must  bear  an  inappreciable  ratio  to  the  distances  of 
the  stars. 

In  the  course  of  his  various  astronomical  labors,  the  late  Sir  William  Her- 
schel  imagined  at  one  time  that  he  had  ascertained  among  the  apparent  changes 
incidental  to  the  firmament,  indications  of  a  movement  of  the  solar  system 
toward  a  point  of  the  universe  occupied  by  some  of  the  stars  composing  the 
constellation  of  Hercules.  This  conjecture  has  not,  however,  been  sustained 
by  subsequent  surveys  of  the  heavens ;  and  the  opinion  among  astronomers 
now  is  that  no  sufficient  data  have  yet  been  attained  to  warrant  any  distinct 
conclusion  regarding  the  progressive  motion  common  to  the  bodies  of  our 
system. 

The  late  astronomer  royal  of  England  (Mr.  Pond),  suggested  a  mode  of 
investigating  the  motion  of  the  solar  system,  marked  by  singular  ingenuity 
and  refinement.  It  is  known  that  the  motion  of  light  combined  with  that  of 
the  earth  in  its  annual  orbit,  produces  an  effect  on  the  apparent  places  of  all  ob- 
jects in  the  heavens,  by  which  they  are  seen  advanced  beyond  their  true  po- 
sitions, always  in  the  direction  in  which  the  earth  is  moving,  and  the  extent 
of  this  apparent  displacement  depends  on  the  proportion  which  the  earth's 
orbitual  velocity  bears  to  the  velocity  of  light.  This  effect  is  called  the 
aberration  of  light.  Now  if  the  sun,  together  with  the  planets,  have  any  pro- 
gressive motion  through  space,  the  velocity  of  such  motion  would  probably  be 
much  greater  than  the  orbitual  velocity  of  the  earth.  Such  a  motion  would 
then  be  attended  with  an  aberration  of  the  stars,  greater  in  amount  than  that 
which  is  due  to  the  earth's  motion.  Such  an  aberration  would  cause  all  the 
stars  to  be  displaced  to  the  same  extent  and  in  the  same  direction,  and  con- 
sequently it  would  cause  no  change  in  their  relative  positions.  We  should 
under  such  circumstances  have  no  means  of  detecting  it.  But  if,  in  the  lapse 
of  ages,  the  velocity  of  the  solar  system  were  to  undergo  any  change  of  suf- 
ficient amount,  or  if  the  direction  of  its  motion  were  to  be  changed  (as  would 
certainly  happen  if  our  system  were  moving  in  an  orbit  round  any  other,  owing 
to  any  combination  like  those  of  the  double  stars),  then  the  quantity  or  direc- 
tion of  the  consequent  aberration  would  be  changed,  the  relative  position  of 
the  stars  would  be  consequently  disturbed,  and  the  effects  would  become  per- 
ceptible. Such  effects  have  not  been  yet  observed,  but  this  suggestion 
may  afford  future  astronomers  the  means  of  ascertaining  the  motion  of  our 
system. 

But  although  no  appearances  have  been  discovered,  such  as  a  progressive 
motion  of  our  system  would  produce,  yet  other  phenomena  have  been  un- 
folded which  prove  that  the  fixed  stars  are  not  absolutely  stationary,  and 
which  indicate  physical  powers  in  active  operation  in  distant  regions  of 
the  universe,  on  a  scale  commensurate  with  the  enormous  distances  and 
magnitudes  which  telescopic  research  has  unfolded.  The  stars,  examined 
individually  with  instruments  of  sufficient  power  and  precision,  have  been 
found  to  be  subject  to  changes  of  position  which,  though  small,  are  very 
perceptible,  and  are  certainly  ascertained.  These  changes  are  called  the 
proper  motions  of  the  stars. 

These  proper  motions  are  not  the  same  in  all  stars.     In  some  no  such,  motion 


THE   STELLAR  UNIVERSE. 


is  discovered.  In  most  of  those  in  which  it  has  been  discovered,  its  amount, 
even  after  the  lapse  of  years,  is  still  but  small.  In  one  or  two  it  is  suf- 
ficiently great  to  be  detected  by  very  ordinary  means  of  astronomical  obser- 
vation. The  greatest  proper  motion  which  has  hitherto  been  observed  in 
any  single  star  is  found  in  the  star  h  in  the  constellation  of  Cassiopeice.  The 
annual  displacement  of  this  star  amounts  to  3"-74,  so  that  in  500  years  it 
will  be  removed  from  the  place  it  now  occupies  by  a  space  equal  to  the  ap- 
parent diameter  of  the  moon.  The  annual  proper  motion  of  Arcturus  is 
about  half  that  of  f  Cassiopeim.  In  the  following  table  is  collected  the  proper 
motions  as  they  affect  the  declination  and  right  ascension  of  some  of  the 
stars  in  which  this  phenomenon  is  most  conspicuous.  The  sign  4-  prefixed 
to  the  annual  variation,  shows  that  it  is  to  be  added,  and  —  that  it  is  to  be 
subtracted,  to  find  the  true  place  of  the  object  at  any  time  : — 


Names  of  the  Stars. 

Magnitude. 

Annual  Motion  in 
R.  A. 

Annual  Motion  in 
Dec. 

Capella 

Sirius 

1 
1 
1 

1.2 
2 

1.2 
3 
1 

1.2 
1 
1 

Seconds. 
-f0.21 
—0.42 
—0.15 
—0.80 
—0.74 
—0.57 
-f-0.74 
—1.26 
-fO.48 
-1-0.23 
0^00 

Seconds. 
4-0.44  N. 
-f-1.04  S. 
-1-0.44  S. 
-^0.95  S. 

0.00 
-f  0.07  S. 
—0.24  S. 
—1.72  S. 
—0.54  N. 
—0.27  N. 
—0.26  N. 

Pollux 

/?  Leonis 

/3  Virginis 

Arcturus 

Altair 

Antares 

But  it  is  among  the  double  stars  that  we  find  the  most  remarkable  examples 
of  proper  motion.  These  systems,  while  their  component  stars  revolve  one 
round  the  other,  or  rather  round  their  common  centre  of  gravity,  seem  to 
be  carried  forward  in  some  determinate  direction  with  a  motion  in  which 
they  both  participate.  Thus  the  individuals  which  compose  the  double  star 
61  Cygni  (of  which  Professor  Bessel  has  discovered  the  annual  parallax), 
have  remained  constantly  at  nearly  the  same  distance  from  each  other  for 
sixty  years  last  past,  but  have  at  the  same  time  been  continually  shifting  their 
position  on  the  firmament,  and  are  now  about  five  minutes  from  the  place 
they  occupied  sixty  years  ago.  In  350  years  this  double  star  will  move  over 
a  space  on  the  firmament  equal  to  the  diameter  of  the  moon. 

The  only  conceivable  explanation  of  the  phenomena  of  the  proper  motions 
of  the  stars,  is  the  supposition  that  these  bodies  actually  have  real  motions 
through  space,  such  as  to  produce  the  apparent  changes  of  position  which  we 
observe.  If  the  distance  of  any  star  having  a  proper  motion  be  known,  the 
rate  at  which  it  moves  may  be  easily  calculated.  Thus,  if  we  assume  the 
distance  of  61  Cygni,  as  determined  by  Bessel's  observations  on  the  parallax, 
to  be  60,000,000,000,000  miles,  their  motion  must  be  at  the  rate  of  one  hun- 
dred and  seventy-seven  thousand  miles  an  hour,  in  order  to  produce  the  ap- 
parent annual  displacement  which  has  been  observed.  This  velocity  would 
be  double  that  of  the  orbitual  motion  of  the  earth. 

Among  the  proper  motions  of  the  stars,  there  is  no  apparent  relation — noth- 
ing to  lead  to  the  conjecture  that  these  phenomena  are  ascribable  to  any 
common  physical  cause  affecting  at  once  all  these  bodies.  We  must  then 
infer  that  they  are  independent  motions  affecting  these  distant  systems — inde- 
pendent at  least,  so  far  as  our  present  knowledge  extends. 


THE  STELLAR  UIIVERSE. 


(SECOND    LECTURE.) 


Form  and  Arrangement  of  the  Mass  of  visible  Stars. — Sir  W.  Herschel's  Analysis  of  the  Heavens. — 
The  Milky  Way. — The  vast  Numbers  of  Stars  in  it. — Form  and  Dimensions  of  this  Mass  of  Stars. — 
Nebulae  and  Clusters. — Various  Forms  and  Appearance  of  Nebulae. — Great  Nebula  in  Orion. — 
Megallanic  Clouds. — Planetary  Nebulse. — Vast  Number  of  Nebulae. — Herschel's  Catalogrue.^ 
Structure  of  the  Universe. — Laplace's  nebular  Hypothesis. — Examination  of  its  moral  Tendency. 


377 


TIE   STELLAR  UIIYERSE 


(SECOND    LECTURE.) 


The  extent  of  the  survey  of  the  universe  which  is  commanded  by  our  natural 
vision,  unaided  by  those  expedients  which  the  inventions  in  optics  have  sup- 
plied, has  been  on  another  occasion  fully  explained.*  We  have  shown  that 
objects  placed  around  us  within  the  scope  of  a  radius  of  such  a  length  that 
light  would  take  about  a  hundred  and  twenty  years  to  move  over  it,  are  thus 
perceivable  by  us.  It  does  not,  however,  follow,  therefore,  that  all  objects 
within  that  radius  are  visible.  There  may  be  within  it  stars  which  fail  to  be 
seen  ;  not  because  of  their  comparative  remoteness,  but  because  of  their  com- 
paratively inferior  intrinsic  splendor ;  and  we  may  infer  that  interminable 
realms  of  space  must  extend  beyond  that  limit,  teeming  with  innumerable  suns 
and  systems,  like  those  which  are  so  abundantly  manifested  within  it. 

An  attempt  was  made  by  the  late  Sir  William  Herschel  to  ascertain  by  im- 
mediate observation  the  manner  in  which  those  stars  which  are  individually 
visible  to  us,  whether  by  the  naked  eye  or  by  the  telescope,  are  distributed 
through  space.  Are  they  casually  scattered  in  all  directions,  without  any 
definite  limit  of  distance,  or  any  definite  form  1  Has  their  entire  mass  any 
ascertainable  shape  or  dimensions  1  Is  it  of  a  regular  form,  such  as  a  sphere 
or  a  cube  ?  And  if  it  have  definite  limits,  how  has  it  pleased  Omnipotence  to 
manifest  itself  in  those  unfathoraed  regions  which  stretch  in  all  directions 
around  that  finite  and  limited  mass  of  systems  1 

It  will  be  recollected  that  on  a  former  occasion  it  was  shown,  that  by  the 
successive  application  of  telescopes  of  augmented  space-penetrating  power, 
we  are  enabled  to  bring  into  view  individual  stars  more  and  more  remote. 
Denominating  the  nearest  and  brightest  stars  to  be  at  the  first  order  of  distance  ; 
those  within  twice  that  radius  to  be  in  the  second  order  of  distance  ;  those  with- 
in three  times  that  radius  to  be  in  the  third  order  of  distance,  and  so  on ;  the 

•  See  the  lecture  on  "  The  Visible  Stars." 


naked  eye  being  capable  of  perceiving  stars  until  we  attain  to  the  twelfth  or- 
der of  distance.  The  telescope  then  carries  our  view  still  further,  and  by  the 
highest  powers  to  which  it  has  hitherto  attained,  it  brings  within  our  view 
stars  which  may  be  considered  to  lie  at  the  2,400th  order  of  distance,  and  from 
which  light  would  therefore  take  24,000  years  to  come. 

Armed  with  such  powers,  Sir  William  Herschel  commenced  the  unparalleled 
enterprise  of  a  general  survey  of  the  stellar  universe.  It  was  easily  rendered 
apparent  that  our  system  is  placed  within  a  mass  of  suns  of  vast  extent  and 
countless  number.  The  few  which  immediately  surround  us  appear  by  their 
comparative  proximity  largest,  or  rather  brightest,  and  are  accordingly  classed 
as  stars  of  the  first  magnitude.  Those  which  lie  immediately  beyond  them, 
occupying  a  wider  circle,  and  proportionally  more  numerous,  are,  by  reason 
of  their  greater  distance,  of  inferior  magnitude.  Thus,  the  greater  the  dis- 
tance we  contemplate,  and  the  wider  the  circle  over  which  the  stars  are  dis- 
tributed, the  greater  they  are  found  to  be  in  number,  and  the  less  intense  in 
splendor. 

These  observations  are  not  applicable  alone  to  the  stars  visible  to  the  naked 
eye.  Direct  the  most  ordinary  telescope  to  any  quarter  of  the  heavens,  and 
move  it  slowly  about  so  as  to  sweep  a  small  portion  of  the  firmament,  and  it 
will  be  found  that  many  stars  will  be  visible  in  it  which  were  before  not  ob- 
servable.    Such  stars  lie  beyond  the  sphere  of  natural  vision. 

But  is  the  system  to  which  the  earth  is  attached  surrounded  by  an  equal 
depth  of  stars  in  every  direction  ?  Is  it  in  the  centre  of  a  globular  mass  of 
stars?  and  if  so,  what  order  of  distance  is  to  be  assigned  to  the  most  remote 
of  these  surrounding  suns  1  If  not,  must  we  not  expect  to  find  stars  smaller 
and  more  thickly  crowded  together  in  those  directions  where  they  extend  to 
more  remote  distances  than  in  those  where  they  are  more  limited  in  their  dis- 
tance ? — Just  as  we  should  find  the  appearance  of  the  stems  of  the  trees  if  we 
stood  in  the  middle  of  a  wood  which  is  narrow  in  one  direction  and  long  in 
another  ?  These  questions  can  be  satisfactorily  resolved  only  by  a  general 
examination  of  the  entire  firmament,  and  by  observing  whether  the  stars  are 
more  numerous,  smaller,  and  more  thickly  crowded  together  in  some  regions 
than  in  others. 

There  is  a  remarkable  band,  or  zone,  which  surrounds  the  firmament,  form- 
ing very  nearly  a  great  circle  of  the  heavens,  and  presenting  to  the  naked  eye 
the  appearance  of  a  cloudy  or  nebulous  whiteness.  This  has  been  called  the 
Via  Lactea  or  the  Milky  Way.  Its  course,  which  however  is  not  regular,  has 
a  direction  nearly  at  right  angles  to  the  celestial  equator,  intersecting  that 
circle  at  two  points,  one  of  which  is  near  the  belt  of  Orion,  and  the  other  near 
the  constellation  of  Aquila,  rendered  conspicuous  by  the  bright  star  Atair,  just 
on  the  verge  of  the  Milky  Way.  If  we  take  a  general  view  of  the  heavens, 
even  without  the  aid  of  a  telescope,  we  shall  find  that  in  those  regions  most 
remote  from  this  remarkable  zone,  the  stars  are  thinly  scattered,  and  as 
we  approach  it  in  any  direction  they  become  smaller,  more  numerous,  and 
more  crowded.  This  becomes  still  more  strongly  manifested  when  we  resort 
to  telescopes,  by  which  (when  of  sufficient  space-penetrating  power),  the  as- 
tonishing fact  is  disclosed  that  this  whitish  zone  consists  entirely  of  small  stars, 
too  minute  to  be  individually  distinguished  without  a  telescope,  and  which  are 
scattered  by  countless  millions,  "  like  glittering  dust  on  the  black  ground  of 
the  general  heavens." 

"  A  broad  and  ample  road,  whose  dust  is  gold, 

And  pavement  stars,  as  stars  to  us  appear ; 

Seen  in  the  galaxy  that  Milky  Way, 
.    Like  to  a  circling  zone  powdered  vdth  stars." — Milton. 


THE   STELLAR  UNIVERSE. 


379 


These  phenomena  led  Sir  William  Herschel  to  the  conclusion  that  the  stars 
of  our  firmament,  instead  of  being  scattered  in  all  directions  indifferently 
through  space,  form  a  stratum,  of  which  the  thickness  is  small  in  comparison 
with  its  length  and  breadth ;  and  in  which  the  earth  occupies  a  place  some- 
where about  the  middle  of  its  thickness,  and  near  the  point  where  it  subdivides 
into  two  principal  laminae,  inclined  at  a  small  angle  to  each  other.     For  it  is 


certain  that,  to  an  eye  so  situated,  the  apparent  density  of  the  stars,  supposing 
them  pretty  equally  scattered  through  the  space  they  occupy,  would  be  least 
in  a  direction  of  the  visual  ray  (as  S  A),  perpendicular  to  the  laminae,  and 
greatest  in  that  of  its  breadth,  as  S  B,  S  C,  S  D ;  increasing  rapidly  in  pass- 
ing from  one  to  the  other  direction,  just  as  we  see  a  slight  haze  in  the  atmo- 
sphere thicking  into  a  decided  fog-bank  near  the  horizon,  by  the  rapid  increase 
of  the  mere  length  of  the  visual  ray.  Accordingly,  such  is  the  view  of  the 
construction  of  the  starry  firmament  taken  by  Sir  William  Herschel,  whose 
powerful  telescopes  have  effected  a  complete  analysis  of  this  wonderful  zone, 
and  demonstrated  the  fact  of  its  entirely  consisting  of  stars.  So  crowded  are 
they  in  some  parts  of  it,  that  by  counting  the  stars  in  a  single  field  of  his  tele- 
scope, he  was  led  to  conclude  that  50,000  had  passed  under  his  review  in  a 
zone  two  degrees  in  breadth,  during  a  single  hour's  observation.  The  im- 
mense distances  at  which  the  remoter  regions  must  be  situated,  will  sufficiently 
account  for  the  vast  predominance  of  small  magnitudes  which  are  observed 
in  it.* 

The  appearance  which  this  mass  of  stars  would  present  if  viewed  from  a 
position  directly  above  its  general  plane,  and  at  a  sufficient  distance  to  allow 
its  entire  outline  to  be  discerned,  was  represented  by  Sir  William  Herschel  as 
resembling  the  annexed  drawing,  fig.  2. 

He  considered  that  it  was  probable  that  the  thickness  of  this  bed  of  stars  was 
equal  to  about  eighty  times  the  distance  of  the  nearest  of  the  fixed  stars  from 
our  system ;  and  supposing  our  sun  to  be  at  the  middle  of  this  thickness,  it 
would  follow  that  the  stars  on  its  surface  in  a  direction  perpendicular  to  its 
general  plane  would  be  at  the  fortieth  order  of  distance  from  us.  The  stars 
placed  in  the  more  remote  edges  of  its  length  and  breadth  he  estimated  to  be 
in  some  places  at  the  nine-hundredth  order  of  distance  from  us,  so  that  its  ex- 
treme length  may  be  said  to  be  in  round  numbers  about  two  thousand  times 
the  distance  of  the  nearest  fixed  scars  from  our  system.  Such  a  space  light 
would  take  twenty  thousand  years  to  move  over,  moving  all  that  time  at  the 
rate  of  two  hundred  thousand  miles  between  every  two  ticks  of  a  common 
clock ! 

The  great  splendor  of  that  part  of  the  Milky  Way  which  passes  through 
the  southern  hemisphere,  and  some  other  peculiarities  which  he  has  remarked 
in  it,  has  suggested  to  Sir  John  Herschel  a  corroboration  of  his  father's  theory 
of  its  form.  "The  general  aspect  of  the  southern  circumpolar  region,"  says 
Sir  John,  "  including  in  that  expression  60°  or  70°  of  S.  P.  D.,t  is  in  a  high 
degree  rich  and  magnificent,  owing  to  the  superior  brilliancy  and  larger  de- 
velopment of  the  Milky  Way :  which,  from  the  constellation  of  Orion  to  that 
of  Antinous,  is  in  a  blaze  of  light,  strangely  interrupted,  however,  with  vacant 


'  Herschel's  Astronomy,  chap.  x. 


t  Soutbem  polar  distance. 


THE   STELLAR  UNIVERSE. 


Fig.  2. 


and  almost  starless  patches,  especially  in  Scorpio,  near  «  Centauri  and  the 
cross ;  while  to  the  north  it  fades  away  pa\e  and  dim,  and  is  in  comparison 
hardly  traceable.  I  think  it  is  impossible  to  view  this  splendid  zone,  with  the 
astonishingly  rich  and  evenly-distributed  fringe  of  stars  of  the  third  and  fourth 
magnitudes,  which  form  a  broad  skirt  to  its  southern  border,  like  a  vast  cur- 
tain— without  an  impression,  amounting  to  a  conviction,  that  the  Milky  Way 
is  not  a  mere  stratum,  but  an  annulus  ;  or,  at  least,  that  our  system  is  placed 
within  one  of  the  poorer  and  almost  vacant  parts  of  its  general  mass,  and  that 
eccentrically,  so  as  to  be  much  nearer  to  the  parts  about  the  cross,  than  to 
that  diametrically  opposed  to  it." 

When  a  telescope  is  directed  to  the  heavens,  the  actual  space  it  renders 
visible  at  one  time,  technically  called  z.  field  of  view,  is  small  in  the  same  pro- 
portion as  the  magnifying  power  of  the  instrument  is  great.  Thus  a  telescope 
of  a  certain  magnifying  power  will  present  to  the  observer  the  complete  disk 


THE    STELLAR  UNIVERSE. 


of  the  moon,  which  may  perhaps  occupy  its  entire  field  of  view.  A  magnify- 
ing power  twice  as  great  will  show  at  once  only  half  the  moon's  apparent 
diameter,  and  therefore  only  a  fourth  of  its  entire  disk.  One  of  three  times  the 
power  would  show  only  one  third  of  its  diameter,  and  one  ninth  of  its  disk, 
and  so  on.  Let  it  be  remembered,  then,  that  some  of  the  magnifying  powers 
with  which  the  researches  of  Sir  William  Herschel  were  made,  gave  a  field 
of  view  not  so  great  as  a  fourth  part  of  the  moon's  disk,  and  we  shall  form  some 
idea,  however  inadequate  and  obscure,  of  the  profusion  of  evidences  of  creative 
power  which  the  firmament  presented  to  that  observer.  He  states  that  in 
those  parts  of  the  Milky  Way  in  which  the  stars  were  most  thinly  scattered, 
he  saw  upon  an  average  eighty  stars  in  each. field.  In  an  hour,  fifteen  de- 
grees of  the  firmament  were  carried  before  his  telescope,  showing  successively 
sixty  distinct  fields.  Allowing  eighty  stars  for  each  field,  there  were  thus  ex- 
hibited to  his  astonished  view  in  a  single  hour  without  moving  the  telescope 
four  thousand  eight  hundred  distinct  stars  !  But  by  moving  the  instrument  at 
the  same  time  in  the  vertical  direction,  he  found  that  in  a  space  of  the  firma- 
ment not  more  than  fifteen  degrees  long  by  four  degrees  broad,  he  was  able 
to  observe  fifty  thousand  stars  large  enough  to  be  individually  visible  and 
distinctly  counted !  The  surprising  character  of  this  result  will  be  more 
adequately  appreciated  if  it  is  remembered  that  this  number  of  stars  thus  seen 
in  a  space  of  the  heavens  not  more  than  thirty  diameters  of  the  moon's  disk 
in  length  and  eight  in  breadth,  is  fifty  times  greater  than  all  the  stars  taken 
together  which  the  naked  eye  can  perceive  at  any  one  time  in  the  heavens  on 
the  most  serene  and  unclouded  night !  And  this,  be  it  observed,  is  in  that 
part  of  the  Milky  Way  which  is  most  sparsely  strewn  with  stars !  What  are 
we  to  say  of  the  richer  parts  ? 

On  presenting  the  telescope  to  the  richer  portions  of  the  Via  Lactea,  Her- 
schel found,  as  might  be  expected,  much  greater  numbers  of  stars.  In  a  single 
field  he  was  able  to  count  588  stars,  and  for  fifteen  minutes,  the  firmament 
being  moved  before  his  telescope  by  the  diurnal  motion,  no  diminution  of 
number  was  apparent,  so  that  he  estimated  that  in  that  space  of  time  116,000 
stars  must  have  passed  in  review  before  him ;  the  number  seen  at  any  one 
time  being  greater  than  can  be  seen  by  the  naked  eye  on  the  entire  firmament, 
except  on  the  clearest  nights. 

It  appears,  then,  that  our^sun  is  an  individual  star,  forming  only  a  single 
unit  in  a  cluster  or  mass  of  many  millions  of  other  similar  stars  ;  that  this 
cluster  has  limited  dimensions,  has  ascertainable  length,  breadth,  and  thick- 
ness, and  in  short,  forms  what  may  be  expressed  by  a  universe  of  solar  sys- 
tems. The  mind,  still  unsatisfied,  is  as  urgent  as  before  in  its  questions  re- 
garding the  remainder  of  immensity !  However  vast  the  dimensions  of  this 
mass  of  suns  be,  they  are  nevertheless  finite.  However  stupendous  be  the 
space  included  within  them,  it  is  still  nothing  compared  to  the  immensity 
which  lies  outside  !  Is  that  immensity  a  vast  solitude  ?  Are  its  unexplored 
realms  dark  and  silent  ?  Has  Omnipotence  circumscribed  its  agency,  and 
has  infinite  Beneficence  left  those  unfathomed  regions  destitute  of  evidence  of 
its  power  ? 

That  the  infinitude  of  space  should  exist  without  a  purpose,  unoccupied  by 
any  works  of  creation,  is  plainly  incompatible  with  all  our  notions  of  the 
character  and  attributes  of  the  Author  of  the  universe,  whether  derived  from 
the  voice  of  revelation  or  from  the  light  of  nature.  We  should  therefore  infer, 
even  in  the  absence  of  direct  evidence,  that  some  works  of  creation  are  dis- 
persed through  those  spaces  which  lie  beyond  the  limits  of  that  vast  stellar 
cluster  in  which  our  system  is  placed.  Nay,  we  should  be  led  by  the  most 
obvious   analogies,  to  conjecture   that  other  stellar  clusters  like   our  own,  are 


dispersed  through  immensity,  separated  probably  by  distances  as  much  greater 
than  those  which  intervene  between  star  and  star  as  the  latter  are  greater 
than  those  which  separate  the  bodies  of  the  solar  system.  But  if  such  distant 
clusters  existed,  it  may  be  objected,  that  they  must  be  visible  to  us  ;  that 
although  diminished,  perhaps,  to  mere  spots  on  the  firmament,  they  would 
still  be  rendered  apparent,  were  it  only  as  confused  whitish  patches,  by  the 
telescope ;  that,  as  the  stars  of  the  Milky  Way  assume  to  the  naked  eye  the 
appearance  of  mere  whitish  nebulosity,  so  the  far  more  distant  stars  of  other 
clusters,  which  can  not  be  perceived  at  all  by  the  naked  eye,  would,  to  tele- 
scopes of  adequate  power,  present  the  same  whitish  nebulous  appearance  ; 
and  that  we  might  look  forward  without  despair  to  such  augmentation  of  the 
powers  of  the  telescope  as  may  even  enable  us  to  perceive  them  to  be  actual 
clusters  of  stars. 

Such  anticipations  have  accordingly  been  realized.     In  various  parts  of  the 
firmament  objects  are  seen  which,  to  the  naked  eye,  appear  like   stars  seen 

Fig.  3. 


THE   STELLAR  UNIVERSE. 


383 


through  a  mist,  and  sometimes  as  nebulous  specks,  which  might  be,  and  not 
unfrequently  are,  mistaken  for  comets.  With  ordinary  telescopes  these  ob- 
jects are  visible  in  very  considerable  numbers,  and  were  observed  nearly  a 
century  ago.  In  the  Connaissance  des  Temps,  for  1784,  Messier,  then,  so  cele- 
brated for  his  observations  on  comets,  published  a  catalogue  of  103  objects  of 
this  class,  of  many  of  which  he  gave  drawings,  and  with  which  all  observers 
who  search  for  comets  ought  to  be  familiar  to  avoid  being  misled  by  their  re- 
semblance to  them.  The  improved  powers  of  the  telescope  speedily  disclosed 
to  astronomers  the  nature  of  these  objects,  which,  when  examined  by  sufficient 
magnifying  powers,  prove  to  be  masses  of  stars  clustered  together  in  a  manner 
identical  with  that  cluster  in  which  our  sun  appears  to  be  placed.  They  ap- 
pear as  they  do,  mere  specks  of  whitish  light,  because  of  their  enormous  dis- 
tance. Many  of  them  are  of  a  round  figure,  and  convey  the  idea  of  a  globular 
space  filled  full  of  stars  insulated  in  the  heavens,  and  constituting  in  itself  a 
family  or  society  apart  from  the  rest,  and  subject  only  to  its  own  internal  laws. 
The  task  were  vain  to  attempt  to  count  the  stars  in  one  of  these  globular  clus- 
ters. They  are  not  to  be  reckoned  by  hundreds  ;  and  on  a  rough  calculation, 
grounded  on  the  apparent  intervals  between  them  at  the  borders  (where  they 
are  not  seen  projected  on  each  other),  and  the  angular  diameter  of  the  whole 
group,  it  would  appear  that  many  clusters  of  this  description  must  contain  at 
least  from  ten  to  twenty  thousand  stars,  compacted  and  wedged  together  in  a 
round  space  whose  visible  magnitude  is  not  a  tenth  part  of  that  of  the  disk  of 
the  moon.* 

One  of  these  objects  (the  13th,  in  Messier's  catalogue),  is  represented  in  the 
annexed  diagram,  fig.  3. 

This,  as  Sir  John  Herschel  observes,  is  exhibited  by  the  telescope  as  con- 
sisting entirely  of  stars  crowded  together  so  as  to  present  an  almost  definite 
outline,  and  to  run  up  to  a  blaze  of  light  in  the  centre,  where  their  condensa- 
tion is  usually  greatest.  This  beautiful  object  was  first  seen  by  Halley,  in 
1714.  It  is  visible  to  the  naked  eye  between  the  stars  /^  and  ^  in  the  constel- 
lation of  Hercules.  If  an  imaginary  line  be  drawn  from  the  star  (first  magni- 
tude) a  Lyrse,  to  the  star  P  (second  magnitude),  in  the  constellation  of  Hercules, 
it  will  pass  through  this  nebula  near  the  latter  star. 

In  fig.  4,  annexed,  is  exhibited  a  sketch  of  one  of  the  most  remarkable 
nebulae  in  the  firmament.  This  is  the  27lh  in  Messier's  catalogue.  Its  form 
may  be  likened  to  an  hour-glass,  a  double-headed  shot,  or  a  dumb-bell,  surrounded 
by  a  thin  hazy  atmosphere.  This  belongs  to  a  class  of  nebulae  which  show 
an  evident  symmetry  of  form.  It  consists,  according  to  Sir  John  Herschel's 
observations,  of  two  bright  and  highly-condensed  round  or  rather  oval  nebulae, 
united  by  a  short  neck  of  nearly  the  same  density.  A  faint  nebulous  atmo- 
sphere completes  the  figure,  enveloping  them  both,  and  filling  up  the  outline 
of  a  circumscribing  ellipse,  whose  shorter  axis  is  the  symmetrical  axis  of  the 
system,  or  the  line  passing  through  the  centres  of  both  the  chief  nebulous 
masses. 

In  fig.  5  is  presented  a  nebula  of  an  elliptical  form,  which  is  visible  to  the 
naked  eye.  In  the  latitude  of  New  York  it  passes  near  the  zenith  at  about 
nine  o'clock  at  night  in  the  month  of  November,  and  in  the  follovying  months 
may  be  seen  in  the  evenings  in  the  northwest,  at  a  considerable  altitude.  It 
appears  like  a  dull,  cloudy,  undefined  spot  upon  the  concave  of  the  firmament, 
and  has  sometimes  been  compared  to  the  light  of  a  small  candle  seen  through 
horn.  Its  central  parts  appear  brightest,  but  its  light  gradually  fades  away 
toward  each  extremity.  A  few  small  stars  appear  adjacent  to  it,  but  they  have  i 
no  immediate  connexion  with  the  nebula.     Its  length  is   nearly  equal  to   the  ' 

*  Herschel.  chap.  xii.  ( 


384 


THE   STELLAR  UNIVERSE. 


Figs.  4—8. 


apparent  diameter  of  the  moonj  and  its  greatest  breadth  a  little  less  than  half 
its  length. 

It  has  been  conjectured  that  this,  and  similar  nebulae,  are  in  reality  flat  cir- 
cular strata  of  stars,  which  are  rendered  elliptical  by  projection,  being  seen  in  a 
direction  oblique  to  their  plane,  and  having  their  diameters  foreshortened  into 
the  lesser  axis  of  the  ellipse. 

In  fig.  6,  is  represented  an  elliptical  spindle-shaped  nebula,  placed  very 
near  that  represented  in  fig.  5.  This  form  of  nebulae  is  very  common,  and  is 
generally  supposed  to  be  produced  by  an  annular  mass  or  ring  of  stars,  which, 
being  seen  very  obliquely,  appears  of  the  elongated  form  here  depicted.  Two 
annular  nebulae,  seen  in  directions  nearly  perpendicular  to  their  planes,  and 
therefore  not  foreshortened,  are  represented  in  fig.  7  and  fig.  8.  The  former 
is  situated  between  the  stars  a  and  0  Lyrm,  and  may  be  seen  with  a  telescope 
of  moderate  power.     It  is  well-defined,  and  is  slightly  elliptical  in  its  form. 


THE   STELLAR  UNIVERSE. 


385 


Figs.  9—10. 


The  open  space  within  the  ring  is  not  entirely  dark,  but  seems  filled  with  a 
faint  hazy  nebulosity.  The  nebula  represented  in  fig.  8,  is  situated  near  the 
star  y,  in  the  constellation  of  the  Swan,  and  is  seen  on  the  meridian  about  the 
10th  September,  at  nine  in  the  evening.  In  the  parallel  of  New  York  it 
passes  the  meridian  about  ten  degrees  south  of  the  zenith. 

A  sketch  is  given  in  fig.  9  of  one  of  the  most  remarkable  nebulae  in  the 
heavens.  This  object  is  situate  about  five  degrees  south  by  west  of  i,  the  last 
star  in  the  tail  of  the  Great  Bear.  It  consists  of  a  bright  round  nebulous  cen- 
tral spot,  surrounded  at  a  great  distance  by  a  nebulous  ring,  which  seems  to 
be  split  into  two  throughout  nearly  half  of  its  circumference,  the  two  portions 
being  separated  by  an  angle  of  about  45^.  This  object  is  thus  noticed  by  Sir 
John  Herschel,  in  his  Memoir  on  Nebula;  (Phil.  Trans.,  1833): — 

"This  very  singular  object  is  thus  described  by  Messier :  '  Nebuleuse  sans 
etoiles,     On  ne  pent  la  voir  que  difficilement  avec  une  lunette  ordinaire  de  3;^ 

vol,.  II.-35 


386 


THE   STELLAR  UNIVERSE. 


pieds.  EUe  est  double,  ayant  chacime  un  centre  brillant  eloigne  I'un  de 
i'auire  de  4'  35".  Les  deux  atmospheres  se  touchent.'  By  this  description 
it  is  evident  that  the  peculiar  phenomena  of  the  nebulous  ring  which  encircles 
the  central  nucleus  had  escaped  his  observation,  as  might  have  been  expected 
from  the  interior  light  of  his  telescopes.  My  father  describes  it  in  his  obser- 
vations of  Messier's  nebulse  (which  are  not  included  in  his  catalogues),  as  a 
bright  round  nebula,  surrounded  with  a  halo  of  glory  at  a  distance  from  it,  and 
accompanied  by  a  companion  ;  but  I  do  not  find  that  the  partial  subdivision  of 
the  ring  into  two  branches  throughout  its  south  following  limb  was  noticed  by 
him.  This  is,  however,  one  of  its  most  remarkable  and  interesting  features. 
Supposing  it  to  consist  of  stars,  the  appearance  it  would  present  to  a  spectator 
placed  on  a  planet  attendant  on  one  of  them,  eccentrically  situated  toward  the 
north  preceding  quarter  of  the  central  mass,  would  be  exactly  similar  to  that 
of  our  Milky  Way,  traversing  in  a  manner  precisely  analogous  the  firmament 

Fisfs.  11—12. 


THE   STELLAIl  UNIVERSE. 


of  large  stars,  into  which  the  central  cluster  would  be  seen  projected,  and 
(owing  to  its  greater  distance)  appearing,  like  it,  to  consist  of  stars  much 
smaller  than  those  in  other  parts  of  the  heavens.  Can  it  then  be  that  we  have 
here  a  real  brother  system,  bearing  a  real  ph^^sical  resemblance  and  strong 
analogy  of  structure  to  our  own  ?  The  elliptic  form  of  the  inner  subdivided 
portion  indicates  with  extreme  probability  an  elevation  of  that  portion  above 
the  plane  of  the  rest,  so  that  the  real  form  must  be  that  of  a  ring  split  through 
half  its  circumference,  and  having  the  split  portions  set  aS-under  at  about  an 
angle  of  45°  each  to  the  plane  of  the  other." 

A  representation  of  this,  as  it  might  appear  if  seen  as  we  see  our  own  Milky 
Way  in  the  direction  of  its  plane,  is  giv^n  ia  fig.  11,  by  which  its  analogy  to 
the  Milky  Way,  will  be  rendered  still  more  apparent.  ' 

In  figures  11  and  12,  are  represented  two  nebulae,  one  of  which  belongs  to 
the  class  which  is  distinctly  resolvable  into  stars,  and  in  which  the  condensa- 
tion at  the  centre  is  so  great  that  it  becomes  at  that  point  a  perfect  blaze   of 

FiiT.  13. 


THE    STELLAR  UNIVERSE. 


light.  The  other,  fig.  12,  is  at  a  distance  so  much  greater,  that  even  to  the 
most  powerful  instruments  it  presents  only  the  appearance  of  a  faint  nebulous 
patch. 

One  of  the  most  splendid  objects  of  this  class  to  be  seen  in  the  heavens,  is 
the  great  nebula  in  the  constellation  of  Orion.  Let  the  eye  be  directed  to  the 
three  well-known  stars  composing  what  is  called  the  Belt.  Immediately  below 
these,  and  verv  nearly  parallel  to  them  in  direction,  will  be  seen  three  stars 
at  nearly  equal  distances  asunder,  the  two  lower  of  the  third  and  the  upper  of 
the  fourth  magnitude.  If  the  middle  star  of  these  three  be  attentively  viewed 
with  the  naked  eye,  the  observer  will  find  that  it  wants  distinctness.  It  will 
be  found  to  present  a  hazy  appearance.  If  a  common  telescope  be  directed  to 
it,  it  will  be  evidently  perceived  to  be  a  nebula.  In  fine,  the  appearance  it 
presents  in  a  twenty-feet  reflector  is  exhibited  in  the  annexed  drawing,  fig.  13. 

The  following  are  the  observations  of  Sir  John  Herschel  upon  this  ob- 
ject: — 

"  I  know  not  how  to  describe  it  better  than  by  comparing  it  with  a  curdling 
liquid,  or  a  surface  strewed  over  vi^ith  flocks  of  wool,  or  to  the  breaking  up  of 
a  mackerel  sky,  when  the  clouds  of  which  it  consists  begin  to  assume  a 
cirrous  appearance.  It  is  not  very  unlike  the  mottling  of  the  sun's  disk,  only, 
if  I  may  so  express  myself,  the  grain  is  much  coarser  and  the  intervals  darker, 
and  ihe  flocculi,  instead  of  being  generally  round,  are  drawn  into  little  wisps. 
They  present,  however,  an  appearance  of  being  composed  of  stars,  and  their 
aspect  is  altogether  different  from  that  of  resolvable  nebulce.  In  the  latter  we 
fancy  by  glimpses  that  we  see  stars,  or  that,  could  we  strain  our  sight  a  little 
more,  we  would  see  them  ;  but  the  former  suggests  no  idea  of  stars,  but  rather 
of  something  quite  distinct  from  them." 

Sir  William  Herschel,  who  had  previously  examined  it,  says  :^ . 

"  In  the  year  1774,  the  4th  of  March,  I  observed  the  nebulous  star  which  is 
the  43d  of  the  Connaissance  des  Temps,  and  is  not  many  minutes  north  of  the 
great  nebulae :  but  at  the  same  time  I  also  took  notice  of  two  similar,  but  much 
smaller  nebulous  stars,  one  on  each  side  of  the  large  one,  and  at  nearlj^  equal 
distances  from  it.  In  1783  I  examined  the  nebulous  star,  and  found  it  to  be 
faintly  surrounded  with  a  circular  glory  of  whitish  nebulosity,  faintly  joining 
it  to  the  great  nebula.  About  the  latter  end  of  that  year  I  remarked  that  it 
was  not  equally  surrounded,  but  most  nebulous  toward  the  south.  In  1784  I 
began  to  entertain  an  opinion  that  the  star  was  not  connected  with  the  nebu- 
losity of  the  great  nebula  of  Orion,  but  was  one  of  those  which  are  scattered 
over  that  part  of  the  heavens.  In  1801 ,  1806,  and  1810,  this  opinion  was  fully 
confirmed  by  the  gradual  change  which  happened  in  that  great  nebula  to  which 
the  nebulosity  surrounding  the  star  belongs  ;  for  the  intensity  of  light  about 
the  nebulous  star  had  by  this  time  been  considerably  reduced  by  the  attenua- 
tion or  dissipation  of  the  nebulovTS  matter,  and  it  seemed  now  to  be  pretty 
evident  that  the  star  is  far  behind  the  nebulous  matter,  and  that,  consequently, 
its  light  in  passing  through  it  is  scattered  and  deflected  so  as  to  produce  the 
appearance  of  a  nebulous  star."  .  .  .  .  "  When  I  viewed  this  interesting  ob- 
ject in  December,  1810,  I  directed  my  attention  particularly  to  the  two  nebu- 
lous stars  by  the  sides  of  the  large  one,  and  found  they  were  perfectly  free 
from  every  nebulous  appearance,  which  confirmed  not  only  my  former  surmis*^ 
of  the  great  attenuation  of  the  nebulosity,  but  also  proved  that  their  former 
nebulous  appearance  had  been  entirely  the  effect  of  the  passage  of  their  feeble 
light  through  the  nebulous  matter  spread  out  before  them.  The  19th  of  Jan- 
uary, 1811,  I  had  another  critical  examination  of  the  same  object,  in  a  very 
clear  view,  through  the  forty-feet  telescope  ;  but,  notwithstanding  the  superior 
light  of  this  instrument,  I  could  not  perceive  any  remains  of  nebulosity  about 


THE   STELLAR  UNIVERSE. 


389 


the  two  small  stars,  which  were  perfectly  clear,  and  in  the  same  situation 
where,  about  thirty-seven  years  before,  I  had  seen  them  involved  in  nebulosity. 
If,  then,  the  light  of  these  three  stars  is  thus  proved  to  have  undergone  a  visible 
modification  in  its  passage  through  the  nebulous  matter,  it  follows  that  its 
situation  among  the  stars  is  less  distant  from  us  than  the  largest  of  the  three, 
which  I  suppose  to  be  of  the  eighth  or  ninth  magnitude.  The  farthest  distance, 
therefore,  at  which  we  can  place  the  faintest  part  of  the  great  nebula  in  Orion, 
to  which  the  nebulosity  surrounding  the  star  belongs,  can  not  well  exceed  the 
region  of  the  stars  of  the  seventh  or  eighth  magnitude." 


Fig.  14. 


In  fig.  14,  annexed,  is  represented  a  nebulous  patch,  differing  in  appearance 
from  those  already  described.  It  is  taken  from  a  telescopic  drawing  made  by 
Mr.  Dunlop  at  Paramatta.  Sir  John  Herschel,  respecting  these  Megallanic 
clouds,  as  they  are  called,  says  : — 

"The  nubecula,  major   and    minor,  are  very  extraordinary  objects.     The 


390 


THE    STELLAR  UNIVERSE. 


greater  is  a  congeries  of  clusters  of  irregular  form,  globular  clusters,  and 
nebulse  of  various  magnitudes  and  degrees  of  condensation,  among  which  is 
interspersed  a  large  portion  of  irresolvable  nebulae,  which  may  be,  and  prob- 
ably is  star-dust,  but  which  the  power  of  the  twenty-feet  telescope  shows 
only  as  a  general  illumination  of  the  field  of  view,  forming  a  bright  ground  on 
which  other  objects  are  scattered.  Some  of  the  objects  in  it  are  of  very  sin- 
gular and  incomprehensible  forms ;  the  chief  one,  especially  (30  Doradus), 
which  consists  of  a  number  of  loops  united  in  a  kind  of  unclear  centre  or  knot, 
like  a  bunch  of  ribands,  disposed  in  what  is  called  a  true-lover^s  knot !  There 
is  no  part  of  the  heavens  where  so  many  nebulse  and  clusters  are  crowded  into 
so  small  a  space  as  this  '  cloud.'  The  nubecula  junior  is  a  much  less  striking 
object.  It  abounds  more  in  irresolvable  nebulous  light ;  but  the  nebulee  and 
clusters  in  it  are  fewer  and  fainter,  though  immediately  joining  to  it  is  one  of 
the  richest  and  most  magnificent  clusters  in  the  hemisphere." 


Pig.  15. 


THE    STELLAR  UNIVERSE. 


391 


In  some  parts  of  the  firmament  appearances  have  been  observed  which  have 
led  to  the  conjecture  that  nebulous  patches  may  be  in  a  state  of  progressive 
formation  into  stellar  clusters.  An  illustration  of  this  is  presented  in  fig.  15, 
annexed,  in  which  a  patch  in  the  constellation  of  Virgo  is  represented.  This 
portion  of  the  heavens  is  strewn  over  with  small  round  telescopic  clusters,  the 
stars  of  which  seem  to  be  closely  condensed  together. 

"  Planetary  nebula,"  says  Sir  John  Herschel,  "  are  very  extraordinary  ob- 
jects. They  have,  as  their  name  imports,  exactly  the  appearance  of  planets  ; 
round  or  slightly  oval  disks,  in  some  instances  quite  sharply  terminated,  in 
others  a  little  hazy  at  the  borders,  and  of  a  light  exactly  equable  or  only  a  very 
little  mottled,  wdiich,  in  some  of  them,  approaches  in  vividness  to  that  of  actual 
planets.  Whatever  be  their  nature,  they  must  be,  of  enormous  magnitude. 
One  of  them  is  to  -be  found  in  the  parallel  of  ^  Aquarii,  and  about  5m.  preceding 
that  star.  Its  apparent  diameter  is  about  20".  Another,  in  the  constellation 
Andromeda,  presents  a  visible  disk  of  12",  perfectly  defined  and  round.  Grant- 
ing these  objects  to  be  equally  distant  from  us  with  the  stars,  their  real  dimen- 
sions must  be  such  as  would  fill,  on  the  lowest  computation,  the  whole  orbit 
of  Uranus.,  It  is  no  less  evident  that,  if  they  be  solid  bodies  of  a, solar  nature, 
the  intrinsic  splendor  of, their  surfaces  must  be  almost  infinitely  inferior  to  that 
of  the  sun's.  A  circular  portion  of  the  sun's  disk,  subtending  an  angle  of  20", 
would  give  a  light  equal  to  100  full  moons ;  while  the  objects  in  question  are 
hardly,  if  at  all,  discernable  with  the  naked  eye.  The  uniformity  of  their 
disks,  and  their  want  of  apparent  central  condensation,  would  certainly  augur 
their  light  to  be  merely  superficial,  and  in  the  nature  of  a  hollow  spherical 
shell  ;  but  whether  filled  with  solid  or  gaseous  matter  or  altogether  empty,  it 
would  be  waste  of  time  to  conjecture. 

"  The  nebuke  furnish,  in  every  point  of  view,  an  inexhaustible  field  of  spec- 
ulation and  conjecture.  That  by  far  the  larger  share  of  them  consist  of  stars 
there  can  be  little  doubt;  and  in  the  interminable -range  of  system  upon  sys- 
tem, and  firmament  upon  firmament,  which  we  thus  catch  a  glimpse  of,  the 
imagination  is  bewildered  and  lost.  On  the  other  hand,  if  it  be  true,  as,  to  say 
the  least,  it  seems  extremely  probable,  that  a  phosphorescent  or  self-luminous 
matter  also  exists,  disseminated  through  extensive  regions  of  space,  in  the 
manner  of  a  cloud  or  fog — now  assuming  capricious  shapes,  like  actual  clouds 
drifted  by  the  wind,  and  now  concentrating  itself  like  a  cometic  atmosphere 
around  particular  stars  ;  what,  we  naturally  ask,  are  the  nature  and  destination 
of  this  nebulous  matter  ?  Is  it  absorbed  by  the  stars  in  whose  neighborhood 
it  is  found,  to  furnivsh,  by  its  condensation,  their  supply  of  light  and  heat  ?  or 
is  it  progressively  concentrating  itself  by  the  effect  of  its  own  gravity  into 
masses,  and  so  laying  the  foundation  of  new  sidereal  systems  or  of  insulated 
stars  1  It  is  easier  to  propound  such  questions  than  to  offer  any  probable  reply 
to  them.  Meanwhile,  appeal  to  fact,  by  the  method  of  constant  and  diligent 
observation,  is  open  to  us ;  and,  as  the  double  stars  have  yielded  to  this  style 
of  questioning,  and  disclosed  a  series  of  relations  of  the  most  intelligible  and 
interesting  description,  we  may  reasonably  hope  that  the  assiduous  study  of 
the  nebulae  will,  ere  long,  lead  to  some  clearer  understanding  of  their  intimate 
nature." 

Having  thus  given  examples  of  the  principal  varieties  of  form  and  condition 
exhibited  by  these  objects,  we  may  add  that  their  number  seems  to  be  almost 
as  unlimited  as  that  of  the  stars,  and  that,  like  the  stars,  they  are  not  equally 
distributed  over  every  part  of  the  firmament,  but  prevail  most  in  particular  re- 
gions of  the  heavens.  The  catalogue  of  nebulae  published  by  Sir  John  Her- 
schel in  the  Philosophical  Transactions  for  1833,  contains  a  list  of  2,306 
nebulae  visible  from  the  observatory  at  Slough,  which  do  not  include  the  large 


392 


THE    STELLAR   UNIVERSE. 


number  since  then  observed  during  the  residence  of  that  astronomer  at  the 
cape  of  Good  Hope.  Although  they  are  very  irregularly  scattered  on  the  fir- 
mament, there  seems  to  be  some  ground  for  concluding  that  they  prevail  chiefly 
in  the  direction  of  a  great  circle  of  the  heavens  inclined  at  a  certain  angle 
vrith  the  general  direction  of  the  Milky  Way.  The  following  passages  from 
the  memoirs  of  Sir  William  Herschel  vvrill  better  explain  the  manner  in  which 
they  are  distributed  than  any  mere  general  description  which  could  be  given  : — 

"  The  nebulae  are  arranged  into  strata,  and  run  on  to  a  great  length ;  and 
some  of  them  I  have  been  able  to  pursue,  and  to  guess  pretty  well  at  their 
form  and  direction.  It  is  probable  enough  that  they  may  surround  the  whole 
starry  sphere  of  the  heavens,  not  unlike  the  Milky  Way,  which  undoubtedly 
is  nothing  but  a  stratum  of  fixed  stars.  And  as  this  latter  immense  starry  bed 
is  not  of  equal  breadth  or  lustre  in  every  part,  nor  runs  on  in  one  straight  di- 
rection, but  is  curved  and  even  divided  into  streams  along  a  very  considerable 
portion  of  it,  we  may  likewise  expect  the  greatest  variety  in  the  strata  of  the 
clusters  of  stars  and  nebulae.  One  of  these  nebulous  beds  is  so  rich,  that  in 
passing  through  a  section  of  it,  in  the  time  of  only  thirty-six  minutes,  I  have 
detected  no  less  than  thirty-one  nebulae  all  distinctly  visible  upon  a  fine  blue 
sky.  Their  situation  and  shape,  as  well  as  condition,  seem  to  denote  the 
greatest  variety  imaginable.  In  another  stratum,  or  perhaps  a  different  branch 
of  the  former,  I  have  seen  double  and  treble  nebulae,  variously  arranged  ;  large 
ones  with  small,  seeming  attendants  ;  narrow,  but  much-extended  lucid  nebulae 
or  bright  dashes ;  some  of  the  shape  of  a  fan,  resembling  an  electric  brush 
issuing  from  a  lucid  point ;  others  of  the  cometic  shape,  with  a  seeming  nu- 
cleus in  the  centre,  or  like  cloudy  stars  surrounded  with  a  nebulous  atmosphere. 
,  A  different  sort,  again,  contain  a  nebulosity  of  the  milky  kind,  like  that  won- 
derful, inexplicable  phenomenon  about  9  Orionis ;  while  others  shine  with  a 
fainter  mottled  kind  of  light,  which  denotes  their  being  resolvable  into  stars. 

"  In  my  late  observations  on  nebulae,  I  have  found  that  I  generally  detected 
them  in  certain  directions  rather  than  in  others  ;  that  the  spaces  preceding 
them  were  generally  quite  deprived  of  their  stars,  so  as  often  to  afford  many 
fields  without  a  single  star  in  it ;  that  the  nebulae  generally  appeared  some 
time  after  among  stars  of  a  certain  considerable  size,  and  but  seldom  among 
very  sm.all  stars  ;  and  when  I  came  to  one  nebula,  I  generally  found  several 
more  in  the  neighborhood ;  that  afterward  a  considerable  time  passed  before  I 
came  to  another  parcel.  These  events  being  often  repeated  in  different  alti- 
tudes of  my  instrument,  and  some  of  them  at  considerable  distances  from  each 
other,  it  occurred  to  me  that  the  intermediate  spaces  between  the  sweeps  might 
also  contain  nebulae ;  and  finding  this  to  hold  good  more  than  once,  I  ventured 
to  give  notice  to  my  assistant  at  the  clock  that  '  I  found  myself  on  nebulous 
ground.' " 

The  conclusion,,  therefore,  which  follows  from  a  general  view  of  all  the  phe- 
nomena is,  that  the  sun  is  an  individual  star  of  one  great  cluster,  occupy- 
ing a  position  near  to,  but  not  in  its  centre  ;  that  the  stars  of  this  cluster,  seen 
in  every  direction  around  us,  constitute  the  starry  heavens  as  they  are  visible 
to  us  ;  that  those  which  are  placed  nearest  to  the  sun  in  the  cluster,  present 
to  us  the  appearance  of  stars  of  the  first  magnitude,  and  that  the-  others  appear 
to  be  less  and  less  bright  and  large  as  their  distances  are  greater  and  greater ; 
that  the  most  remote  and  most  numerous  stars  of  the  cluster  are  individually 
lost  to  the  eye  by  their  distance,  but  being  confounded  together  like  grains  of 
powder  thickly  sprinkled  on  the  general  firmament,  form  the  Milky  Way ;  that 
this  cluster  of  ours  is  not  the  only  one  of  the  kind  in  the  universe,  but  that 
there  are  many  thousands  of  others  scattered  through  the  depths  of  immensity  ;  , 
that  those  which  are  nearest  to  our  own   cluster  can  be  seen   by  sufficiently  ' 


THE   STELLAR  UNIVERSE. 


powerful  telescopes  so  plainly  that  the  individual  stars  which  compose  them 
can  be  distinguished,  and  these  accordingly  are  called  resolvable  nebulcs ;  that 
some  more  remote  give  doubtful  appearances  in  the  telescope,  leaving  the  ob- 
server convinced  that  a  little  greater  proximity  of  the  object,  or  a  little  greater 
power  of  the  telescope,  would  render  the  stars  composing  them  distinctly 
visible ;  that  others,  still  more  remote,  are  at  such  enormous  distances  as  to 
present  no  appearance,  even  to  instruments  of  the  greatest  powers,  except  that 
of  a  faint  nebulous  patch  ;  and  finally,  that  every  augmentation  of  the  power 
of  telescopes  will  resolve  a  greater  number  of  these  nebulse  into  distinct  stars, 
and  bring  others  which  now  can  not  be  seen  at  all  into  view ;  and  that  this 
progression  will  go  on  without  limit,  the  universe  still  expanding  wider  and 
more  wide  into  the  depths  of  infinite  space,  before  the  increasing  power  of 
science. 

But  is  this  all  which  can  be  inferred  ?  That  innumerable  clusters  may  ex- 
ist at  such  distances  as  only  to  appear  as  nebulous  patches,  even  under  the 
space-penetrating  power  of  instruments  so  colossal  as  those  of  Sir  William 
Herschel,  and  the  more  recent  ones  constructed  by  Lord  Rosse,  can  not  be 
disputed;  but  are  all  nebulous  appearances  of  this  character?  Distance  is 
indisputably  a  cause  of  nebular  phenomena,  but  is  it  the  only  cause  ?  This  is 
a  question  which  will  require  some  discussion. 

Sir  William  Herschel,  who  was  the  first  to  explain  the  phenomena  of  clus- 
ters and  nebulce  by  the  supposition  of  distinct  and  separate  masses  of  stars 
removed  to  such  a  distance  as  to  subtend  a  small  visual  angle,  was  also  the 
first  to  raise  a  doubt  whether  this  cause  alone  be  sufficient  to  explain  all  the 
nebular  phenomena.  After  long,  patient,  and  minute  surveys  of  the  heavens, 
he  was  at  last  impressed  with  the  belief  that  certain  appearances  indicated  also 
the  actual  existence  of  luminous  matter  in  situations  comparatively  near  to  us, 
and  presenting  the  same  or  nearly  the  same  appearance  as  masses  of  stars 
would  whose  distinctness  would  melt  away  in  the  magnitude  of  their  distance. 
Among  the  phenomena  which  suggested  this  idea,  the  most  prominent  vvere 
those  of  nebulous  stars.  These  objects  appear  as  a  bright  stellar  point,  some- 
times of  the  seventh  or  eighth  magnitude,  surrounded  by  a  faintly  luminous 
atmosphere  of  several  minutes  diameter.  The  star  appears  exactly  in  the 
centre,  and  the  atmosphere  around  it  perfectly  circular  in  its  outline  is  so 
diluted,  faint,  and  equal  throughout,  as  to  suggest  no  idea  of  its  consisting  of 
stars.  "If,"  says  Sir  William  Herschel,  "the  nebulosity  in  this  case  consists 
of  stars,  appearing  nebulous  because  of  their  distance  which  causes  them  to 
run  into  each  other,  what  must  be  the  size  of  the  central  body,  which,  at  so 
enormous  a  distance,  yet  so  far  outshines  all  the  rest  ?  In  the  next  place,  if 
the  central  star  be  no  bigger  than  common,  how  very  small  and  compressed 
must  be  the  other  luminous  points  which  send  us  only  so  faint  a  light?  In 
the  former  case  the  central  body  would  far  exceed  what  we  call  a  star  ;  and 
in  the  latter,  the  shining  matter  about  the  centre  would  be  too  small  to  come 
under  that  designation.  Either,  then,  we  have  a  central  body,  which  is  not  a 
star,  or  a  star  involved  in  a  shining  fluid  of  a  nature  wholly  unknown  to  us." 

There  is  one  other  supposition  which  will  suggest  itself.  The  central 
bright  star  may  be  immeasurably  nearer  to  us  than  the  cluster  which,  by  its 
enormous  distance,  is  reduced  to  a  luminous  haze,  and  may  be  projected  upon 
it  in  the  direction  of  the  visual  ray.  Against  this  is  to  be  advanced  the  im- 
probability of  such  a  casual  projection,  throwing  the  nearer  star  into  the 
mathematical  centre  of  the  distant  cluster.  Such  an  accident  might  possibly 
occur  in  one  or  two  instances,  but  we  find  it  taking  place  in  all  cases  of  nebu- 
lous stars.  In  some  parts  of  the  heavens  these  stars  appear  in  considerable 
numbers.     Sir  John  Herschel  mentions  the  nebulas  surrounding  the  quadruple 


394 


THE    STELLAR  UNIVERSE. 


or  rather  sextuple  star  s  in  Orion,  and  the  star  i,  in  the  constellation  called 
Robur  Caroli,  as  examples  of  nebulous  appearances  not  easily  explicable  by 
the  supposition  of  distant  masses  of  stars.  "  The  nebulous  character  of  these 
objects,"  says  he,  "  at  least  of  the  former,  is  very  different  from  what  might 
be  supposed  to  arise  from  the  congregation  of  an  immense  collection  of  small 
stars.  It  is  formed  of  little  flocky  masses,  like  wisps  of  cloud;  and  such 
wisps  seem  to  adhere  to  many  small  stars  at  its  outskirts,  and  especially  to 
one  considerable  star  (represented,  in  the  figure,  below  the  nebula),  which  it 
envelopes  with  a  nebulous  atmosphere  of  considerable  extent  and  singular 
figure.  Several  astronomers,  on  comparing  this  nebula  with  the  figures  of  it 
handed  down  to  us  by  its  discoverer,  Huygens,  have  concluded  that  its  form  has 
undergone  a  perceptible  change.  But  when  it  is  considered  how  difficult  it 
is  to  represent  such  an  object  duly,  and  how  entirely  its  appearance  will  dif- 
fer, even  in  the  same  telescope,  according  to  the  clearness  of  the  air,  or  other 
temporary  causes,  we  shall  readily  admit  that  we  have  no  evidence  of  change 
that  can  be  relied  on." 

The  impression  of  the  necessity  of  admitting  the  existence  of  a  subtle, 
self-luminous,  nebulous  fluid  in  the  universe,  gradually  stole  upon  the  mind 
of  Sir  William  Herschel,  and  appears  to  be  admitted  by  him  with  that  re- 
luctance which  is  felt  when  we  are  forced  to  admit  something  which  a  favor- 
ite hypothesis  fails  to  explain. 

"  When  I  pursued  these  researches/'  says  he,  "I  was  in  the  situation  of  a 
natural  philosopher  who  follows  the  various  species  of  animals  and  insects  from 
the  height  of  their  perfection  down  to  the  lowest  ebb  of  life ;  when  arriving 
at  the  vegetable  kingdom,  he  can  scarcely  point  out  the  precise  boundary 
where  the  animal  ceases  and  the  plant  begins,  and  may  even  go  so  far  as  to 
suspect  them  not  to  be  essentially  different.  But  recollecting  himself,  he 
compares-,  for  instance,  one  of  the  human  species  with  a  tree,  and-all  doubt 
upon  the  subject  vanishes  before  him.  In  the  same  manner  we  pass  by 
gentle  steps  from  a  coarse  cluster  dovyn  through  others  more  remote,  and 
therefore  of  finer  texture,  without  any  hesitation,  till  we  find  ourselves  brought 
to  an  object  such  as  the  nebula  in  Orion,  when  we  are  still  inclined  to  remain 
in  our  once  adopted  idea  of  stars  exceedingly  remote  and  inconceivably  crowd- 
ed, as  being  the  occasion  of  that  remarkable  occurrence.  It  seems,  therefore, 
to  require  a  more  dissimilar  object  to  bring  us  right  again.  ,  A  glance  like 
that  of  the  naturalist,  who  casts  his  eye  from  the  perfect  vegetable  to  the  per- 
fect animal,  is  wanting  to  remove  the  veil  from  the  mind  of  astronomers." 

We  must  then  conclude  that  appearances  have  been  observed  in  the 
heavens  which  can  not  be  satisfactorily  explained  by  the  supposition  of  dis- 
tant masses  of  stars.  The  supposition  of  self-luminous  nebulous  matter,  dif- 
fused in  certain  regions  of  the  universe,  has  consequently  been  proposed  as 
the  only  other  mode  of  explaining  them.  That  such  matter,  if  it  exist,  is  in 
the  state  or  condition  of  vapor  or  fluid,  is  difficult  to  admit,  since  there  ap- 
pears more  permanency  about  the  nebular  phenomena  than  could,  be  easily 
reconciled  with  such  a  state.  The  most  eminent  mathematician  and  natural 
philosopher  of  the  present  century,  has  however  adopted  the  supposition  of 
a  widely-diffused  nebulosity,  and  has  made  it  the  basis  of  one  of  the  boldest 
and  most  remarkable  conjectures  of  modern  times.  Laplace  has  suggested 
that  systems  such  as  that  of  our  sun  and  planets,  might  be  conceived  to  be 
produced  by  the  mere  operation  of  mechanical  laws  out  of  such  a  nebular 
chaos !  The  gradual  changes  which  this  supposition  compels  us  to  admit 
must  be  imagined  to  be  so  slow,  that  in  the  whole  duration  of  our  experience 
or  observation  of  the  heavens  they  have  not  been  perceptible.  In  other 
words,  the  period  of  time   over  which  astronomical  observation   extends,  is 


THE    STELLAR  UNIVERSE. 


395 


but  a  moment  in  the  growth  of  a  system.  The  length  of  some  astronomical 
periods,  the  reality  of  which  is  not  disputed,  is  adduced  to  justify  this.  The 
planet  Herschel  or  TJrayius,  has  not  yet  completed  a  single  period  since  its 
discovery ;  and  several  of  the  binary  stars  have  been  observed  to  move 
through  only  a  small  arc  of  their  entire  course.  In  the  solar  system  many 
secular  changes  have  been  discovered,  the  completion  of  which  will  occupy 
many  thousand  years.  Yet  the  reality  of  these  is  not  the  less  certain.  It  is 
contended,  therefore,  that  the  gradual  change  of  a  nebula  into  a  system,  not 
having  been  actually  seen,  is  no  conclusive  argument  against  its  possible  ex- 
istence. 

In  the  celebrated  nebular  hypothesis,  which  its  illustrious  author  propounds 
as  a  mere  conjecture,  and  with  great  diffidence  it  is  supposed — that  the  sun 
has  been  formed  by  the  gradual  condensation  and  solidification  of  a  mass  of 
nebulous  matter ;  that  it  revolved  together  with  the  nebulous  atmosphere 
around  it  in  the  direction'  in  which  the  planets  now  revolve,  and  which  atmo- 
sphere, by  the  operation  of  an  excessive  degree  of  heat,  extended  to  a  distance 
from  the  common  centre  much  greater  than  that  of  the  most  remote  planet ; 
that  as  this  heat  gradually  diminished,  the  solar  atmosphere  contracted  accord- 
ing to  the  common  law  of  cooling  bodies;  that  in  accordance  with  the  laws 
of  revolving  bodies,  the  velocity  of  its  rotation  increased;  that  an  exterior 
zone  of  vapor  was  detached  from  the  rest,  where  the  centrifugal  force  pro- 
duced by  the  central  motion,  exceeded  the  central  attraction  ;  that  such  zone 
of  vapor  might  collect  into  a  ring  like  those  of  Saturn;  might  aggregate  into 
several  masses  revolving  nearly  in  the  same  circle,  like  the  new  planets  ;  or, 
finally,  might  coalesce  into  a  single  mass.  Thus  would  be  formed  a  number 
of  planets  which  at  first  would  be  vapor.  These  planets  would,  according  to 
the  laws  of  mechanics,  have  rotatory  motions  on  their  axes  ;  these  rotatory 
motions  would  be  all  in  the  same  direction,  and  as  the  vapor  would  gradually 
cool  down,  each  planet  might  form  round  its' own  centre  satellites  or  rings,  in 
the  same  manner  as  the  planets  themselves  would  be  formed  round  the  central 
sun.  _ 

This  supposition  will  evidently  explain  the  most  obvious  provisions  of  our 
system.  If  it  did  not,  it  would  never  have  been  proposed  by  its  author.  It  is 
evident  that  all  the  principal  motions  of  such  a  system  would  be  nearly  circu- 
lar, and  nearly  in  the  plane  of  the  original  motion  of  rotation  of  the  nebulous 
mass.  It  is  easily  proved  also  that  the  motions  of  J;he  satellites  round  the 
planets  respectively,  and  the  motion  of  both  planets  and  satellites  on  their 
axes  would  be  in  one  common  direction  and  one  common  plane.  Thus  it 
may  be  admitted  that  the  most  important  laws  on  which  the  stability  of  the 
solar  system  depends  w^ould  be  explained. 

But  some  modern  writers  on  this  subject  have  ascribed  to  this  conjecture  of 
a  distinguished  man  a  much  more  serious  character  than  the  author  himself 
claimed  for  it.  Laplace  too  well  understood  the  rigorous  canons  of  inductive 
philosophy  to  view  his  guesses  as  anything  higher  than  an  extremely  refined 
and  ingenious  conjecture,  certainly  not  deserving  the  name  of  a  theory,  and 
scarcely  proper  to  be  called  even  an  hypothesis. 

It  is  not  worth  while  here  to  notice  the  innumerable  arrangements  which  it 
fails  to  explain,  arrangements  certainly  not  less  important  than  those  which 
have  been  selected  as  the  basis  of  the  conjecture.  But  it  may  be  well  to  warn 
those  who  are  little  familiar  with  such  inquiries  against  the  errors  to  which  such 
an  hypothesis  might  give  birth.  It  is  the  general  tendency  of  every  ascent 
in  the  analysis  of  causation  to  give  the  appearance  of  superseding  the  supposi- 
tion of  an  omnipotent  agent  above  matter  and  its  laws,  the  fountain  of  the  intel- 
ligence, wisdom,  design,  and  beneficence,  manifested  in   the  visible  creation. 


396 


THE    STELLAR  UNIVERSE. 


Yet  the  admission  of  the  nebular  hypothesis,  were  it  conclusively  established, 
would  have  no  more  real  effect  on  the  source  of  the  wisdom  of  creation  than 
the  admission  of  the  theory  of  gravitation.  Every  step  we  make  in  the  gene- 
ralization of  the  phenomena  supplied  by  observation  only  transfers  our  view 
of  the  first  cause  one  degree  higher.  In  the  present  case  we  may  ask  with  a 
distinguished  contemporary,  "  how  came  the  sun  and  its  atmosphere  to  have 
such  materials,  such  motions,  such  a  constitution,  that  these  beneficent  conse- 
quences followed  from  their  primordeal  condition  ?  How  came  the  parent 
vapor,  if  vapor  it  were,  to  be  thus  capable  of  coherence,  separation,  contraction, 
solidification  ?  How  came  the  laws  of  its  motion,  attraction,  repulsion,  con- 
densation, to  be  so  fixed  as  to  lead  to  a  beautiful  and  harmonious  system  in 
the  end  1  How  came  it  to  be  neither  too  fluid  nor  too  tenacious,  to  contract 
neither  too  quickly  nor  too  slowly  for  the  successive  formation  of  the  several 
planetary  bodies  ?  How  came  that  substance  which  at  one  time  was  a  lu- 
minous vapor  to  be  at  a  subsequent  period  solids  and  fluids  of  many  various 
kinds  ?  What  but  design  and  intelligence  prepared  and  tempered  this  vari- 
ously-existing element,  so  that  it  should  by  its  natural  changes  produce  such 
an  orderly  system  ?" 

And,  we  may  further  ask,  to  what  else  except  intelligence  and  wisdom, 
prompted  by  beneficence,  can  be  ascribed  the  fact  that  the  source  of  light  and 
heat  should  be  placed  in  the  central  mass,  while  the  detached  revolving  masses 
are  deprived  of  this  quality  ?  or  why  is  it  that  the  apparatus  for  reflected  light 
augments  in  power  and  efiiciency  as  the  planet  to  be  supplied  with  light  is 
more  and  more  remote  from  the  sun  ?  and  why  is  the  distribution  of  land  and 
water  on  the  surface  of  the  globe  so  admirably  regulated  1  and  whence  has 
arisen  the  wonderful  adaptation  of  the  quantity  and  density  of  the  air,  and  the 
mechanical  and  physical  and  chemical  properties  of  that  fluid  to  all  the  ex- 
isting qualities  and  conditions  of  the  world  ?  These  are  manifestations  of 
design,  wisdom,  goodness,  and  power,  which  are  not  reached  or  pretended  to 
be  reached,  by  any  theory  or  hypothesis  that  the  human  mind  has  ever  yet 
devised,  save  that  only  which  we  find  in  the  character  of  the  Most  High, 
whether  imparted  by  the  voice  of  revelation  or  written  on  the  face  of  nature 


C^- 


THE    STEAM-ENGINE 

(FIRST    LECTURE.) 


The  Steam-Engine  a  Subject  of  popular  Interest. — Effects  of  Steam. — Great  Power  of  Steam. — 
Mechanical  Properties  of  Fluids. — Elastic  aud  inelastic  Fluids. — Elasticity  of  Gases.^Effects  of 
Heat. — Savery's  Engine. — Boilers  and  their  Appendages. — Working  Apparatus. — Mode  of  Op- 
eration.— Defects  of  Savery's  Engine. — Newcomen  and  Cawley's  Patent.— Accidental  Discov- 
ery of  Condensation  by  Injection. — Potter's  Invention  of  the  Method  of  working  the  Valves. — 
His  Contrivance  improved  by  the  Substitution  of  the  Plug-Frame. 


THE    STEAM-ENGINE. 


399 


THE  STEAl-ENGOE. 


(FIRST    LECTURE.) 


That  the  history  of  the  invention  of  a  piece  of  mechanism,  and  the  de- 
scription of  its  structure,  operation,  and  uses,  should  be  capable  of  being  ren- 
dered a  subject-matter  destined  not  alone  for  the  instruction  of  engineers  or 
machinists,  but  for  the  information  and  amusement  of  the  public  in  general,  is 
a  statement  which  at  no  very  remote  period  would  have  been  deemed  extrava- 
gant and  incredible. 

Advanced  as  we  are  in  the  art  of  rendering  kno-wledge  popular,  and  culti- 
vated as  the  public  taste  is  in  the  appreciation  of  the  expedients  by  which 
science  ministers  to  the  uses  of  life,  there  is  still  perhaps  but  one  machine  of 
which  such  a  proposition  can  be  truly  predicated  :  it  is  needless  to  say  that 
that  machine  is  the  steam-engine.  There  are  many  circumstances  attending 
this  extraordinary  piece  of  mechanism  which  impart  to  it  an  interest  so  uni- 
versally felt.  Whether  we  regard  the  details  of  its  structure  and  operation, 
the  physical  principles  which  it  calls  into  play,  and  tl\e  beautiful  contrivances 
by  which  these  physical  principles  are  rendered  available  :  or,  passing  over 
these  means,  we  direct  our  attention  to  the  ends  which  they  attain,  we  are 
equally  filled  with  astonishment  and  admiration.  The  history  of  the  steam- 
engine  offers  to  our  notice  a  series  of  contrivances  which,  for  exquisite  and 
refined  ingenuity,  stand  without  any  parallel  in  the  annals  of  mechanical  sci- 
ence. These  admirable  inventions,  urdike  other  results  of  scientific  inquiry, 
have  also  this  peculiarity,  that,  to  understand  their  excellence  and  to  perceive 
their  beauty,  no  previous  or  subsidiary  knowledge  is  necessary,  save  what  may 
be  imparted  with  facility  and  clearness  in  the  progress  of  the  explanation  and 
development  of  the  machine  itself.  A  simple  and  clear  exposition,  divested 
of  needless  technicalities,  and  aided  by  well-selected  diagrams,  is  all  that  is 
necessary  to  render  the  construction  and  operation  of  the  steam-engine,  in  all 
its  forms,  intelligible  to  persons  of  plain  understanding  and  moderate  informa- 
tion. 


400 


THE    STEAM-ENGINE. 


But  if  the  contrivances  by  which  this  vast  power  is  brought  to  bear  on  the 
arts  and  manufactures,  be  rendered  attractive  by  their  great  mechanical  beauty, 
how  much  more  imposing  will  the  subject  become  when  the  effects  which  the 
steam-engine  has  produced  upon  the  well-being  of  the  human  race  are  consid- 
ered !  It  has  penetrated  the  crust  of  the  earth,  and  drawn  from  beneath  it 
boundless  treasures  of  mineral  wealth,  which,  without  its  aid,  would  have 
been  rendered  inaccessible  ;  it  has  drawn  up,  in  measureless  quantity,  the  fuel 
on  which  its  own  life  and  activity  depend  ;  it  has  relieved  men  from  their  most 
slavish  toils,  and  reduced  labor  in  a  great  degree  to  light  and  easy  superin- 
tendence. To  enumerate  its  present  effects,  would  be  to  count  almost  every 
comfort  and  every  luxury  of  life.  It  has  increased  the  sum  of  human  happi- 
ness, not  only  by  calling  new  pleasures  into  existence,  but  by  so  cheapening 
former  enjoyments  as  to  render  them  attainable  by  those  who  before  could 
never  have  hoped  to  share  them  :  the  surface  of  the  land,  and  the  face  of  the 
waters,  are  traversed  with  equal  facility  by  its  power  ;  and  by  thus  stimulating 
and  facilitating  the  intercourse  of  nation  with  nation,  and  the  commerce  of 
people  with  people,  it  has  knit  together  remote  countries  by  bonds  of  amity 
not  likely  to  be  broken.  Streams  of  knowledge  and  information  are  kept  flow- 
ing between  distant  centres  of  population,  those  more  advanced  diffusing  civi- 
lization and  improvement  among  those  that  are  more  backward.  The  press 
itself,  to  which  mankind  owes  in  so  large  a  degree  the  rapidity  of  their  im- 
provement in  modern  times,  has  had  its  power  and  influence  increased  in  a 
manifold  ratio  by  its  union  with  the  steam-engine.  It  is  thus  that  literature  is 
cheapened,  and,  by  being  cheapened,  diffused  ;  it  is  thus  that  reason  has  taken 
the  place  of  force,  and  the  pen  has  superseded  the  sword  ;  it  is  thus  that  war 
has  almost  ceased  upon  the  earth,  and  that  the  differences  which  inevitably 
arise  between  people  and  people  are  for  the  most  part  adjusted  by  peaceful 
negotiation. 

The  steam-engine  is  a  mechanical  contrivance,  by  which  coal,  wood,  or 
other  fuel,  is  rendered  capable  of  executing  any  kind  of  labor.  Coals  are  by 
it  made  to  spin,  weave,  dye,  print,  and  dress  silks,  cottons,  woollens,  and  other 
cloths  ;  to  make  paper,  and  print  books  upon  it  when  made  ;  to  convert  corn 
into  flour  ;  to  express  oil  from  the  olive,  and  wine  from  the  grape  ;  to  draw  up 
metal  from  the  bowels  of  the  earth  ;  to  pound  and  smelt  it,  to  melt  and  mould 
it ;  to  forge  it ;  to  roll  it,  and  to  fashion  it  into  every  desirable  form  ;  to  trans- 
port these  manifold  products  of  its  own  labor  to  the  doors  of  those  for  whose 
convenience  they  are  produced  ;  to  carry  persons  and  goods  over  the  waters 
of  rivers,  lakes,  seas,  and  oceans,  in  opposition  alike  to  the  natural  difficulties 
of  wind  and  water  ;  to  carry  the  wind-bound  ship  out  of  port  ;  to  place  her  on 
the  open  deep  ready  to  commence  her  voyage  ;  to  throw  its  arms  around  the 
ship-of-war,  and  place  Her  side  by  side  with  the  enemy  ;  to  transport  over  the 
surface  of  the  deep  persons  and  information,  from  town  to  town,  and  from 
country  to  country,  with  a  speed  as  much  exceeding  that  of  the  ordinary  wind, 
as  the  ordinary  wind  exceeds  that  of  a  common  pedestrian. 

Such  are  the  virtues,  such  the  powers,  which  the  steam-engine  has  con- 
ferred upon  COALS.  The  means  of  calling  these  powers  into  activity  are  sup- 
plied by  a  substance  which  nature  has  happily  provided  in  unbounded  quantity 
in  every  part  of  the  earth  ;  and  though  it  has  no  price,  it  has  inestimable  value  : 
this  substance  is  water. 

A  pint  of  water  may  be  evaporated  by  two  ounces  of  coals.  In  its  evapo- 
ration it  swells  into  two  hundred  and  sixteen  gallons  of  steam,  with  a  me- 
chanical force  sufficient  to  raise  a  weight  of  thirty-seven  tons  a  foot  high. 
The  steam  thus  produced  has  a  pressure  equal  to  that  of  common  atmospheric 
air  ;  and  by  allowing  it  to  expand,  by  virtue  of  its  elasticity,  a  further  mechani- 


THE  STEAM-ENGINE. 


cal  force  may  be  obtained,  at  least  equal  in  amount  to  the  former.  A  pint  of 
water,  therefore,  and  two  ounces  of  common  coal,  are  thus  rendered  capable 
of  doing  as  much  work  as  is  equivalent  to  seventy-four  tons  raised  a  foot 
high. 

The  circumstances  under  which  the  steam-engine  is  worked  on  a  railway 
are  not  favorable  to  the  economy  of  fuel.  Nevertheless  a  pound  of  coke  burned 
in  a  locomotive-engine  will  evaporate  about  five  pints  of  water.  In  their 
evaporation  they  will  exert  a  mechanical  force  sufficient  to  draw  two  tons 
weight  on  the  railway  a  distance  of  one  mile  in  two  minutes.  Four  horses 
working  in  a  stage-coach  on  a  common  road  are  necessary  to  draw  the  same 
weight  the  same  distance  in  six  minutes. 

A  train  of  coaches  weighing  about  eighty  tons,  and  transporting  two  hun- 
dred and  forty  passengers  with  their  luggage,  has  been  taken  from  Liverpool  to 
Birmingham,  and  back  from  Birmingham  to  Liverpool,  the  trip  each  way  ta- 
king about  four  and  a  quarter  hours,  stoppages  included.  The  distance  be- 
tween these  places  by  the  railway  is  ninety-five  miles.  This  double  journey 
of  one  hundred  and  ninety-miles  is  effected  by  the  mechanical  force  pro- 
duced in  the  combustion  of  four  tons  of  coke,  the  value  of  which  in  England 
is  about  five  pounds.  To  carry  in  England  the  same  number  of  passengers 
daily  between  the  same  places  by  stage-coaches  on  a  common  road,  would 
require  twenty  coaches  and  an  establishment  of  three  thousand  eight  hundred 
horses,  with  which  the  journey  in  each  direction  would  be  performed  in  about 
twelve  hours,  stoppages  included. 

The  circumference  of  the  earth  measures  twenty-five  thousand  miles  ;    and 
if  it  were  begirt  with  an  iron  railway,  such  a  train  as  above  described,  carry- 
ing two  hundred  and  forty  passengers,  would  be  drawn  round  it  by  the  com- 
bustion of  about  thirty  tons  of  coke,  and  the  circuit  would  be  accomplished  in  { 
five  weeks. 

In  the  drainage  of  the  Cornish  mines  the  economy  of  fuel  is  much  attended 
to,  and  coals  are  there  made  to  do  more  work  than  elsewhere.  A  bushel  of 
coals  usually  raises  forty  thousand  tons  of  water  a  foot  high  ;  but  it  has  on 
some  occasions  raised  sixty  thousand  tons  the  same  height.  Let  us  take  its 
labor  at  fifty  thousand  tons  raised  one  foot  high.  A  horse  worked  in  a  fast 
stage-coach  pulls  against  an  average  resistance  of  about  a  quarter  of  a  hundred 
weight.  Against  this  he  is  able  to  work  at  the  usual  speed  through  about  eight 
miles  daily  ;  his  work  is  therefore  equivalent  to  about  five  hundred  tons  raised 
one  foot.  A  bushel  of  coals,  consequently,  as  used  in  Cornwall,  performs  as 
much  labor  as  a  day's  work  of  one  hundred  such  horses. 

The  great  pyramid  of  Egypt  stands  upon  a  base  measuring  seven  hundred 
feet  each  way,  and  is  five  hundred  feet  high,  its  weight  being  twelve  thousand, 
seven  hundred  and  sixty  millions  of  pounds.  Herodotus  states,  that,  in  con- 
structing it,  one  hundred  thousand  men  were  constantly  employed  for  twenty 
years.  The  materials  of  this  pyramid  would  be  raised  from  the  ground  to 
their  present  position  by  the  combustion  of  about  four  hundred  and  eighty  tons 
of  coals. 

The  Menai  bridge  consists  of  about  two  thousand  tons  of  iron,  and  its  height 
above  the  level  of  the  water  is  one  hundred  and  twenty  feet.  Its  mass  might 
be  lifted  from  the  level  of  the  water  to  its  present  position  by  the  combustion 
of  four  bushels  of  coals. 

The  enormous  consumption  of  coals   produced  by  the  application  of  the 
steam-engine  in  the  arts  and  manufactures,  as  well  as  to  railways  and  navi- 
gation, has  of  late  years  excited  the  fears  of  many  as  to  the  possibility  of  the 
exhaustion  of  our  coal-mines.     Such  apprehensions  are,  however,  altogether 
,  groundless.     If  the  present  consumption  of  coal  be  estimated  at  twenty  millions 

VOL.  II.-36 


402 


THE  STEAM-ENGINE. 


of  tons  annually,  it  is  demonstrable  that   the  coal-fields  of  England  and  the 
United  States  would  not  be  exhausted  for  many  centuries 

But  in  speculations  like  these,  the  probable  if  not  certain  progress  of  im- 
provement and  discovery  ought  not  to  be  overlooked  ;  and  we  may  safely  pro- 
nounce that,  long  before  such  a  period  of  time  shall  have  rolled  away,  other 
and  more  powerful  mechanical  agents  will  supersede  the  use  of  coal.  Phi- 
losophy already  directs  her  finger  at  sources  of  inexhaustible  power  in  the 
phenomena  of  electricity  and  magnetism.  The  alternate  decomposition  and 
recomposition  of  water,  by  magnetism  and  electricity,  has  too  close  an  analogy 
to  the  alternate  processes  of  vaporization  and  condensation,  not  to  occur  at 
once  to  every  mind  :  the  development  of  the  gases  from  solid  matter  by  the 
operation  of  the  chemical  affinities,  and  their  subsequent  condensation  into  the 
liquid  form,  has  already  been  essayed  as  a  source  of  power.  In  a  word,  the 
general  state  of  physical  science  at  the  present  moment,  the  vigor,  activity, 
and  sagacity,  with  which  researches  in  it  are  prosecuted  in  every  civilized  coun- 
try, the  increasing  consideration  in  which  scientific  men  are  held,  and  the  per- 
sonal honors  and  rewards  which  begin  to  be  conferred  upon  them,  all  justify 
the  expectation  that  we  are  on  the  eve  of  mechanical  discoveries  still  greater 
than  any  which  have  yet  appeared  ;  and  that  the  steam-engine  itself,  with  the 
gigantic  powers  conferred  upon  it,  will  dwindle  into  insignificance  in  compari- 
son with  the  energies  of  nature  which  are  still  to  be  revealed  ;  and  that  the 
day  will  come  when  that  machine,  which  is  now  extending  the  blessings  of 
civilization  to  the  most  remote  skirts  of  the  globe^  will  cease  to  have  existence 
except  in  the  page  of  history. 

In  explaining  the  different  forms  of  steam-engine  which  have  been  proposed 
in  the  course  of  the  progressive  improvement  of  that  machine  from  its  early 
rude  and  imperfect  state  to  its  present  comparatively  perfect  form,  it  will  be 
necessary  to  advert  to  physical  phenomena  and  mechanical  principles,  which, 
however  obvious  to  those  who  are  conversant  with  matters  of  science,  must 
necessarily  be  at  least  imperfectly  known  by  the  great  majority.  To  refer  for 
information  on  such  topics  to  other  works  on  mechanics  and  general  physics, 
would  be  with  most  readers  inefi:ectual,  and  with  all  unsatisfactory.  We  shall 
therefore  pause  as  we  proceed,  where  these  difficulties  occur,  to  give  such  ex- 
planation and  illustration  as  may  seem  best  suited  to  render  them  intelligible 
and  interesting  to  the  unscientific  reader. 

Fluid  bodies  are  of  two  kinds — inelastic  fluids,  or  liquids,  and  elastic  fluids, 
or  gases.  Of  the  former  of  these  classes,  water  is  the  most  familiar  example  ; 
and  of  the  latter,  air. 

These  two  species  of  fluids  are  each  distinguished  by  peculiar  mechanical 
properties. 

The  constituent  particles  of  a  liquid  are  distinguished  from  those  of  solids 
by  having  little  or  no  coherence  ;  so  that  unless  the  mass  be  confined  by  the 
sides  of  the  vessel  which  contains  it,  the  particles  will  fall  asunder  by  their 
gravity.     A  mass  of  liquid,  therefore,  unlike  a  solid,  can  never  retain  any  par-  [ 
ticular  form,  but  will  accommodate  itself  to  the  form  of  the  vessel  in  which  it  ' 
is  placed.     It  will  press  against  the  bottom  of  the  vessel  which  contains  it  , 
with  the  whole  force  of  its  weight,  and  it  will  press  against  the  sides  with  a  ' 
force  proportional  to  the  depth  of  the  particles  in  contact  with  the  sides  meas-  i 
ured  from  the  surface  of  the  liquid  above.     This  lateral  pressure  also  distin- 
guishes liquids  from  solids.     Let  us  take  for  illustration  the  case  of  a  square 
or  a  cubical  vessel,  A  B  C  D,  fig.  1.     If  a  solid  body,  such  as  a  piece  of  lead, 
be  cut  to  the  shape  of  this  vessel,  so  as  to  fit  in  it  without  pressing  with  any 
force  against  its  sides,  the  mechanical  effect  which  would  be  produced  by  it 
when  placed  in  the  vessel,  would  be  merely  a  pressure  upon  the  bottom  B  C, 


THE  STEAM-ENGINE. 


403 


Fig.  1. 


=^==^\D 


the  amount  of  which  would  be  equal  to  the  weight  of  the  metallic  mass.  No 
pressure  would  be  exerted  against  the  sides  ;  for  the  coherence  of  the  parti- 
cles of  the  solid  maintaining  them  in  their  position,  the  removal  of  the  sides 
would  not  subject  the  solid  body  contained  in  the  vessel  to  any  change. 

Now  let  us  suppose  this  solid  mass  of  lead  to  be  rendered  liquid  by  being 
melted.  The  constituent  particles  will  then  be  deprived  of  that  cohesion  by 
which  they  were  held  together  ;  they  will  accordingly  have  a  tendency  to  sep- 
arate, and  fall  asunder  by  their  gravity,  and  will  only  be  prevented  from  ac- 
tually doing  so  by  the  support  afforr'ed  to  them  by  the  sides,  A  B,  D  C,  of  the 
vessel.  They  will  therefore  produ  e  a  pressure  against  the  sides,  which  was 
not  produced  by  the  lead  in  its  solid  state.  This  pressure  will  vary  at  differ- 
ent depths  :  thus  a  part  of  the  side  of  the  vessel  at  P  will  receive  a  pressure 
proportional  to  the  depth  of  the  point  P  below  the  surface  of  the  lead.  If,  for 
example,  we  take  a  square  inch  of  the  inner  surface  of  the  side  of  the  vessel 
at  P,  it  will  sustain  an  outward  pressure  equal  to  the  weight  of  a  column  of 
lead  having  a  square  inch  for  its  base,  and  a  height  equal  to  P  A.  And,  in 
like  manner,  every  square  inch  of  the  sides  of  the  vessel  will  sustain  an  out- 
ward pressure  equal  to  the  weight  of  a  column  of  lead  having  a  square  inch 
for  its  base,  and  a  height  equal  to  the  depth  of  the  point  below  the  surface  of 
the  lead. 

We  have  here  proceeded  upon  the  supposition  that  no  force  acts  on  the  up- 
per surface,  A  D,  of  the  lead.  If  any  force  presses  A  D  downward,  that  force 
would  be  transferred  to  the  bottom  by  the  lead,  and  would  produce  a  pressure 
on  the  bottom  B  C,  equal  to  its  own  amount,  in  addition  to  the  weight  of  the 
lead ;  and  if  the  lead  were  solid,  this  would  be  the  only  additional  mechanical 
effect  which  such  a  force  acting  on  the  surface,  A  D,  of  the  lead  would  pro- 
duce. But  if,  on  the  other  hand,  the  lead  were  liquefied,  then  the  force  now 
adverted  to,  acting  on  the  surface,  A  D,  would  not  only  produce  a  pressure 
on  the  bottom  B  C,  equal  to  its  own  amount  in  addition  to  the  weight  of  the 
lead,  but  it  would  also  produce  a  pressure  against  every  part  of  the  sides  of 
the  vessel,  equal  to  that  which  it  would  produce  upon  an  equal  magnitude  of 
the  surface  A  D. 

Thus,  if  we  suppose  any  mechanical  cause  producing  a  pressure  on  the  sur- 
face A  D  amounting  to  ten  pounds  on  each  square  inch,  the  effect  which  would 
be  produced,  if  the  lead  were  solid,  would  be  an  additional  pressure  on  the 
base  B  C  amounting  to  ten  pounds  per  square  inch.  But  if  the  lead  were 
liquid,  besides  this  pressure  on  each  square  inch  of  the  base  B  C,  there  would 
likewise  be  a  pressure  of  ten  pounds  on  every  square  inch  of  the  sides  of  the 
vessel. 

All  that  has  been  here  stated  with  respect  to  a  square  or  a  cubical  vessel, 
will  be  equally  applicable  to  a  vessel  of  any  other  form. 

The  second  class  of  fluids  are  distinguished  from  liquids  by  the  particles 
not  merely  being  destitute  of  cohesion,  but  having  a  tendency  directly  the 
reverse,  to  repel  each  other,  and  fly  asunder  with  more  or.  less  force.     Thus, 


404 


THE   STEAM-ENGINE. 


if  a  vessel,  such  as  that  represented  in  fig.  1,  were  filled  with  a  fluid  of  this 
kind,  being  open  at  the  top,  and  not  being  restrained  by  any  pressure  incum- 
bent upon  it,  the  particles  of  the  fluid  would  not  rest  in  the  vessel  by  their 
gravity,  as  those  of  the  liquid  would  do  ;  but  they  would,  by  their  mutual 
repulsion,  fly  asunder,  and  rise  out  of  the  vessel,  as  smoke  is  seen  to  rise  from 
a  chimney,  or  steam  from  the  spout  of  a  kettle.  Let  us  suppose,  then,  that 
the  vessel  in  which  an  elastic  fluid  is  contained  is  closed  on  every  side  by 
solid  surfaces.  In  fact,  let  us  imagine  that  the  square  or  cubical  vessel  rep- 
resented in  fig.  1  is  closed  by  a  square  lid  at  the  top,  A  D,  having  contained 
in  it  an  elastic  fluid,  such  as  atmospheric  air. 

If  such  a  cover,  or  lid,  had  been  placed  upon  a  liquid,  the  cover  would  sus- 
tain no  pressure  from  the  fluid,  nor  would  any  mechanical  effect  be  produced, 
save  those  already  described  in  the  case  of  the  open  vessel ;  but  when  the 
fluid  contained  in  the  vessel  is  elastic,  as  is  the  case  with  air,  then  the  elas- 
ticity (by  which  name  is  expressed  the  tendency  of  the  particles  of  the  fluid 
to  fly  asunder)  will  produce  peculiar  mechanical  effects,  which  have  no  exist- 
ence whatever  in  the  case  of  a  liquid. 

It  is  true  that,  supposing  the  fluid  to  be  air  or  any  other  gas  or  vapor,  a 
pressure  will  be  produced  upon  the  bottom,  B  C,  of  the  vessel  equivalent  to 
the  weight  of  such  fluid,  and  lateral  pressures  will  be  produced  on  the  difl^er- 
ent  points  of  the  sides  by  the  weight  of  that  part  of  the  fluid  which  is  above 
these  points  ;  but  gases  and  vapors  are  bodies  of  such  extreme  levity,  that 
these  effects  due  to  their  weight  are  neglected  in  practice. 

Putting,  then,  the  weight  of  the  air  contained  in  the  vessel  out  of  the  ques- 
tion, let  us  consider  the  effect  of  its  elasticity.  If  the  vessel,  as  already  de- 
scribed, be  supposed  to  contain  atmospheric  air  in  its  ordinary  state,  the  ten- 
dency of  the  constituent  particles  to  fly  asunder  will  be  such  as  to  produce  on 
every  square  inch  of  the  inner  surface  of  the  vessel  a  pressure  amounting  to 
fifteen  pounds  ;  this  pressure  being,  as  already  stated,  quite  independent  of  the 
weight  of  the  air.  In  fact,  this  pressure  would  continue  to  exist  if  the  air  con- 
tained in  the  vessel  actually  ceased  to  have  weight  by  being  removed  from  the 
neighborhood  of  the  earth,  which  is  the  cause  of  its  gravity. 

Different  gases  are  endowed  with  different  degrees  of  elasticity,  and  the 
same  gas  may  have  its  elasticity  increased  or  diminished,  either  by  varying 
the  space  within  which  it  is  confined,  or  by  altering  the  temperature  to  which 
it  is  exposed. 

If  the  space  within  which  an  elastic  fluid  is  enclosed  be  enlarged,  its  elas- 
ticity is  found  to  diminish  in  the  same  proportion.  Thus,  if  the  air  contained 
in  the  vessel  A  B  C  D  (fig.  1)  be  allowed  to  pass  into  a  vessel  of  twice  the 
magnitude,  the  elasticity  of  the  particles  will  cause  them  to  repel  each  other, 
so  that  the  same  quantity  of  air  shall  diff'use  itself  throughout  the  larger  ves- 
sel, assuming  double  its  former  bulk.  Under  such  circumstances,  the  pressure 
which  it  would  exert  upon  the  sides  of  the  larger  vessel  would  be  only  half 
that  which  it  had  exerted  on  the  sides  of  the  smaller  vessel.  If,  on  the  other 
hand,  it  were  forced  into  a  vessel  of  half  the  magnitude  of  A  B  C  D,  as  it 
might  be,  then  its  elasticity  would  be  double,  and  it  would  press  on  the  inner 
surface  of  that  vessel  with  twice  the  force  with  which  it  pressed  on  that  of 
the  vessel  A  B  C  D. 

This  power  of  swelling  and  contracting  its  dimensions  according  to  the 
dimensions  of  the  vessel  in  which  it  is  confined,  or  to  the  force  compressing 
it,  is  a  quality  which  results  immediately  from  elasticity,  and  is  consequently 
one  which  is  peculiar  to  the  gases  or  elastic  fluids,  and  does  not  at  all  apper- 
tain to  liquids.  If  the  liquid  contained  in  the  vessel  A  B  C  D  were  trans- 
ferred to  a  vessel  of  twice  the  magnitude,  it  would  only  occupy  half  the  ca- 


THE   STEAM-ENGINE. 


405 


pacity  of  that  vessel,  and  it  could  not  by  any  means  be  transferred,  as  we  have  < 
supposed  the  air  or  gas  to  be,  to  a  vessel  of  half  the  dimensions,  since  it  is  ] 
inelastic  and  incompressible.  i 

The  elasticity  of  gases  is  likewise  varied  by  varying  the  temperature  to  | 
which  they  are  exposed  ;  thus,  in  general,  if  air  or  any  other  gas  be  augment-  i 
ed  in  temperature,  it  will  likewise  be  increased  in  elasticity  ;  and  if,  on  the  | 
other  hand,  it  be  diminished  in  temperature,  it  will  be  likewise  diminished  in  ( 
its  elastic  force.  The  more  heated,  therefore,  any  air  or  gas  confined  in  a  ] 
vessel  becomes,  the  greater  will  be  the  force  with  which  it  will  press  on  the  * 
inner  surface  of  that  vessel,  and  tend  to  burst  it.  , 

The  same  body  may,  by  the  agency  of  heat,  be  made  to  pass  successively  ' 
through  the  different  states  of  solid,  liquid,  and  gas  or  vapor.  The  most  , 
familiar  and  obvious  example  of  these  successive  transitions  is  presented  by  ' 
water.  Exposed  to  a  certain  temperature,  water  can  only  exist  as  a  solid  ;  as  [ 
the  temperature  is  increased,  the  ice,  or  solid  water,  is  liquefied  ;  and  by  the 
continued  application  of  heat,  this  water  again  undergoes  a  change,  and  as-  ! 
sumes  the  form,  and  acquires  the  mechanical  qualities,  of  air  or  gas  :  in  such 
a  state  it  is  called  steam. 

This  is  a  common  property  of  all  liquids.  If  they  be  exposed  for  a  sufficient 
length  of  time  to  a  sufficient  degree  of  heat,  they  will  always  be  converted 
into  elastic  fluids.  These  are  usually  distinguished  from  air  and  other  perma- 
nent gases,  which  never  are  known  to  exist  in  the  liquid  form,  by  the  term 
vapor,  by  which,  therefore,  must  be  understood  an  elastic  fluid  which  at  com- 
mon temperatures  exists  in  the  liquid  or  solid  state  ;  by  steam  is  expressed  the 
vapor  of  water ;  and  by  gases,  those  elastic  fluids  which,  like  air,  are  never 
known — at  least,  under  ordinary  circumstances — to  exist  in  any  other  but  the 
elastic  form. 

When  a  liquid  is  caused,  by  the  application  of  heat,  to  take  the  form  of  an 
elastic  fluid,  or  is  evaporated,  besides  acquiring  the  property  of  elasticity,  it 
always  yndergoes  a  considerable  change  of  bulk.  The  amount  of  this  change 
is  different  with  different  liquids,  and  even  with  the  same  liquid  it  varies  with 
the  circumstances  under  which  the  change  is  produced. 

When  water  is  evaporated  under  ordinary  circumstances — that  is,  when 
exposed  to  no  other  external  pressure  than  that  of  the  atmosphere — it  in- 
creases its  volume  about  seventeen-hundred-fold.  Thus  a  cubic  inch  of 
liquid  water  would  form  about  seventeen  hundred  cubic  inches  of  common 
steam.  If,  however,  the  water  be  confined  by  a  greater  pressure  than  that 
produced  by  the  common  atmosphere,  then  the  increase  of  volume  which  takes 
place  in  its  evaporation  would  be  less  in  proportion. 

The  steam-engine  contrived  by  Savery  in  the  year  1698,  like  every  other 
which  has  since  been  constructed,  consists  of  two  parts,  essentially  distinct. 
The  first  is  that  which  is  employed  to  generate  the  steam,  which  is  called  the 
boiler  ;  and  the  second,  that  in  which  the  steam  is  applied  as  a  moving  power. 

The  former  apparatus  in  Savery's  engine  consists  of  two  strong  boilers, 
sections  of  which  are  represented  at  D  and  E  in  fig.  2  ;  D  the  greater  boiler, 
and  E  the  less.  The  tubes  T  and  T'  communicate  with  the  working  appar- 
atus, which  we  shall  presently  describe.  A  thin  plate  of  metal,  R,  is  applied 
closely  to  the  top  of  the  great  boiler  D,  turning  on  a  centre  C,  so  that  by 
moving  a  lever  applied  to  the  axis  C  on  the  outside  of  the  top,  the  sliding  plate 
R  can  be  brought  from  the  mouth  of  the  one  tube  to  the  mouth  of  the  other 
alternately.  This  sliding-valve  is  called  the  regulator,  since  it  is  by  it  that 
the  communications  between  the  boiler  and  two  steam-vessels  (hereafter  de- 
scribed) are  alternately  opened  and  closed,  the  lever  which  effects  this  being 
moved  at  intervals  by  the  hand  of  the  attendant. 


Two  gauge-cocks  are  represented  at  G  G',  the  use  of  which  is  to  determine 
the  depth  of  water  in  the  boiler.  One,  G,  has  its  lower  aperture  a  little  above 
the  proper  depth  ;  and  the  other,  G',  a  little  below  it.  Cocks  are  attached  to 
the  upper  ends  G  G',  which  can  be  opened  or  closed  at  pleasure.  The  steam 
collected  in  the  top  of  the  boiler  pressing  on  the  surface  of  the  water,  forces  it 
up  in  the  tubes  G  G'.  if  their  lower  ends  be  immersed.  Upon  opening  the 
cocks  G  G',  if  water  be  forced  from  both,  there  is  too  much  water  in  the  boiler, 
since  the  mouth  of  G  is  below  its  level.  If  steam  issue  from  both,  there  is  too 
little  water  in  the  boiler,  since  the  mouth  of  G'  is  above  its  level.  But  if  steam 
issue  from  G,  and  water  from  G',  the  water  in  the  boiler  is  at  its  proper  level. 
This  ingenious  contrivance  for  determining  the  level  of  the  water  in  the  boiler 
is  the  invention  of  Savery,  and  is  used  in  many  instances  at  the  present  day. 

The  mouth  of  the  pipe  G  should  be  at  a  level  of  a  little  less  than  one  third 
of  the  whole  depth,  and  the  mouth  of  G'  at  a  level  little  lower  than  one  third, 
for  it  is  requisite  that  about  two  thirds  of  the  boiler  should  be  kept  filled  with 
water.  The  tube  I  forms  a  communication  between  the  greater  boiler  D  and 
the  lesser  or  feeding  boiler  E,  descending  nearly  to  the  bottom  of  it.  This 
communication  can  be  opened  and  closed  at  pleasure  by  the  cock  K.  A  gauge 
pipe  is  inserted  similar  to  G  G',  but  extending  nearly  to  the  bottom.  From 
this  boiler  a  tube,  F,  extends,  which  is  continued  to  a  cistern,  C  (fig.  3),  and 
a  cock  is  placed  at  M,  which,  when  opened,  allows  th^  water  from  the  cistern 
to  flow  into  the  feeding  boiler  E,  and  which  is  closed  when  that  boiler  is  filled. 
The  manner  in  which  this  cistern  is  supplied  will  be  described  hereafter. 

Let  us  now  suppose  that  the  principal  boiler  is  filled  to  the  level  between 
the  gauge-pipes,  and  that  the  subsidiary  boiler  is  nearly  full  of  water,  the  cock 
K  and  the  gauge-cocks  G  G'  being  all  closed.  The  fire  being  lighted  beneath 
D,  and  the  water  boiled,  steam  is  produced,  and  is  transmitted  through  one  or 
other  of  the  tubes  T  T',  to  the  working  apparatus.  When  evaporation  has 
reduced  the  water  in  D  below  the  level  of  G,  it  will  be  necessary  to  replenish 
the  boiler  D.  This  is  effected  thus : — A  fire  being  lighted  beneath  the  feed- 
ing-boiler E,  steam  is  produced  in  it  above  the  surface  of  the  water,  which, 
having  no  escape,  presses  on  the  surface  so  as  to  force  it  up  in  the  pipe  I. 
The  cock  K  being  then  opened,  the  boiling  water  is  forced  into  the  principal 
boiler  D,  into  which  it  is  allowed  to  flow  until  water  issues  from  the  gauge- 
cock  G'.  When  this  takes  place,  the  cock  K  is  closed,  and  the  fire  removed 
from  E  until  the  great  boiler  again  wants  replenishing.  When  the  feeding- 
boiler  E  has  been  exhausted,  it  is  replenished  from  the  cistern  C  (fig.  3), 
through  the  pipe  F,  by  opening  the  cock  M. 

We  shall  now  describe  the  working  apparatus  in  which  the  steam  is  used 
as  a  moving  power. 


THE   STEAM-ENGINE. 


407 


Let  V  V  (fig.  3)  be  two  steam-vessels,  communicating  by  the  tubes  T  T' 
(marked  by  the  same  letters  in  fig.  2)  with  the  greater  boiler  D. 

Let  S  be  a  pipe,  called  the  suction-pipe,  descending  into  the  well  or  reser- 
voir iVom  which  the  water  is  to  be  raised,  and  communicating  with  each  of 

Fisr.  3. 


the  steam-vessels  through  tubes  D  D',  by  valves  A  A',  which  open  upward. 
Let  F  be  a  pipe  continued  from  the  level  of  the  engine  to  whatever  higher 
level  it  is  intended  to  elevate  the  water.  The  steam-vessels  V  V  communi- 
cate with  the  force-pipe  F,  by  valves  B  B',  which  open  upward,  through  the 
tubes  E  E'.  Over  the  steam-vessels  and  on  the  force-pipe  is  placed  a  small 
cistern,  C,  already  mentioned,  which  is  kept  filled  with  cold  water  from  the 
force-pipe,  and  from  the  bottom  of  which  proceeds  a  pipe  terminated  with  a 
cock  G.  This  is  called  the  condensing-pipe,  and  can  be  brought  alternately 
over  each  steam-vessel.  From  this  cistern  another  pipe  communicates  with 
the  feeding-boiler  (fig.  2)  by  the  cock  M. 

The  communication  of  the  pipes  T  T'  with  the  boiler  can  be  opened  and 
closed  alternately,  by  the  regulator  R  (fig.  2),  already  described. 

Now,  suppose  the  steam-vessels  and  tubes  to  be  all  filled  with  common 
atmospheric  air,  and  that  the  regulator  be  placed  so  that  the  communication 
between  the  tube  T  and  the  boiler  be  opened,  the  communication  between  the 
other  tube  T'  and  the  boiler  being  closed,  steam  will  flow  into  V  through  T. 
At  first,  while  the  vessel  V  is  cold,  the  steam  will  be  condensed,  and  will  fall 
in  drops  of  water  on  the  bottom  and  sides  of  the  vessel.  The  continued  sup- 
ply of  steam  from  the  boiler  will  at  length  impart  such  a  degree  of  heat  to  the 
vessel  V,  that  it  will  cease  to  condense  it.  Mixed  with  the  heated  air  con- 
tained in  the  vessel  V,  it  will  have  an  elastic  force  greater  than  the  atmo- 
spheric pressure,  and  will  therefore  force  open  the  valve  B,  through  which  a 
mixture  of  air  and  steam  will  be  driven  until  all  the  air  in  the  vessel  V  will 
have  passed  out,  and  it  will  contain  nothing  but  the  pure  vapor  of  water. 

When  this  has  taken  place,  suppose  the  regulator  be  moved  so  as  to  close 
the  communication  between  the  tube  T  and  the  boiler,  and  to  stop  the  further 
supply  of  steam  to  the  vessel  V ;  and  at  the  same  time  let  the  condensing-pipe 
G  be  brought  over  the  vessel  V,  and  the  cock  opened  so  as  to  let  a  stream  of 
cold  water  flow  upon  it.  This  will  cool  the  vessel  V,  and  the  stream  with 
which  it  is  filled  will  be  condensed  and  fall  in  a  few  drops  of  water,  leaving 


408 


THE   STEAM-ENGINE. 


the  interior  of  the  vessel  a  vacuum.  The  valve  B  will  be  kept  closed  by  the 
atmospheric  pressure.  But  the  elastic  force  of  the  air  between  the  valve  A 
and  the  surface  of  the  water  in  the  well,  or  reservoir,  will  open  A,  so  that  a 
part  of  this  air  will  rush  in  and  occupy  the  vessel  V.  The  air  in  the  suction- 
pipe  S,  being  thus  allowed  an  increased  space,  will  be  proportionably  dimin- 
ished in  its  elastic  force,  and  its  pressure  will  no  longer  balance  that  of  the 
atmosphere  acting  on  the  external  surface  of  the  water  in  the  reservoir.  This 
pressure  will  therefore  force  water  up  in  the  tube  S,  until  its  weight,  together 
with  the  elastic  force  of  the  air  above  it,  balances  the  atmospheric  pressure. 
When  this  has  taken  place,  the  water  will  cease  to  ascend. 

Let  us  now  suppose,  that,  by  shifting  the  regulator,  the  communication  is 
opened  between  T  and  the  boiler,  so  that  steam  flows  again  into  V.  The 
condensing-cock  G  being  removed,  the  vessel  will  be  again  heated  as  before, 
the  air  expelled,  and  its  place  filled  by  the  steam.  The  condensing-pipe  be- 
ing again  allowed  to  play  upon  the  vessel  V,  and  the  further  supply  of  steam 
being  stopped,  a  vacuum  will  be  produced  in  V,  and  the  atmospheric  pressure 
will  force  the  water  through  the  valve  A  into  the  vessel  V,  which  it  will 
nearly  fill,  a  small  quantity  of  air,  however,  remaining  above  it. 

Thus  far  the  mechanical  agency  employed  in  elevating  the  water  is  the 
atmospheric  pressure,  and  the  power  of  steam  is  no  further  employed  than 
in  the  production  of  a  vacuum.  But,  in  order  to  continue  the  elevation  of  the 
water  through  the  force-pipe  F,  above  the  level  of  the  steam-vessel,  it  will  be 
necessary  to  use  the  elastic  pressure  of  the  steam.  The  vessel  V  is  now 
nearly  filled  by  the  water  which  has  been  forced  into  it  by  the  atmosphere. 
Let  us  suppose,  that,  the  regulator  being  shifted  again,  the  communication  be- 
tween the  tube  T  and  the  boiler  is  opened,  the  condensing-cock  removed,  and 
that  steam  flows  into  V.  At  first,  coming  in  contact  with  the  cold  surface  of 
the  water  and  that  of  the  vessel,  it  is  condensed  ;  but  the  vessel  is  soon  heated, 
and  the  water  formed  by  the  condensed  steam  collects  in  a  sheet  or  film  upon 
the  surface  of  the  water  in  V,  so  as  to  form  a  surface  as  hot  as  boiling  water.* 
The  steam  then  being  no  longer  condensed,  presses  on  the  surface  of  the 
water  with  its  elastic  force  ;  and  when  that  pressure  becomes  greater  than  the 
atmospheric  pressure,  the  valve  B  is  forced  open,  and  the  water  issuing  through 
it,  passes  through  E  into  the  force-pipe  F  ;  and  this  is  continued  until  the 
steam  has  forced  all  the  water  from  V  and  occupies  its  place. 

The  further  admission  of  steam  through  T  is  once  more  stopped  by  moving 
the  regulator,  and  the  condensing-pipe  being  again  allowed  to  play  on  V,  so  as 
to  condense  the  steam  which  fills  it,  produces  a  vacuum.  Into  this  vacuum, 
as  before,  the  atmospheric  pressure  will  force  the  water  and  fill  the  vessel  V. 
The  condensing-pipe  being  then  closed,  and  steam  admitted  through  T,  the 
water  in  V  will  be  forced  by  its  pressure  through  the  valve  B  and  tube  E  into 
F,  and  so  the  process  is  continued. 

We  have  not  yet  noticed  the  other  steam  vessel  V,  which,  as  far  as  we 
have  described,  would  have  remained  filled  with  common  atmospheric  air,  the 
pressure  of  which  on  the  value  A'  would  have  prevented  the  water  raised  in 
the  suction-pipe  S  from  passing  through  it.  However,  this  is  not  the  case  ; 
for,  during  the  entire  process  which  has  been  described  in  V,  similar  effects 
have  been  produced  in  V,  which  we  have  only  omitted  to  notice  to  avoid  the 
confusion  which  the  two  processes  might  produce.  It  will  be  remembered, 
that  after  the  steam,  in  the  first  instance,  having  flowed  from  the  boiler  through 
T,  has  blown  the  air  out  of  V  through  B,  the  communication  between  T  and 
the  boiler  is  closed.  Now,  the  same  motion  of  the  regulator  which  closes 
this,  opens  the   communication  between  T'  and  the  boiler ;   for  the  sliding- 

*  Hot  water,  being  ligliter  than  cold,  floats  on  the  surface. 


THE   STEAM-ENGINE. 


409 


plate  R  (fig.  2)  is  moved  from  the  one  tube  to  the  other,  and  at  the  same  time, 
as  we  have  already  stated,  the  condensing-pipe  is  brought  to  play  on  V.  While, 
therefore,  a  vacuum  is  being  formed  in  V  by  condensation,  the  steam,  flowing 
through  T',  blows  out  the  air  through  B',  as  already  described  in  the  other  ves- 
sel V  ;  and  while  the  air  in  S  is  rushing  up  through  A  into  V,  followed  by  the 
water  raised  in  S  by  the  atmospheric  pressure,  the  vessel  V''  is  being  filled  with 
steam,  and  the  air  is  completely  expelled  from  it. 

The  communication  between  T  and  the  boiler  is  now  again  opened,  and  the 
communication  between  T'  and  the  boiler  closed  by  moving  the  regulator  R 
(fig.  2)  from  the  tube  T  to  T' ;  at  the  same  time  the  condensing  pipe  is  re- 
moved from  over  V,  and  brought  to  play  upon  V.  While  the  steam  once  more 
expels  the  air  from  V  through  B,  a  vacuum  is  formed  by  condensation  in  V, 
into  which  the  water  in  S  rushes  through  the  valve  A'.  In  the  meantime  V  is 
again  filled  with  steam.  The  communication  between  T  and  the  boiler  is  now 
closed,  and  that  between  T'  and  the  boiler  is  opened,  and  the  condensing  pipe 
removed  from  V,  and  brought  to  play  on  V.  While  the  steam  from  the  boiler 
forces  the  water  in  V^  through  B'  into  the  force-pipe  F,  a  vacuum  is  being 
produced  in  V,  into  which  water  is  raised  by  the  atmospheric  pressure. 

Thus  each  of  the  vessels  V  V  is  alternately  filled  from  S,  and  the  water 
thence  forced  into  F.  The  same  steam  which  forces  the  water  from  the  ves- 
sels into  F,  having  done  its  duty,  is  condensed,  and  brings  up  the  water  from 
S,  by  giving  effect  to  the  atmospheric  pressure. 

During  this  process,  two  alternate  motions  or  adjustments  must  be  constantly 
made  ;  the  communication  between  T  and  the  boiler  must  be  opened,  and  that 
between  T'  and  the  boiler  closed,  which  is  done  by  one  motion  of  the  regula- 
tor. The  condensing  pipe  at  the  same  time  must  be  brought  from  V  to  play 
on  V,  which  is  done  by  the  lever  placed  upon  it.  Again  the  communication 
between  T'  and  the  boiler  is  to  be  opened,  and  that  between  T  and  the  boiler 
closed  ;  this  is  done  by  moving  back  the  regulator.  The  condensing-pipe  is 
brought  from  V  to  V  by  moving  back  the  other  lever,  and  so  on  alternately. 

In  order  duly  to  appreciate  the  value  of  improvements,  it  is  necessary  first 
to  perceive  the  defects  which  these  improvements  are  designed  to  remove. 
Savery's  steam-engine,  considering  how  little  was  known  of  the  value  and 
properties  of  steam,  and  how  low  the  general  standard  of  mechanical  knowl- 
edge was  in  his  day,  is  certainly  highly  creditable  to  his  genius.  Neverthe- 
less it  had  very  considerable  defects,  and  was  finally  found  to  be  inefficient  for 
the  most  important  purposes  to  which  he  proposed  applying  it. 

At  the  time  of  this  invention,  the  mines  in  England  had  greatly  increased  in 
depth,  and  the  process  of  draining  them  had  become  both  expensive  and  diffi- 
cult ;  so  much  so,  that  it  was  found  in  many  instances  that  their  produce  did 
not  cover  the  cost  of  working  them.  The  drainage  of  these  mines  was  the 
most  important  purpose  to  which  Savery  proposed  to  apply  his  steam-engine. 

It  has  been  already  stated  that  the  pressure  of  the  atmosphere  amounts  to 
about  fifteen  pounds  on  every  square  inch.  Now,  a  column  of  water,  whose 
base  is  one  square  inch,  and  whose  height  is  thirty-four  feet,  weighs  about 
fifteen  pounds.  If  we  suppose  that  a  perfect  vacuum  were  produced  in  the 
steam-vessels  V  V  (fig.  3),  by  condensation,  the  atmospheric  pressure  would 
fail  to  force  up  the  water,  if  the  height  of  the  top  of  these  vessels  above  the 
water  to  be  raised  exceeded  thirty-four  feet.  It  is  plain,  therefore,  that  tlife 
engine  cannot  be  more  than  thirty-four  feet  above  the  water  which  it  is  in- 
tended to  elevate.  But  in  fact  it  cannot  be  so  much  ;  for  the  vacuum  produced 
in  the  steam-vessels  V  V  is  never  perfect.  Water,  when  not  submitted  to  the 
pressure  of  the  atmosphere,  will  vaporize  at  a  very  low  temperature,  as  we 
shall  hereafter  explain  ;  and  it  was  found  that  a  vapor  possessing  a  considera- 


ble  elasticity  would,  notwithstanding  the  condensation,  remain  in  the  vessels 

V  V  and  the  pipe  S,  and  would  oppose  the  ascent  of  the  water.  In  conse- 
quence of  this,  the  engine  could  never  be  placed,  with  practical  advantage,  at 
a  greater  height  than  twenty-six  feet  above  the  level  of  the  water  to  be  raised. 

When  the  water  is  elevated  to  the  engine,  and  the  steam-vessels  filled,  if 
steam  be  introduced  above  the  water  in  V,  it  must  first  balance  the  atmospheric 
pressure,  before  it  can  force  the  water  through  the  valve  B.  Here,  then,  is  a 
mechanical  pressure  of  fifteen  pounds  per  square  inch  expended,  without  any 
water  being  raised  by  it.  If  steam  of  twice  that  elastic  force  be  used,  it  will 
elevate  a  column  in  F  of  thirty-four  feet  in  height ;  and  if  steam  of  triple  the 
force  be  used,  it  will  raise  a  column  of  sixty-eight  feet  high,  which,  added  to 
twenty-six  feet  raised  by  the  atmosphere,  gives  a  total  lift  of  ninety-four  feet. 

In  eflfecting  this,  steam  of  a  pressure  equal  to  three  times  that  of  the  atmo- 
sphere acts  on  the  inner  surface  of  the  vessels  V  V,  One  third  of  this  burst- 
ing pressure  is  balanced  by  the  pressure  of  the  atmosphere  on  the  external 
surface  of  the  vessels  ;  but  an  efl^ective  pressure  of  thirty  pounds  per  square 
inch  still  remains,  tending  to  burst  the  vessels.  It  was  found  that  the  appa- 
ratus could  not  be  constructed  to  bear  more  than  this  with  safety ;  and,  there- 
fore, in  practice,,  the  lift  of  such  an  engine  was  limited  to  about  ninety  perpen- 
dicular feet.  In  order  to  raise  the  water  from  the  bottom  of  the  mine  by  these 
engines,  therefore,  it  was  necessary  to  place  one  at  every  ninety  feet  of  the 
depth  ;  so  that  the  water  raised  by  one  through  the  first  ninety  feet  should  be 
received  in  a  reservoir,  from  which  it  was  to  be  elevated  the  next  ninety  feet 
by  another,  and  so  on. 

Beside  this,  it  was  found  that  sufficient  strength  could  not  be  given  to  those 
engines,  if  constructed  upon  a  large  scale. 

They  were,  therefore,  necessarily  very  limited  in  their  dimensions,  and 
were  incapable  of  raising  the  water  with  sufficient  speed.  Hence  arose  a  ne- 
cessity for  several  engines  at  each  level,  which  greatly  increased  the  expense. 

These,  however,  were  not  the  only  defects  of  Savery's  engines.  The  con- 
sumption of  fuel  was  enormous ;  the  proportion  of  heat  wasted  being  much 
more  than  what  was  used  in  either  forcing  up  the  water,  or  producing  a  vacu- 
um. This  will  be  very  easily  understood,  by  attending  to  the  process  of  work- 
ing the  engine  already  described. 

When  the   steam  is  first  introduced  from  the  boiler  into  the  steam-vessels 

V  V,  preparatory  to  the  formation  of  a  vacuum,  it  is  necessary  that  it  should 
heat  these  vessels  up  to  the  temperature  of  the  steam  itself;  for  until  then  the 
steam  will  be  condensed  the  moment  it  enters  the  vessel,  by  the  cold  surface. 
All  this  heat,  therefore,  spent  in  raising  the  temperature  of  the  steam-vessels 
is  wasted.  Again,  when  the  water  has  ascended  and  filled  the  vessels  V  V, 
and  steam  is  introduced  to  force  this  water  through  B  B'  into  F,  it  is  immedi- 
ately condensed  by  the  cold  surface  in  V  V,  and  does  not  begin  to  act  until  a 
quantity  of  hot  water,  formed  by  condensed  steam,  is  collected  on  the  surface 
of  the  cold  water  which  fills  these  vessels.  Hence  another  source  of  the  waste 
of  heat  arises. 

When  the  steam  begins  to  act  upon  the  surface  of  the  water  in  V  V,  and  to 
force  it  down,  the  cold  surface  of  the  vessels  is  gradually  exposed  to  the  steam, 
and  must  be  heated  while  the  steam  continues  its  action  ;  and  when  the  water 
has  been  forced  out  of  the  vessel,  the  vessel  itself  has  been  heated  to  the  tem- 
perature of  the  steam  which  fills  it,  all  which  heat  is  dissipated  by  the  subse- 
quent process  of  condensation.  It  must  thus  be  evident,  that  the  steam  used 
in  forcing  up  the  water  in  F,  and  in  producing  a  vacuum,  bears  a  very  small 
proportion,  indeed,  to  what  is  consumed  in  heating  the  apparatus  after  con- 
densation. 


THE  STEAM-ENGINE. 


411 


There  is  also  another  circumstance  which  increasesnhe  consumption  of 
fuel.  The  water  must  be  forced  through  B,  not  only  against  the  atmospheric  ] 
pressure,  but  also  against  a  column  of  sixty-eight  feet  of  water.  Steam  is  < 
therefore  required  of  a  pressure  of  forty-five  pounds  on  the  square  inch.  Con-  , 
sequently  the  water  in  the  boiler  must  be  boiled  under  this  pressure.  That  < 
this  should  take  place,  it  is  necessary  that  the  water  should  be  raised  to  a  , 
temperature  considerably  above  212°,  even  so  high  as  275°  ;  and  thus  an  in-  < 
creased  heat  must  be  given  to  the  boiler.  Independently  of  the  other  defects,  , 
this  intense  heat  weakened  and  gradually  destroyed  the  apparatus.  ' 

Savery  was  the  first  who  suggested  the  method  of  expressing  the  power  of  , 
an  engine  with  reference  to  that  of  horses.     In  this  comparison,  however,  he  ' 
supposed  each  horse  to  work  but  eight  hours  a  day,  while  the  engine  works  , 
for  twenty-four  hours.     This  method  of  expressing  the  power  of  steam-engines 
will  be  explained  hereafter. 

The  failure  of  the  engines  proposed  by  Captain  Savery  in  the  work  of  * 
drainage,  from  the  causes  which  have  been  just  mentioned,  and  the  increasing 
necessity  for  effecting  this  object,  arising  from  the  large  property  in  mines 
which  became  every  year  unproductive  by  being  flooded,  stimulated  the  inge- 
nuity of  mechanics  to  contrive  some  means  of  rendering  those  powers  of  steam 
exhibited  in  Savery's  engine  available. 

Thomas  Newcomen,  the  reputed  inventor  of  the  atmospheric  engine,  was  an 
ironmonger,  or,  according  to  some,  a  blacksmith,  in  the  town  of  Dartmouth,  in 
Devonshire.  From  his  personal  acquaintance  and  intercourse  with  Dr.  Hooke, 
the  celebrated  natural  philosopher,  it  is  probable  that  he  was  a  person  of  some 
education,  and  therefore  likely  to  be  above  the  position  of  a  blacksmith.  Be- 
ing in  the  habit  of  visiting  the  tin  mines  in  Cornwall,  Newcomen  became  ac- 
quainted with  the  engine  invented  by  Savery,  and  with  the  causes  which  led 
to  its  inefficiency  for  the  purposes  of  drainage. 

John  Cawley,  who  was  the  associate  of  Newcomen  in  his  experiments  and 
inquiries,  was  a  plumber  and  glazier  of  the  same  town.  Newcomen  and  Caw- 
ley obtained  a  patent  for  the  atmospheric  engine,  in  1705,  in  which  Savery 
was  associated,  he  having  previously  obtained  a  patent  for  the  method  of  pro- 
ducing a  vacuum  by  the  condensation  of  steam,  which  was  essential  to  New- 
comen's  contrivance.  It  was  not,  however,  until  about  the  year  171 1,  that  any 
engine  had  been  constructed  under  this  patent. 

Newcomen  resumed  the  old  method  of  raising  the  water  from  the  mines  by 
ordinary  pumps,  but  conceived  the  idea  of  working  these  pumps  by  some 
moving  power  less  expensive  than  that  of  horses.  The  means  whereby  he 
proposed  efi'ecting  this,  was  by  connecting  the  end  of  the  pump-rod  D  (fig.  4) 
by  a  chain  with  the  arch-head  A  of  a  working-beam  A  B,  playing  on  an  axis 
C.  The  other  arch-head  B  of  this  beam  was  connected  by  a  chain  with  the 
rod  K  of  a  solid  piston  P,  which  moA'ed  air-tight  in  a  cylinder  F.  If  a  vacuum 
be  created  beneath  the  piston  P,  the  atmospheric  pressure  acting  upon  it  will 
press  it  down  with  a  force  of  fifteen  pounds  per  square  inch ;  and  the  end  A 
of  the  beam  being  thus  raised,  the  pump-rod  D  will  be  drawn  up.  If  a  pres- 
sure equivalent  to  the  atmosphere  be  then  introduced  below  the  piston,  so  as 
to  neutralize  the  downward  pressure,  the  piston  will  be  in  a  slate  of  indiffer- 
ence as  to  the  rising  or  falling ;  and  if  in  this  case  the  rod  D  be  made  heavier 
than  the  piston  and  its  rod,  so  as  to  overcome  the  friction,  it  will  descend,  and 
elevate  the  piston  again  to  the  top  of  the  cylinder.  The  vacuum  being  again 
produced,  another  descent  of  the  piston,  and  consequent  elevation  of  the  pump- 
rod,  will  take  place  ;  and  so  the  process  may  be  continued. 

Such  was  Newcomen's  first  conception  of  the  atmospheric  engine ;  and  the 
contrivance  had  much,  even  at  the  first  view,  to  recommend  it.     The  power  of 


412 


THE    STEAM-ENGINE. 


Fis.  4. 


})^^. 


such  a  machine  would  depend  entirely  on  the  magnitude  of  the  piston  ;  and 
being  independent  of  highly  elastic  steam,  would  not  expose  the  materials  to 
the  destructive  heat  which  was  necessary  for  working  Savery's  engine.  Sup- 
posing a  perfect  vacuum  to  be  produced  under  the  piston  in  the  cylinder,  an 
effective  downward  pressure  would  be  obtained,  amounting  to  fifteen  times  as 
many  pounds  as  there  are  square  inches  in  the  section  of  the  piston.*  Thus, 
if  the  base  of  the  piston  were  100  square  inches,  a  pressure  equal  to  1,500 
pounds  would  be  obtained. 

In  order  to  accomplish  this,  two  things  were  necessary  :  1.  To  make  a 
speedy  and  effectual  vacuum  below  the  piston  in  the  descent;  and,  2.  To  con- 
trive a  counterpoise  for  the  atmosphere  in  the  ascent. 

The  condensation  of  steam  immediately  presented  itself  as  the  most  effectual 
means  of  accomplishing  the  former ;  and  the  elastic  force  of  the  same  steam 
previous  to  condensation  an  obvious  method  of  affecting  the  latter.  Nothing 
now  remained  to  carry  the  design  into  execution,  but  the  contrivance  of  means 
for  the  alternate  introduction  and  condensation  of  the  steam  ;  and  Newcomen 
and  Cawley  were  accordingly  granted  a  patent  in  1707,  in  which   Savery  was 

*  As  the  calculation  of  the  power  of  an  engine  depends  on  the  number  of  square  inches  in  the 
section  of  the  piston,  it  may  be  useful  to  give  a  rule  for  computing  the  number  of  square  inches  in 
a  circle.  The  follo%ving  rule  will  always  give  the  dimensions  with  sufficient  accuracy :  Multiply 
the  number  of  inches  in  the  diaineter  by  itself ;  divide  the  product  by  14,  and  multiply  the  quotient 
thus  obtained  by  11,  and  the  result  will  be  tlie  number  of  square  ijiches  in  the  circle.  Thus,  if  there 
be  12  inches  in  the  diameter,  this  multiplied  by  itself  gives  144,  which  divided  by  14  gives  lOyjj 
which  multiplied  by  11  gives  113,  neglecting  fractions.  There  are,  therefore,  113  square  inches  in 
a  circle  whose  diameter  is  12  inches. 


THE   STEAM-ENGINE. 


413 


united,  in  consequence  of  the  principle  of  condensation  for  which  he  had  pre- 
viously received  a  patent  being  necessary  to  the  projected  machine.  We  shall 
now  describe  the  atmospheric  engine,  as  first  constructed  by  Newcomen : — 

The  boiler  K  (fig.  4)  is  placed  over  a  furnace  I,  the  flue  of  which  winds 
round  it,  so  as  to  communicate  heat  to  every  part  of  the  bottom  of  it.  In  the 
top,  which  is  hemispherical,  two  gaiige-cocks  G  G'  are  placed,  as  in  Savery's 
engine,  and  s.  puppet  valve  V,  which  opens  upward,  and  is  loaded  at  one  pound 
per  square  inch  ;  so  that  when  the  steam  produced  in  the  boiler  exceeds  the 
pressure  of  the  atmosphere  by  more  than  one  pound  on  the  square  inch,  the 
valve  V  is  lifted,  and  the  steam  escapes  through  it,  and  continues  to  escape 
until  its  pressure  is  sufficiently  diminished,  when  the  valve  V  again  falls  into 
its  seat.     This  valve  performs  the  office  of  the  safety-valve  in  modern  engines. 

The  great  steam-tube  is  represented  at  S,  which  conducts  steam  from  the 
boiler  to  the  cylinder ;  and  a  feeding  pipe  T,  furnished  with  a  cock,  which  is 
opened  and  closed  at  pleasure,  proceeds  from  a  cistern  L  to  the  boiler.  By 
this  pipe  the  boiler  may  be  replenished  from  the  cistern,  when  the  gauge-cock 
G'  indicates  that  the  level  has  fallen  below  it.  The  cistern  L  is  supplied  with 
hot  water,  by  means  which  we  shall  presently  explain. 

To  understand  the  mechanism  necessary  to  work  the  piston,  let  us  consider 
how  the  supply  and  condensation  of  steam  must  be  regulated.  When  the 
piston  has  been  forced  to  the  bottom  of  the  cylinder  by  the  atmospheric  pres- 
sure acting  against  a  vacuum,  in  order  to  balance  that  pressure,  and  enable  it 
to  be  drawn  up  by  the  weight  of  the  pump-rop,  it  is  necessary  to  introduce 
steam  from  the  boiler.  This  is  accomplished  by  opening  the  cock  R  in  the 
steam-pipe  S.  The  steam  being  thus  introduced  from  the  boiler,  its  pressure 
balances  the  action  of  the  atmosphere  upon  the  piston,  which  is  immediately 
drawn  to  the  top  of  the  cylinder  by  the  weight  of  the  pump-rod  D.  It  then 
becomes  necessary  to  condense  this  steam,  in  order  to  produce  a  vacuum.  To 
accomplish  this,  the  further  supply  of  steam  must  be  cut  off,  which  is  done  by 
closing  the  cock  R.  The  supply  of  steam  from  the  boiler  being  thus  suspend- 
ed, the  application  of  cold  water  on  the  external  surface  of  the  cylinder  becomes 
necessary  to  condense  the  steam  within  it.  This  was  done  by  enclosing  the 
cylinder  within  another,  leaving  a  space  between  them.*  Into  this  space  cold 
water  was  allowed  to  flow  from  a  cock  M  placed  over  it,  supplied  by  a  pipe 
from  the  cistern  N.  This  cistern  is  supplied  with  water  by  a  pump  O,  which 
is  worked  by  the  engine. 

The  cold  water  supplied  from  M,  having  filled  the  space  between  the  two 
cylinders,  abstracts  the  heat  from  the  inner  one  ;  and  condensing  the  stream, 
produces  a  vacuum,  into  which  the  piston  is  forced  by  the  atmospheric  pressure. 
Preparatory  to  the  next  descent,  the  water  which  thus  fills  the  space  between 
the  cylinders,  and  which  is  warmed  by  the  heat  abstracted  from  the  steam, 
must  be  discharged,  in  order  to  give  room  for  a  fresh  supply  of  cold  water 
from  M.  An  aperture,  furnished  with  a  cock,  is  accordingly  provided  in  the 
bottom  of  the  cylinder,  through  which  the  water  is  discharged  into  the  cistern 
L  ;  and  being  warm,  is  adapted  for  the  supply  of  the  boiler  through  T,  as  al- 
ready mentioned. 

The  cock  R  being  now  again  opened,  steam  is  admitted  below  the  piston, 
which,  as  before,  ascends,  and  the  descent  is  again  accomplished  by  closing 
the  cock  R,  and  opening  the  cock  M,  admitting  cold  water  between  the  cylin- 
ders, and  thereby  condensing  the  steam  below  the  piston. 

The  condensed  steam,  thus  reduced  to  water,  will  collect  in  the  bottom  of 
the  cylinder,  and  resist  the  descent  of  the  piston.  It  is  therefore,  necessary  to 
provide  an  exit  for  it,  which  is  done  by  a  valve  opening  outward  into  a  tube 

*  The  external  cylinder  is  not  represented  in  the  diagram. 


THE   STEAM-ENGINE. 


which  leads  to  the  feeding  cistern  L,  into  which  the  condensed  steam  is  driven. 
That  the  piston  should  continue  to  be  air-tight,  it  was  necessary  to  keep  a 
constant  supply  of  water  over  it ;  this  was  done  by  a  cock  similar  to  M,  which 
allowed  water  to  flow  from  the  pipe  M  on  the  piston. 

Soon  after  the  first  construction  of  these  engines,  an  accidental  circumstance 
suggested  to  Newcomen  a  much  better  method  of  condensation  than  the  applica- 
tion of  cold  water  on  the  external  surface  of  the  cylinder.  An  engine  was  ob- 
served to  work  several  strokes  with  unusual  rapidity,  and  without  the  regular 
supply  of  the  condensing  water.  Upon  examining  the  piston,  a  hole  was  found 
in  it,  through  which  the  water,  which  was  poured  on  to  keep  it  air-tight,  flowed, 
and  instantly  condensed  the  steam  under  it. 

On  this  suggestion  Newcomen  abandoned  the  external  cylinder,  and  intro- 
duced a  pipe  H,  furnished  with  a  cock  Q,  into  the  bottom  of  the  cylinder,  so 
that,  on  turning  the  cock,  the  pressure  of  the  water  in  the  pipe  H,  from  the 
level  of  the  water  in  the  cistern  N,  would  force  the  water  to  rise  as  a  jet  into 
the  cylinder,  and  would  instantly  condense  the  steam.  This  method  of  con- 
densing by  injection  formed  a  very  important  improvement  in  the  engine,  and 
is  still  used. 

Having  taking  a  general  view  of  the  parts  of  the  atmospheric  engine,  let  us 
now  consider  more  particularly  its  operation. 

When  the  engine  is  not  working,  the  weight  of  the  pump-rod  D  (fig.  4) 
draws  down  the  beam  A,  and  draws  the  piston  to  the  top  of  the  cylinder, 
where  it  rests.  Let  us  suppose  all  the  cocks  and  valves  closed,  and  the 
boiler  filled  to  the  proper  depth.  The  fire  being  lighted  beneath  it,  the  water 
is  boiled  until  the  steam  acquires  sufficient  force  to  lift  the  valve  V.  When 
this  takes  place,  the  engine  may  be  started.  For  this  purpose  the  regulating 
valve  R  is  opened.  The  steam  rushes  in,  and  is  first  condensed  by  the  cold 
cylinder.  After  a  short  time  the  cylinder  acquires  the  temperature  of  the 
steam,  which  then  ceases  to  be  condensed,  and  mixes  with  the  air  which  filled 
the  cylinder.  The  steam  and  heated  air,  having  a  greater  force  than  the 
atmospheric  pressure,  will  open  a  valve  placed  at  the  end  X  of  a  small  tube  in 
the  bottom  of  the  cylinder,  and  which  opens  outward.  From  this  (which  is 
called  the  blowing  valve*)  the  steam  and  air  rush  in  a  constant  stream,  until  all 
the  air  has  been  expelled,  and  the  cylinder  is  filled  with  the  pure  vapor  of 
water.     This  process  is  called  blowing  the  engine  preparatory  to  starting  it. 

When  it  is  about  to  be  started,  the  engine-man  closes  the  regulator  R,  and 
thereby  suspends  the  supply  of  steam  from  the  boiler.  At  the  same  time  he 
opens  the  condensing  valve  H  ;t  and  thereby  throws  up  a  jet  of  cold  water  into 
the  cylinder.  This  immediately  condenses  the  steam  contained  in  the  cylinder, 
and  produces  the  vacuum.  (The  atmosphere  cannot  enter  the  blowing  valve, 
because  it  opens  outward,  so  that  no  air  can  enter  to  vitiate  the  vacuum.) 
The  atmospheric  pressure  above  the  piston  now  takes  effect,  and  forces  it  down 
in  the  cylinder.  The  descent  being  completed,  the  engine-man  closes  the 
condensing  valve  H,  and  opens  the  regulator  R.  By  this  means  he  stops  the 
play  of  the  jet  within  the  cylinder,  and  admits  the  steam  from  the  boiler.  The 
first  efl'ect  of  the  steam  is  to  expel  the  condensing  water  and  condensed  steam 
which  are  collected  in  the  bottom  of  the  cylinder,  through  the  tube  Y,  contain- 
ing a  valve  which  opens  outward  (called  the  eduction  valve),  which  leads  to  the 
hot  cistern  L,  into  which  this  water  is  therefore  discharged. 

When  the  steam  admitted  through  R  ceases  to  be  condensed,  it  balances 
the  atmospheric  pressure  above  the  piston,  and  thus  permits  it  to  be  drawn  to 

*  Also  called  the  snifling  valve,  from  the  peculiar  noise  made  by  the  air  and  steam  escaping 
from  it. 
t  Also  called  the  injection  valve. 


THE  STEAM-ENGINE. 


the  top  of  the  cylinder  by  the  weight  of  the  rod  D.  This  ascent  of  the  piston 
is  also  assisted  by  the  circumstance  of  the  steam  being  somewhat  stronger  than 
the  atmosphere. 

When  the  piston  has  reached  the  top,  the  regulating  valve  R  is  closed,  and 
the  condensing  valve  H  opened,  and  another  descent  produced,  as  before,  and 
so  the  process  is  continued. 

The  manipulation  necessary  in  working  this  engine  was,  therefore,  the 
alternate  opening  and  closing  of  two  valves  ;  the  regulating  and  condensing 
valves.  When  the  piston  reached  the  top  of  the  cylinder,  the  former  was  to 
be  closed,  and  the  latter  opened  ;  and,  on  reaching  the  bottom,  the  former  was 
to  be  opened,  and  the  latter  closed. 

The  duty  of  working  the  engine  requiring  no  great  amount  of  labor,  or  skill, 
was  usually  intrusted  to  boys,  called,  cock  boys.     It  happened  that  one  of  the 
most   important  improvements  which  has  ever  been  made  in  the  working  of 
steam-engines  was  due  to  the  ingenuity  of  one  of  these  boys.     It  is  said  that 
a  lad,  named  Humphrey  Potter,  was   employed  to   work  the  cocks  of  an  at- 
mospheric engine,  and  being  tempted  to  escape  from  the  monotonous  drudgery 
.  to  which  his  duty  confined  him,  his  ingenuity  was  sharpened  so  as  to  prompt 
(  him  to  devise  some  means  by  which  he  might  indulge  his  disposition  to  play 
without  exposing  himself  to  the  consequences  of  suspending  the  performance 
of  the    engine.     On  observing  the  alternate  ascending  and  descending  motion 
of  the  beam  above  him,  and  considering  it  in  reference  to  the  labor  of  his  own 
hands,  in  alternately  raising  and  lowering  the  levers  which  governed  the  cocks, 
he   perceived  a  relation  which  served  as  a  clue  to  a  simple  contrivance,  by 
which  the  steam-engine,  for  the  first  time,  became  an  automaton.     When  the 
^  beam  arrived  at  the  top  of  its  play,  it  was  necessary  to  open  the  steam-valve 
I  by  raising  a  lever,  and  to  close  the  injection  valve  by  raising  another.     This 
I  he  saw  could   be  accomplished  by  attaching  strings  of  proper  length  to  these 
I  levers,  and  tying  them  to  some  part  of  the  beam.     These  levers  required  to  be 
i)  moved  in  the  opposite  direction  when  the  beam  attained  the  lowest  point  of  its 
play.     This  he  saw  could  be  accomplished  by  strings,  either  connected  with 
the  outer  arm  of  the  beam,  or  conducted  over  rods  or  pulleys.     In  short,  he 
contrived  means  of  so  connecting  the  levers  which  governed  the  two  cocks  by 
strings   with   the  beam,  that  the   beam  opened  and  closed  these  cocks  with 
the  most  perfect  regularity  and  certainty  as  it  moved  upward  and  downward. 

Besides  rendering  the  machine  independent  of  manual  superintendence,  this 
process  conferred  upon  it  much  greater  regularity  of  performance  than  any 
manual  superintendence  could  insure. 

This  contrivance  of  Potter  was  very  soon  improved  by  the  substitution  of  a 
bar,  called  ^plug-frame,  which  was  suspended  from  the  arm  of  the  beam,  and 
which  carried  upon  it  pins,  by  which  the  arms  of  the  levers  governing  the  cocks 
were  struck  as  the  plug-frame  ascended  and  descended,  so  as  to  be  opened  and 
closed  at  the  proper  times. 

The  engine  thus  improved  required  no  other  attendance  except  to  feed  the 
boiler  occasionally  by  the  cock  T,  and  to  attend  the  furnace. 

However  the  merit  of  the  discovery  of  the  physical  principles  on  which  the 
mechanical  application  of  steam  depends  may  be  awarded,  it  must  be  admitted 
that  the  engine  contrived  by  Newcomen  and  his  associates,  considered  as  a 
practical  machine,  was  immeasurably  superior  to  that  which  preceded  it ; 
superior,  indeed,  to  such  a  degree,  that  while  the  one  was  incapable  of  any 
permanenily  useful  application,  the  other  soon  became  a  machine  of  extensive 
utility  in  the  drainage  of  mines  ;  and,  even  at  the  present  time,  the  atmospheric 
engine  is  not  unfrequently  used  in  preference  to  the  modern  steam-engine,  in 
districts  where  fuel  is  abundant  arid  cheap ;  the  expense  of  constructing  and 


416 


THE  STEAM-ENGINE. 


maintaining  it  being  considerably  less  than  that  of  an  improved  steam-engine. 
The  low  pressure  of  the  steam  used  in  working  it,  rendered  it  perfectly  safe. 
While  Savery's  engine,  to  work  with  effect,  required  that  the  steam  confined 
in  the  vessels  should  have  a  bursting  pressure  amounting  to  about  thirty  pounds 
per  square  inch,  the  pressure  of  steam  in  the  boiler  and  cylinder  of  the  at- 
mospheric engine  required  only  a  pressure  about  one  pound  per  square  inch. 
The  high  pressure  also  of  the  steam  used  in  Savery's  engine,  was  necessarily 
accompanied,  as  we  shall  presently  explain,  by  a  greatly  increased  temperature. 
The  effect  of  this  was,  to  weaken  and  gradually  destroy  the  vessels,  especially 
those  which,  like  the  steam-vessels  V  and  V  (fig.  3),  were  alternately  heated 
and  cooled. 

Besides  these  defects,  the  power  of  Savery's  engines  was  also  very  restricted, 
both  as  to  the  quantity  of  water  raised  and  as  to  the  height  to  which  it  was 
elevated.  On  the  other  hand,  the  atmospheric  engine  was  limited  in  its  power 
only  by  the  dimensions  of  its  piston.  Another  considerable  advantage  which 
the  atmospheric  engine  possessed  over  that  of  Savery,  was  the  facility  with 
which  it  was  capable  of  driving  machinery  by  means  of  the  working-beam. 
The  merit,  however,  of  Newcomen's  engine,  regarded  as  an  invention,  and 
apart  from  merely  practical  considerations,  must  be  ascribed  principally  to  its 
mechanism  and  combinations.  We  find  in  it  no  new  principle,  and  scarcely 
even  a  novel  application  of  a  principle.  The  agency  of  the  atmospheric  pres- 
sure acting  against  a  vacuum,  or  partial  vacuum,  had  been  long  known  :  the 
method  of  producing  a  vacuum  by  the  condensation  of  steam  had  been  suggested 
by  Papin,  and  carried  into  practical  effect  by  Savery.  The  mechanical  power 
obtained  from  the  direct  pressure  of  the  elastic  force  of  steam,  used  in  the 
atmospheric  engine  to  balance  the  atmosphere  during  the  ascent  of  the  piston, 
was  suggested  by  De  Caus  and  Lord  Worcester.  The  boiler,  gauge-pipes,  and 
the  regulator,  were  all  borrowed  from  the  engine  of  Savery.  The  idea  of  using 
the  atmospheric  pressure  against  a  vacuum  or  partial  vacuum,  to  work  a  piston 
in  a  cylinder,  had  been  suggested  by  Otto  Guericke,  an  ingenious  German 
philosopher,  who  invented  the  air-pump  ;  and  this,  combined  with  the  produc- 
tion of  a  vacuum  by  the  condensation  of  steam,  was  subsequently  suggested  by 
Papin.  The  use  of  a  working-beam  could  not  have  been  unknown.  Never- 
theless, the  judicious  combination  of  these  scattered  principles  must  be  ac- 
knowledged to  deserve  considerable  credit.  In  fact,  the  mechanism  contrived 
by  Newcomen  rendered  a  machine  which  was  before  altogether  inefficient, 
highly  efficient :  and,  as  observed  by  Tredgold,  such  a  result,  considered  in  a 
practical  sense,  should  be  more  highly  valued  than  the  fortuitous  discovery  of 
a  physical  principle. 


THE    STEAM-EIGOE, 


(SECOND    LECTURE.) 


Mechanical  Force  of  Steam. — Facta  to  be  remembered.— Watt  finds  Condensation  in  the  Cylinder 
incompatible  with  a  due  Economy  of  Fuel. — Conceives  the  notion  of  Condensing  out  of  the  Cyl- 
inder.—Discovers  separate  Condensation. — Invents  the  Air-Pump. — Substitutes  Steam  Pressure 
for  Atmospheric  Pressure.— Invents  the  Steam  Case  or  Jacket.— His  first  Experiments  to  realize 
these  Inventions. — His  Experimental  Apparatus. — His  Models  at  Delft  House. — Difficulties  of 
bringing  the  improved  Engines  into  Use. — Watt  employed  by  Roebuck.— His  Partnership. — His 
first  Patent. — His  Single-Acting  Engine. — Discovery  of  the  Expansive  Action  of  Steam. — Its 
Mechanical  Effects. — Its  Variable  Action. — Methods  of  Equalizing  it. — Its  extensive  Application 
in  the  Cornish  Engines. — Extension  of  the  Steam-Engine  to  Manufactures. — Attempts  of  Papiu, 
Savery,  Hull,  Champion,  Stewart,  and  Washborough. — Watt's  second  Patent. — Sunand-Planet 
Wheels. — Valves  of  Double-Acting  Engine. 


VOL..  II.— 37 


THE   STEAM-ENGINE. 


419 


THE   STEAM-ENGINE. 


(SECOND    LECTURE.) 


Having  explained  in  a  fornner  lecture  the  conditions  under  which,  by  sup- 
plying heat  to  water,  it  is  converted  into  steam,  and,  by  abstracting  heat  from 
steam,  it  may  be  reconverted  into  water,  let  us  now  consider  the  mechanical 
force  which  is  developed  in  these  phenomena. 

Fig.  5. 


Let  A  B  (fig.  5)  be  a  tube,  or  cylinder,  the  base  of  which  is  equal  to  a 
square  inch,  and  let  a  piston  P  move  in  it  so  as  to  be  steam-tight.  Let  it  be 
supposed,  that  under  this  piston  there  is,  in  the  bottom  of  the  cylinder,  a  cubic 
inch  of  water  between  the  bottom  of  the  piston  and  the  bottom  of  the  tube ; 


420 


THE    STEAM-ENGINE. 


let  the  piston  be  counterbalanced  by  a  weight  W  acting  over  a  pulley,  which  [ 
will  be  just  sufficient  to  counterpoise  the  weight  of  the  piston,  so  as  leave  no  ^ 
force  tending  to  keep  the  piston  down,  except  the  force  of  the  atmosphere 
acting  above  it.  Under  the  circumstances  here  supposed,  the  piston  being  in 
contact  with  the  water,  and  all  air  being  excluded,  it  will  be  pressed  down  by 
the  weight  of  the  atmosphere,  which  we  will  suppose  to  be  fifteen  pounds,  the 
magnitude  of  the  piston  being  a  square  inch. 

Now  let  the  flame  of  a  lamp  be  applied  at  the  bottom  of  the  tube  ;  the  water 
under  the  piston  having  its  temperature  thereby  gradually  raised,  and  being 
submitted  to  no  pressure  save  that  of  the  atmosphere  above  the  piston,  it  will 
begin  to  be  converted  into  steam  when  it  has  attained  the  temperature  of  212°. 
According  as  it  is  converted  into  steam,  it  will  cause  the  piston  to  ascend  in 
the  tube  until  all  the  water  has  been  evaporated.  If  the  tube  were  constructed 
of  sufficient  length,  the  piston  then  would  be  found  to  have  risen  to  the  height 
of  about  seventeen  hundred  inches,  or  one  hundred  and  forty-two  feet ;  since, 
as  has  been  already  explained,  water  passing  into  steam  under  the  ordinary 
pressure  of  the  atmosphere  undergoes  an  increase  of  bulk  in  the  proportion  of 
about  seventeen  hundred  to  one. 

Now  in  this  process,  the  air  above  the  piston,  which  presses  on  it  with  a 
force  equal  to  fifteen  pounds,  has  been  raised  one  hundred  and  forty-two  feet. 
It  appears,  therefore,  that,  by  the  evaporation  of  a  cubic  inch  of  water  under  a 
pressure  equal  to  fifteen  pounds  per  square  inch,  a  mechanical  force  of  this 
amount  is  developed. 

It  is  evident  that  fifteen  pounds  raised  one  hundred  and  forty-two  feet  suc- 
cessively, is  equivalent  to  one  hundred  and  forty-two  times  fifteen  pounds 
raised  one  foot.  Now,  one  hundred  and  forty-two  times  fifteen  is  two  thousand 
one  hundred  and  thirt}%  and  therefore  the  force  thus  obtained  is  equal  to  two 
thousand  one  hundred  and  thirty  pounds  raised  one  foot  high.  This  being 
within  about  110  pounds  of  a  ton,  it  may  be  stated,  in  round  numbers,  that,  by 
the  evaporation  of  a  cubic  inch  of  water  under  these  circumstances,  a  force  is 
obtained  equal  to  that  which  would  raise  a  ton  weight  a  foot  high. 

The  augmentation  of  volume  which  water  undergoes  in  passing  into  steam 
under  the  pressure  here  supposed,  may  be  easily  retained  in  the  memory,  from 
the  accidental  circumstance  that  a  cubic  inch  of  water  is  converted  into  a  cubic 
foot  of  steam,  very  nearly.  A  cubic  foot  contains  one  thousand  seven  hundred 
and  twenty-eight  cubic  inches — which  is  little  different  from  the  proportion 
which  steam  bears  to  water,  when  raised  under  the  atmosphoric  pressure. 

It  will,  therefore,  be  an  advantage  to  retain  in  memory  the  following  general 
facts  : — 

1.  A  cubic  inch  of  water  evaporated  under  the  ordinary  atmospheric  pres- 
sure, is  converted  into  a  cubic  foot  of  steam. 

2.  A  cubic  inch  of  water  evaporated  under  the  atmospheric  pressure,  gives  a 
mechanical  force  equal  to  what  would  raise  about  a  ton  weight  afoot  high. 

Let  us,  again,  suppose  the  piston  P  (fig.  5)  to  be  restored  to  its  original 
position,  with  the  liquid  water  beneath  it ;  and,  in  addition  to  the  weight  of 
the  atmosphere  which  before  pressed  it  down,  let  us  suppose  another  weight 
of  fifteen  pounds  laid  upon  it,  so  that  the  water  below  shall  be  pressed  by 
double  the  weight  of  the  atmosphere.  If  the  lamp  were  now  applied,  and  at 
the  same  time  a  thermometer  were  immersed  in  the  water,  it  would  be  found 
that  the  water  would  not  begin  to  be  converted  into  steam  until  it  attained  the 
temperature  of  about  250°.  The  piston  would  then  begin,  as  before,  to  ascend, 
and  the  water  to  be  gradually  converted  into  vapor.  The  water  being  com- 
pletely evaporated,  it  would  be  found  that  the  piston  would  be  raised  to  a  height 
little  more  than  half  its  former  height,  or  72  feet.     The   mechanical   effect, 


THE  STEAM-ENGINE. 


421 


therefore,  thus  obtained,  will  be  equivalent  to  double  the  former  weight  raised 
half  the  former  height. 

In  like  manner,  if  the  piston  were  loaded  with  thirty  pounds  in  addition  to 
the  atmosphere,  the  whole  pressure  on  the  water  being  then  three  times  the 
pressure  first  supposed,  the  piston  would  be  raised  to  somewhat  more  than  one 
third  of  its  first  height  by  the  evaporation  of  the  water.  This  would  give  a 
mechanical  force  equivalent  to  three  times  the  original  weight  raised  a  little 
more  than  one  third  of  the  original  height. 

In  general,  as  the  pressure  on  the  piston  is  increased,  the  height  to  which 
the  piston  would  be  raised  by  the  evaporation  of  the  water  will  be  diminished 
in  a  proportion  somewhat  less  than  the  proportion  in  which  the  pressure  on 
the  pistop  is  increased.  If  the  temperature  at  which  the  water  is  converted 
into  steam  under  these  different  pressures  were  the  same,  then  the  height  to 
which  the  piston  would  be  raised  by  the  evaporation  of  the  water  would  be 
diminished  in  precisely  the  same  proportion  as  the  pressure  on  the  piston  is 
increased  ;  and,  in  that  case,  the  whole  mechanical  force  developed  by  the 
evaporation  of  the  water  would  remain  exactly  the  same  under  whatever  pres- 
sure the  water  might  be  boiled.  We  shall  explain  hereafter  the  extent  to 
which  the  variation  of  temperature  in  the  water  and  steam  corresponding  to 
the  variation  of  pressure  modifies  this  law  ;  but,  as  the  effect  of  the  difference 
of  temperatures  is  not  considerable,  it  will  be  convenient  to  register  in  the 
memory  the  following  important  practical  conclusion  : — 

A  cubic  i?ich  of  water  converted  into  steam  ivill  supply  a  mechanical  force  very 
nearly  equal  to  a  ton  weight  raised  afoot  high;  and  this  force  will  not  be  subject 
to  considerable  variation,  whatever  be  the  temperature  or  pressure  at  which  the 
water  may  be  evaporated. 

At  the  period  to  which  we  have  now  brought  the  history  of  the  invention  of 
the  steam-engine,  Watt  had  directed  his  attention  to  the  subject,  and  had  obtain- 
ed chiefly  by  his  own  experiments,  a  sufficient  knowledge  of  the  phenomena 
which  have  been  just  explained,  to  enable  him  to  arrive  at  the  conclusion  that 
a  very  small  proportion  of  the  whole  mechanical  effect  attending  the  evapora- 
tion was  really  rendered  available  by  the  atmospheric  engine  ;  and  that,  there- 
fore, extensive  and  injurious  sources  of  waste  existed  in  its  machinery. 

He  perceived  that  the  principal  source  of  this  wasteful  expenditure  of  power 
consisted  in  the  quantity  of  steam  which  was  condensed  at  each  stroke  of  the 
piston,  in  heating  the  cylinder  previous  to  the  ascent  of  the  piston.  Yet,  as 
it  was  evident  that  that  ascent  could  not  be  accomplished  in  a  cold  cylinder, 
it  was  apparent  that  this  waste  of  power  must  be  inevitable,  unless  soine  ex- 
pedient could  be  devised,  by  which  a  vacuum  could  be  maintained  in  the  cylinder, 
without  cooling  it.  But,  to  produce  such  a  vacuum,  the  steam  must  be  con- 
densed ;  and,  to  condense  the  steam,  its  temperature  must  be  lowered  to  such 
a  point  that  the  vapor  proceeding  from  it  shall  have  no  injurious  pressure  ;  yet,  if 
condensed  steam  be  contained  in  a  cylinder  at  a  high  temperature,  it  will  re- 
turn to  the  temperature  of  the  cylinder,  recover  its  elasticity,  and  resist  the 
descent  of  the  piston. 

Having  reflected  on  these  circumstances,  it  became  apparent  to  Watt,  that 
a  vice  was  inherent  in  the  structure  of  the  atmospheric  engine,  which  rendered 
a  large  waste  of  power  inevitable  ;  this  vice  arising  from  the  fact,  that  the 
condensation  of  the  steam  was  incompatible  with  the  condition  of  maintaining 
the  elevated  temperature  of  the  cylinder  in  which  that  condensation  took  place. 
It  followed,  therefore,  either  that  the  steam  must  be  imperfectly  condensed,  or 
that  the  condensation  could  not  take  place  in  the  cylinder.  It  was  in  1765, 
that,  pondering  on  these  circumstances,  the  happy  idea  occurred  to  him,  that 
the  production  of  a  vacuum  could  be  equally  effected,  thougl 


ph 


422 


THE   STEAM-ENGINE. 


the  condensation  of  the  steam  took  place  were  not  the  cylinder  itself.  He 
saw,  that  if  a  vessel  in  which  a  vacuum  was  produced  were  put  into  communi- 
cation with  another  containing  an  elastic  fluid,  the  elastic  fluid  would  rush  into 
the  vacuum,  and  diff'use  itself  through  the  two  vessels  ;  but  if,  on  rushing  into 
such  vacuum,  this  elastic  fluid,  being  vapor,  were  there  condensed,  or  restored 
to  the  liquid  form,  that  then  the  space  within  the  two  vessels  would  be  equally 
rendered  a  vacuum  ;  that,  under  such  circumstances,  one  of  the  vessels  might 
be  maintained  at  any  temperature,  however  high,  while  the  other  might  be  kept 
at  any  temperature,  however  low.  This  felicitous  conception  formed  the  first 
step  in  that  splendid  career  of  invention  and  discovery  which  has  conferred 
immortality  on  the  name  of  Watt.  He  used  to  say,  that  the  moment  the  idea 
of  separate  condensation  occurred  to  him — that  is,  of  condensing,  in  one  vessel 
kept  cold,  the  steam  coming  from  another  vessel  kept  hot — all  the  details  of 
his  improved  engine  rushed  into  his  mind  in  such  rapid_ succession,  that,  in  the 
course  of  a  day,  his  invention  was  so  complete  that  he  proceeded  to  submit  it 
to  experiment. 

To  explain  the  first  conception  of  this  memorable  invention ;  let  a  tube  or 
pipe,  S  (fig.  6),  be  imagined  to  proceed  from  the  bottom  of  the  cylinder  A  B 


Fig.  6. 


to  a  vessel,  C,  having  a  stop-cock,  D,  by  which  the  communication  between 
the  cylinder  and  the  vessel  C  may  be  opened  or  closed  at  pleasure.  If  we 
suppose  the  piston  P  at  the  top  of  the  cylinder,  and  the  space  below  it  filled 
with  steam,  the  cyUnder  and  steam  being  at  the  usual  temperature,  while  the 
vessel  C  is  a  vacuum,  and  maintained  at  a  low  temperature.  Then,  on  opening 
the  cock  D,  the  steam  will  rush  from  the  cylinder  A  B  through  the  tube  S, 
and,  passing  into  the  cold  vessel  C,  will  be  condensed  by  contact  with  its  cold 
sides.  This  process  of  condensation  will  be  rendered  instantaneous  if  a  jet 
of  cold  water  is  allowed  to  play  in  the  vessel  C.  When  the  steam  thus  rushing 
into  C,  has  been  destroyed,  and  the  space  in  the  cylinder  A  B  becomes  a 
vacuum,  then  the  pressure  of  the  atmosphere  being  unobstructed,  the  piston 
will  descend  with  the  force  due  to  the  excess  of  the  pressure  of  the  atmosphere 
above  the  friction.  When  it  has  descended,  suppose  the  stop-cock  D  closed, 
and  steam  admitted  from  the  boiler  through  a  proper  cock  or  valve  below  the 
piston,  the  cylinder  and  piston  being  still  at  the  same  temperature  as  before. 
The  steam  on  entering  the  cylinder,  not  being  exposed  to  contact  with  any 


THE   STEAM-ENGINE. 


423 


surface  below  its  own  temperature,  will  not  be  condensed,  and  therefore  will 
immediately  cause  the  piston  to  rise,  and  the  piston  will  have  attained  the  top 
of  the  cylinder  when  as  much  steam  shall  have  been  supplied  by  the  boiler  as 
will  fill  the  cylinder.  When  this  has  taken  place,  suppose  the  communication 
with  the  boiler  cut  off,  and  the  cock  D  once  more  opened ;  the  steam  will 
ac-ain  rush  through  the  pipe  S  into  the  vessel  C,  where  encountering  the  cold 
surface  and  the  jet  of  cold  water,  it  will  be  condensed,  and  the  vacuum,  as  be- 
fore, will  be  produced  in  the  cylinder  A  B  ;  that  cylinder  still  maintaining  its 
temperature,  the  piston  will  again  descend,  and  so  the  process  may  be  continued. 

Having  carried  the  invention  to  this  point.  Watt  saw  that  the  vessel  C  would 
gradually  become  heated  by  the  steam  which  would  be  continually  condensed 
in  it.  To  prevent  this,  as  well  as  to  supply  a  constant  jet  of  cold  water,  he 
proposed  to  keep  the  vessel  C  submerged  in  a  cistern  of  cold  water,  from  which 
a  pipe  should  conduct  a  jet  to  play  within  the  vessel,  so  as  to  condense  the 
steam  as  it  would  pass  from  the  cylinder. 

But  here  a  difficulty  presented  itself,  against  which  it  was  necessary  to 
provide.  The  cold  water  admitted  through  the  jet  to  condense  the  steam, 
mixed  with  the  condensed  steam  itself,  would  gradually  collect  in  the  vessel 
C,  and  at  length  choke  it.  To  prevent  this.  Watt  proposed  to  put  the  vessel 
C  in  communication  with  a  pump  F,  which  might  be  wrought  by  the  engine 
itself,  and  by  which  the  water,  which  would  collect  in  the  bottom  of  the  vessel 
C,  would  be  constantly  drawn  off.  This  pump  would  be  evidently  rendered 
the  more  necessary,  since  more  or  less  atmospheric  air,  always  combined  with 
water  in  its  common  state,  would  enter  the  vessel  C  by  the  condensing  jet. 
This  air  would  be  disengaged  in  the  vessel  C  by  the  heat  of  the  steam  con- 
densed therein  ;  and  it  would  rise  through  the  tube  S,  and  vitiate  the  vacuum 
in  the  cylinder  ;  an  effect  which  would  be  rendered  the  more  injurious,  inas- 
much as,  unlike  steam,  this  elastic  fluid  would  be  incapable  of  being  condensed 
by  cold.  The  pump  F,  therefore,  by  which  Watt  proposed  to  draw  off  the 
water  from  the  vessel  C,  might  also  be  made  to  draw  off  the  air,  or  the  princi- 
pal part  of  it. 

The  vessel  C  was  subsequently  called  a  condenser  ;  and,  from  the  circum- 
stances just  adverted  to,  the  pump  F  has  been  called  the  air-pump. 

These — namely,  the  cylinder,  the  condenser,  and  the  air-pump — were  the 
three  principal  parts  in  the  invention,  as  it  first  presented  itself  to  the  mind  of 
Watt — and  even  before  it  was  reduced  to  a  model,  or  submitted  to  experiment. 
But,  in  addition  to  these,  other  two  improvements  offered  themselves  in  the 
very  first  stage  of  its  progress. 

In  the  atmospheric  engine,  the  piston  was  maintained  steam-tight  in  the 
cylinder  by  supplying  a  stream  of  cold  water  above  it,  by  which  the  small 
interstices  between  the  piston  and  cylinder  would  be  stopped.  It  is  evident 
that  the  effect  of  this  water  as  the  piston  descended  would  be  to  cool  the  cyl- 
inder, besides  which  any  portion  of  it  which  might  pass  between  the  piston 
and  cylinder  and  which  would  pass  below  the  piston,  would  boil  the  moment 
it  would  fall  into  the  cylinder,  which  itself  would  be  maintained  at  the  boiling 
temperature.  This  water,  therefore,  would  produce  steam,  the  pressure  of 
which  would  resist  the  descent  of  the  piston. 

Watt  perceived,  that  even  though  this  inconvenience  were  removed  by  the 
use  of  oil  or  tallow  upon  the  piston,  still,  that  as  the  piston  would  descend  in 
the  cylinder,  the  cold  atmosphere  would  follow  it ;  and  would,  to  a  certain 
extent,  lower  the  temperature  of  the  cylinder.  On  the  next  ascent  of  the  pis- 
ton, this  temperature  would  have  to  be  again  raised  to  212°  by  the  steam 
coming  from  the  boiler,  and  would  entail  upon  the  machine  a  proportionate 
waste  of  power. 


424 


THE   STEAM-ENGINE. 


atmos 


e  of  the  engine-house  could  be  kept  heated  to  the  tempera-  < 
ture  of  boiling  water,  this  inconvenience  would  be  removed.     The  piston  would  ) 
then  be  pressed  down  by  air  as  hot  as  the  steam  to  be  subsequently  introduced  I 
into  it.     On  further  consideration,  however,  it  occurred  to  Watt  that  it  would  ) 
be  still  more  advantageous  if  the  cylinder  itself  could  be  worked  in  an  at-  ( 
mosphere  of  steam,  having  only  the  same  pressure  as  the  atmosphere.     Such  ) 
steam  would  press  the  piston  down  as  effectually  as  the  air  would  ;  and  it  would  ( 
have  the  further  advantage  over  air,  that  if  any  portion  of  it  leaked  through  be-  ) 
tween  the  piston  and  cylinder,  it  would  be  condensed,  which  could  not  be  the 
the  case  with  atmospheric  air.     He  therefore  determined  on  surrounding  the  j 
cylinder  by  an  external  casing,  the  space  between  which  and  the  cylinder  he  j 
proposed  to  be  filled  with  steam  supplied  from  the  boiler.     The  cylinder  would  i 
thus  be  enclosed  in  an  atmosphere  of  its  own,  independent  of  the  external  air,  ] 
and  the  vessel  so  enclosing  it  would  only  require  to  be  a  little  larger  than  the  i 
cylinder,  and  to  have  a  close  cover  at   the  top,  the  centre  of  which  might  be  ' 
perforated  with  a  hole  to  admit  the  rod  of  the  piston  to  pass  through,  the  rod  , 
being  made  smooth,  and  so  fitted  to  the  perforation  that  no  steam  should  escape  * 
between   them.     This  method  would   be  attended  also  with  the  advantage  of 
keeping  the  cylinder  and  piston  always  heated,  not  only  inside  but  outside  ; 
and  Watt  saw  that  it  would  be  further  advantageous  to  employ  the  pressure  of 
steam  to  drive  the  piston  in  its  descent  instead  of  the  atmosphere,  as  its  inten- 
sity or  force  would  be  much  more  manageable  ;  for,  by  increasing  or  diminish- 
ing the  heat  of  the  steam  in  which  the  cylinder  was  enclosed,  its  pressure  might 
be  regulated  at  pleasure,  and  might  be  made  to  urge  the  piston  with  any  force 
that  might  be  required.     The  power  of  the  engine  would  therefore  be  completely 
under  control,  and  independent  of  all  variations  in  the  pressure  of  the  atmosphere. 

This  was  a  step  which  totally  changed  the  character  of  the  machine,  and 
which  rendered  it  a  steam-engine  instead  of  an  atmospheric  engine.  Not 
only  was  the  vacuum  below  the  piston  now  produced  by  the  property  of  steam, 
in  virtue  of  which  it  is  reconverted  into  water  by  cold  ;  but  the  pressure  which 
urged  the  piston  into  this  vacuum  was  due  to  the  elasticity  of  steam. 

The  external  cylinder,  within  which  the  working  cylinder  was  enclosed,  was 
called  THE  JACKET,  and  is  still  very  generally  used. 

The  first  experiment  in  which  Watt  attempted  to  realize,  on  a  small  scale, 
his  conceptions,  was  made  in  the  following  manner.  The  cylinder  of  the  engine 
was  represented  by  a  brass  syringe,  A  B  (fig.  7),  an  inch  and  a  third  in  diameter, 
and  ten  inches  in  length,  to  which  a  top  and  a  bottom  of  tin  plate  was  fitted. 
Steam  was  conveyed  by  a  pipe,  S,  from  a  small  boiler  into  the  lower  end  of 
this  syringe,  a  communication  being  made  with  the  upper  end  of  the  syringe 
by  a  branch  pipe,  D.  For  the  greater  convenience  of  the  experiment,  it  was 
found  desirable  to  invert  the  position  of  the  cylinder,  so  that  the  steam  should 
press  the  piston  P  upward  instead  of  downward.  The  piston-rod  R  therefore 
was  presented  downward.  An  eduction  pipe,  E,  was  also  inserted  in  the  top 
of  the  cylinder,  which  was  carried  to  the  condenser.  The  piston-rod  was 
made  hollow,  or  rather  a  hole  was  drilled  longitudinally  through  it,  and  a  valve 
was  fitted  at  its  lower  end,  to  carry  off  the  water  produced  by  the  steam,  which 
would  be  condensed  in  the  cylinder  in  the  commencement  of  the  process. 
The  condenser  used  in  this  experiment  operated  without  injection,  the  steam 
being  condensed  by  the  contact  of  cold  surfaces.  It  consisted  of  two  thin 
pipes,  F  G,  of  tin,  ten  or  twelve  inches  in  length,  and  the  sixth  of  an  inch  in 
diameter,  standing  beside  each  other  perpendicularly,  and  communicating  at 
the  top  wiih  the  eduction  pipe,  which  was  provided  with  a  valve  opening  up- 
ward. At  the  bottom  these  two  pipes  communicated  with  another  tube,  I,  of 
about  an  inch  in  diameter,  by  a  horizontal  pipe,  having  in  it  a  valve,  M,  open- 


THE   STEAM-ENGINE. 


425   ' 


Fig.  7. 


"^^ 


■dfl'^ 


ing  toward  I,  fitted  with  a  piston  K,  which  served  the  office  of  the  air-pump, 
being  worked  by  the  hand.  This  piston,  K,  had  valves  in  its  opening  upward. 
These  condensing  pipes  and  air-pump  were  immersed  in  a  small  cistern,  filled 
with  cold  water.  The  steam  was  conveyed  by  the  steam-pipe  S  to  the  bottom 
of  the  cylinder,  a  communication  between  the  top  and  bottom  of  the  cylinder 
being  occasionally  opened  by  a  cock,  C,  placed  in  the  branch  pipe.  The 
eduction  pipe  leading  to  the  condenser  also  had  a  cock,  L,  by  which  the  com- 
munication between  the  top  of  the  cylinder  and  the  condenser  might  be  opened 
and  closed  at  pleasure.  In  the  commencement  of  the  operation,  the  cock  N 
admitting  steam  from  the  boiler,  and  the  cock  L  opening  a  communication  be- 
tween the  cylinder  and  the  condenser,  and  the  cock  C  opening  a  communica- 
tion between  the  top  and  bottom  of  the  cylinder,  being  all  open,  steam  rushed 
from  the  boiler,  passing  through  all  the  pipes,  and  filling  the  cylinder.  A 
current  of  mixed  air  and  steam  was  thus  produced  through  the  eduction  pipe 
E,  through  the  condensing  pipes  F  and  G,  and  through  the  air-pump  I,  which 
issued  from  the  valve  H  in  the  eduction  pipe,  and  from  the  valve  in  the  air- 
pump  piston,  all  of  which  opened  upward.  The  steam  also  in  the  cylinder 
passed  through  the  hole  drilled  in  the  piston-rod,  and  escaped,  mixed  with  air, 
through  the  valve  in  the  lower  end  of  that  rod.  This  process  was  continued 
until  all  the  air  in  the  cylinder,  pipes,  and  condenser,  was  blown  out,  and  all 
these  spaces  filled  with  pure  steam.  The  cocks  L,  C,  and  N,  were  then  closed, 
and  the  atmospheric  pressure  closed  the  valve  H  and  the  valves  in  the  air-pump 
piston.  The  cold  surfaces  condensing  the  steam  in  the  pipes  F  and  G,  and  in 
the  lower  part  of  the  air-pump,  a  vacuum  was  produced  in  these  spaces.  The 
cock  C  being  now  closed,  and  the  cocks  L  and  N  being  open,  the  steam  in 
the  upper  part  of  the  cylinder  rushed  through  the  pipe  E  into  the  condenser. 


426 


THE    STEAM-ENGINE. 


where  it  was  reduced  to  water,  so  that  a  vacuum  was  left  in  the  upper  part  of 
the  cylinder.  The  steam  from  the  boiler  passing  below  the  piston,  pressed  it 
upward  with  such  force,  that  it  lifted  a  weight  of  eighteen  pounds  hung  from 
the  end  of  the  piston-rod.  When  the  piston  reached  the  top  of  the  cylinder, 
the  cocks  L  and  N  were  closed,  and  the  cock  C  opened.  All  communication 
between  the  cylinder  and  the  boiler,  as  well  as  between  the  cylinder  and  the 
condenser,  were  now  cut  off,  and  the  steam  in  the  cylinder  circulated  freely 
above  and  below  the  piston,  by  means  of  the  open  tube  D.  The  piston,  being 
subject  to  equal  forces  upward  and  downward,  would  therefore  descend  by  its 
own  weicfht,  and  would  reach  the  bottom  of  the  cylinder.  The  air-pump 
piston  meanwhile  being  drawn  up,  the  air  and  the  condensed  steam  in  the 
tubes  F  and  G  were  drawn  into  the  air-pump  I,  through  the  open  horizontal 
tube  at  the  bottom.  Its  return  was  stopped  by  the  valve  M.  By  another 
stroke  of  the  air-pump,  this  water  and  air  were  drawn  out  through  valves  in 
the  piston,  which  opened  upward.  The  cock  C  was  now  closed,  and  the 
cocks  L  and  N  opened,  preparatory  to  another  stroke  of  the  piston.  The 
steam  in  the  upper  part  of  the  cylinder  rushed,  as  before,  into  the  tubes  F  and 
G,  and  was  condensed  by  their  cold  surfaces,  while  steam  from  the  boiler 
coming  through  the  pipe  S,  pressed  the  piston  upward.  The  piston  again 
ascended  with  the  same  force  as  before,  and  in  the  same  manner  the  process 
was  continually  repeated. 

The  quantity  of  steam  expended  in  this  experimental  model  in  the  produc- 
tion of  a  given  number  of  strokes  of  the  piston  was  inferred  from  the  quantity 
of  water  evaporated  in  the  boiler  ;  and  on  comparing  this  with  the  magnitude 
of  the  cylinder  and  the  weight  raised  by  the  pressure  of  the  steam,  the  contri- 
vance was  proved  to  affect  the  economy  of  steam,  as  far  as  the  imperfect  con- 
ditions of  such  a  model  could  have  permitted.  A  larger  model  was  next  con- 
structed, having  an  outer  cylinder,  or  steam  case,  surrounding  the  working 
cylinder,  and  the  experiments  made  with  it  fully  realized  Watt's  expectations, 
and  left  no  doubt  of  the  great  advantages  which  would  attend  his  invention. 
The  weights  raised  by  the  piston  proved  that  the  vacuum  in  the  cylinder  pro- 
duced by  the  condensation  was  almost  perfect ;  and  he  found  that  when  he 
used  water  in  the  boiler  which  by  long  boiling  had  been  well  cleared  of  air, 
the  weight  raised  was  not  much  less  than  the  whole  amount  of  the  pressure 
of  the  steam  upoii  the  piston.  In  this  large  model,  the  cylinder  was  placed 
in  the  usual  position,  with  a  working  lever  and  other  apparatus  similar  to  that 
employed  in  the  atmospheric  engine. 

It  was  in  the  beginning  of  the  year  1765,  Watt  being  then  in  the  twenty- 
ninth  year  of  his  age,  that  he  arrived  at  these  great  discoveries.  The  experi- 
mental models  just  described,  by  which  his  invention  was  first  reduced  to  a 
rude  practical  test,  were  fitted  up  at  a  place  called  Delft  house,  in  Glasgow. 
It  will  doubtless  at  the  first  view,  be  a  matter  of  surprise  that  improvements 
of  such  obvious  importance  in  the  economy  of  steam  power,  and  capable  of 
being  verified  by  tests  so  simple,  were  not  immediately  adopted  wherever  at- 
mospheric engines  were  used.  At  the  time,  however,  referred  to.  Watt  was 
an  obscure  artisan,  in  a  provincial  town,  not  then  arrived  at  the  celebrity  to 
which  it  has  since  attained,  and  the  facilities  by  which  inventions  and  improve- 
ments became  public  were  much  less  than  they  have  since  become.  It  should 
also  be  considered  that  all  great  and  sudden  advances  in  the  useful  arts  are 
necessarily  opposed  by  the  existing  interests  with  which  their  effects  are  in 
conflict.  From  these  causes  of  opposition,  accompanied  with  the  usual  influence 
of  prejudice  and  envy.  Watt  was  not  exempt,  and  was  not  therefore  likely  sud- 
denly to  revolutionize  the  arts  and  manufactures  of  the  country  by  displacing 
the  moving  powers  employed  in  them,  and  substituting  an  engine,  the  efficacy 


THE  STEAM-ENGINE. 


and  power  of  which  depended  mainly  on  physical  principles,  then  altogether 
new  and  but  imperfectly  understood. 

Not  having  the  command  of  capital,  and  finding  it  impracticable  to  inspire 
those  who  had,  with  the  same  confidence  in  the  advantages  of  his  invention 
which  he  himself  felt,  he  was  unable  to  take  any  step  toward  the  construction 
of  engines  on  a  large  scale.  Soon  after  this,  he  gave  up  his  shop  in  Glasgow, 
and  devoted  himself  to  the  business  of  a  civil  engineer.  In  this  capacity  he 
was  engaged  to  make  a  survey  of  the  river  Clyde,  and  furnished  an  elaborate 
and  valuable  report  upon  its  projected  improvements.  He  was  also  engaged 
in  making  a  plan  of  the  canal,  by  which  the  produce  of  the  Monkland  Colliery 
was  intended  to  be  carried  to  Glasgow,  and  in  superintending  the  execution 
of  that  work.  Besides  these,  several  other  engineering  enterprises  occupied 
his  attention,  among  which  may  be  mentioned,  the  navigable  canal  across  the 
isthmus  of  Crinan,  afterward  completed  by  Rennie  ;  improvements  proposed 
in  the  ports  of  Ayr,  Glasgow,  and  Greenock  ;  the  construction  of  the  bridges 
at  Hamilton,  and  at  Rutherglen  ;  and  the  survey  of  the  country  through  which 
the  celebrated  Caledonian  canal  was  intended  to  be  carried. 

"  If,  forgetful  of  my  duties  as  the  organ  of  this  academy,"  says  M.  Arago 
(whose  eloquent  observations  on  the  delays  of  this  great  invention,  addressed 
to  the  assembled  members  of  the  National  Institute  of  France,  we  cannot  for- 
bear to  quote),  "  I  could  think  of  making  you  smile,  rather  than  expressing 
useful  truths,  I  would  find  here  matter  for  a  ludicrous  contrast.  I  would  call 
to  your  recollection  the  authors,  who  at  our  weekly  sittings  demand  with  all 
their  might  and  main  (a  cor  et  a  cris)  an  opportunity  to  communicate  some 
little  remark — ^some  small  reflection — some  trifling  note,  conceived  and  written 
the  night  before  ;  I  would  represent  them  to  you  cursing  their  fate,  when  ac- 
cording to  your  rules,  the  reading  of  their  communication  is  postponed  to  the 
next  meeting,  although  during  this  cruel  week,  they  are  assured  that  their  im- 
portant communication  is  deposited  in  our  archives  in  a  sealed  packet.  On 
the  other  hand,  I  would  point  out  to  you  the  creator  of  a  machine,  destined  to 
form  an  epoch  in  the  annals  of  the  world,  undergoing  patiently  and  without 
murmur,  the  stupid  contempt  of  capitalists — conscious  of  his  exalted  genius, 
yet  stooping  for  eight  years  to  the  common  labor  of  laying  down  plans,  taking 
levels,  and  all  the  tedious  calculations  connected  with  the  routine  of  common 
engineering.  While  in  this  conduct  you  cannot  fail  to  recognise  the  serenity, 
the  mederation,  and  the  true  modesty  of  his  character,  yet  such  indifference, 
however  nobly  may  have  been  its  causes,  has  something  in  it  not  altogether 
blameless.  It  is  not  without  reason  that  society  visits  with  severe  reprobation 
those  who  withdraw  gold  from  circulation  and  hoard  it  in  their  coffers.  Is  he 
less  culpable  who  deprives  his  country,  his  fellow-citizens,  his  age,  of  treasures 
a  thousand  times  more  precious  than  the  produce  of  the  mine  ;  who  keeps  to 
himself  his  immortal  inventions,  sources  of  the  most  noble  and  purest  enjoyment 
of  the  mind,  who  abstains  from  conferring  upon  labor  those  powers,  by  which 
would  be  multiplied  in  an  infinite  proportion  the  products  of  industry,  and  by 
whi'ch,  with  advantage  to  civilization  and  human  nature,  he  would  smooth  away 
the  inequalities  of  the  conditions  of  man."* 

Although  Watt  was  thus  attracted  by  pursuits  foreign  to  his  recent  investiga- 
tions respecting  the  improvement  of  steam  power,  he  never  lost  sight  of  that 
object.  It  was  not  until  the  year  1768,  three  years  after  his  great  discoveries, 
that  any  step  was  taken  to  enable  him  to  carry  them  into  effect  on  a  large  scale. 
At  that  time  his  friends  brought  him  into  communication  with  Dr.  Roebuck,  the 
proprietor  of  the  Carron  Iron  Works,  who  rented  extensive  coal  works  at  Kin- 
neal  from  the  duchess  of  Hamilton.     Watt  was  first  employed  by  Roebuck  as 

*  Eloge,  p.  308. 


428 


THE  STEAM-ENGINE. 


a  civil  engineer  ;  but  when  he  made  known  to  him  the  improvements  he  had 
projected  in  the  steam-engine,  Roebuck  proposed  to  take  out  a  patent  for  an 
engine  on  the  principle  of  the  model  which  had  been  fitted  up  at  Delft  house, 
and  to  join  Watt  in  a  partnership,  for  the  construction  of  such  engines.  Sensi- 
ble of  the  advantages  to  be  derived  from  the  influence  of  Roebuck,  and  from 
his  command  of  capita].  Watt  agreed  to  cede  to  him  two  thirds  of  the  advantages 
to  be  derived  from  the  invention.  A  patent  was  accordingly  taken  out  on  the 
fifth  of  January,  1769,  nearly  four  years  after  the  invention  had  been  completed; 
and  an  experimental  engine  on  a  large  scale  was  constructed  by  him,  and  fitted 
up  at  Kinneal  house.  In  the  first  trial  this  machine  more  than  fulfilled  Watt's 
anticipations.  Its  success  was  complete.  In  the  practical  details  of  its  con- 
struction, however,  some  difficulties  were  still  encountered,  the  greatest  of 
which  consisted  in  packing  the  piston,  so  as  to  be  steam-tight.  The  principle 
of  the  new  engine  did  not  admit  of  water  being  kept  upon  the  piston,  to  prevent 
leakage,  as  in  the  old  engines  ;  he  was  therefore  obliged  to  have  his  cylinders 
much  more  accurately  bored,  and  more  truly  cylindrical,  and  to  try  a  great 
variety  of  soft  substances  for  packing  the  piston,  which  would  make  it  steam- 
tight  without  great  friction,  and  maintain  it  so  in  a  situation  perfectly  dry,  and 
at  the  temperature  of  boiling  water. 

While  Watt  was  endeavoring  to  overcome  these  and  other  difficulties,  in 
the  construction  of  the  machine,  his  partner,  Dr.  Roebuck,  became  embar- 
rassed, by  the  failure  of  his  undertaking  in  the  Borrowstowness  coal  and  salt 
works  ;  and  he  was  unable  to  supply  the  means  of  prosecuting  with  the  neces- 
sary vigor  the  projected  manufacture  of  the  new  engines. 

The  important  results  of  Wall's  labors  having  happily  at  this  time  become 
more  publicly  known,  Mr.  Matthew  Boulton,  whose  establishment  at  Soho, 
near  Birmingham,  was  at  that  time  the  most  complete  manufactory  for  metal- 
work  in  England,  and  conducted  with  unexampled  enterprise  and  spirit,  pro- 
posed to  purchase  Dr.  Roebuck's  interest  in  the  patent.  This  arrangement 
was  eff'ected  in  the  year  1773,  and  in  the  following  year  Mr.  Watt  removed  to 
Soho,  where  a  portion  of  the  establishment  was  allotted  to  him,  for  the  erec- 
tion of  a  foundry,  and  other  works  necessary  to  realize  his  inventions  on  a 
grand  scale. 

The  patent  which  had  been  granted  in  1769  was  limited  to  a  period  of 
fourteen  years,  and  would  consequently  expire  about  the  year  1783.  From 
the  small  progress  which  had  hitherto  been  made  in  the  construction  of  engines 
upon  the  new  principle,  and  from  the  many  difficulties  still  to  be  encountered, 
and  the  large  expenditure  of  capital  which  must  obviously  be  incurred  before 
any  return  could  be  obtained,  it  was  apparent  that,  unless  an  extension  of  the 
patent-right  could  be  obtained,  Boulton  and  Watt  could  never  expect  any  ad- 
vantage adequate  to  the  risk  of  their  great  enterprise.  In  the  year  1774  an 
application  was  accordingly  made  to  parliament  for  an  extension  of  the  patent, 
which  was  supported  by  the  testimony  of  Dr.  Roebuck,  and  Mr.  Boulton,  and 
others,  as  to  the  merits  and  probable  utility  of  the  invention.  An  act  was  ac- 
cordingly passed,  in  1775,  extending  the  term  of  the  patent  until  the  year  1800. 

Thus  protected  and  supported,  Watt  now  directed  the  whole  vigor  of  his 
mind  to  perfect  the  practical  details  of  his  invention ;  and  the  result  was  the 
construction,  on  a  large  scale,  of  the  engine  which  has  since  been  called  his 
Single  acting  Stkam-Engine. 

It  is  necessary  to  recollect  that,  notwithstanding  the  extensive  and  various 

application  of  steam  power  in  the  arts  and  manufactures  at  the  time  to  which 

our  narrative  has  now  reached,  the  steam-engine  had  never  been  employed  for 

any  other  purpose,  save  that  of  raising  water  by  working  pumps.     The  motion, 

I  therefore,  which  was  required  was  merely  an  upward  force,  such  as  was  ne- 


THE   STEAM-ENGINE. 


429 


cessary  to  elevate  the  piston  of  a  pump,  loaded  with  the  column  of  water  which 
it  raised.  The  following,  then,  is  a  description  of  the  improved  engine  of  Watt, 
by  which  such  work  was  proposed  to  be  performed  : — 

Fig.  8. 


^XWiiV^f^^  ^  --W  %\      \     \\\      \\\  %     \%XJ»,'\-X,S:.\-;.«SXXKiiv 


In  the  cylinder  represented  at  C  (fig.  8),  the  piston  P  moves  steam-tight. 
It  is  closed  at  the  top,  and  the  piston-rod,  being  accurately  turned,  runs  in  a 
steam-tight  collar  B,  furnished  with  a  stuffing-box,  and  is  constantly  lubricated 
with  melted  tallow.  A  funnel  is  screwed  into  the  top  of  the  cylinder,  throuo'h 
which,  by  opening  a  stop-cock,  melted  tallow  is  permitted  from  time  to  time  to 
fall  upon  the  piston  within  the  cylinder,  so  as  to  lubricate  it,  and  keep  it  steam- 
tight.     Two   boxes,  A  A,  called  the   upper  and  lower  steam-boxes,   confain 


THE   STEAM-ENGINE. 


valves  by  which  steam  from  the  boiler  may  be  admitted  and  withdrawn.  These  ] 
steam-boxes  are  connected  by  a  tube  of  communication  T,  and  they  communi-  ' 
cate  with  the  cylinder  at  the  top  and  bottom  by  short  tubes  represented  in  the  ', 
figure.  The  upper  steam-box  A  contains  one  valve,  by  which  a  communication 
with  the  boiler  may  be  opened  or  closed  at  pleasure.  The  lower  valve-box 
contains  two  valves.  The  lower  valve  I  communicates  with  the  tube  T',  lead- 
ing to  the  condenser  D,  which  being  opened  or  closed,  a  communication  is  ' 
made  or  cut  off  at  pleasure,  between  the  cylinder  C  and  the  condenser  D.  A 
second  valve,  or  upper  valve  H,  which  is  represented  closed  in  the  figure,  may 
be  opened  so  as  to  make  a  free  communication  between  the  cylinder  C  and  the 
tube  T,  and  by  that  means  between  the  cylinder  C,  below  the  piston,  and  the 
space  above  the  piston.  The  condenser  D  is  submerged  in  a  cistern  of  cold 
water.  At  the  side  there  enters  it  a  tube,  E,  governed  by  a  cock,  which,  being 
opened  or  closed  to  any  required  extent,  a  jet  of  cold  water  may  be  allowed  to 
play  in  the  condenser,  and  may  be  regulated  or  stopped  at  pleasure.  This  jet, 
when  playing,  throws  the  water  upward  in  the  condenser  toward  the  mouth  of 
the  tube  T',  as  water  issues  from  the  nose  of  a  watering-pot.  The  tube  S  pro- 
ceeds from  the  boiler,  and  terminates  in  the  steam-box  A,  so  that  the  steam 
supplied  from  the  boiler  constantly  fills  that  box.  The  valve  G  is  governed  by 
levers,  whose  pivots  are  attached  to  the  framing  of  the  engine,  and  is  opened 
or  closed  at  pleasure,  by  raising  or  lowering  the  lever  G'.  The  valve  G,  when 
open,  will  therefore  allow  steam  to  pass  from  the  boiler  through  the  short  tube 
to  the  top  of  the  piston,  and  this  steam  will  also  fill  the  tube  T.  If  the  lower 
valve  H  be  closed,  its  circulation  beyond  that  point  will  be  stopped ;  but  if  the 
valve  H  be  open,  the  valve  1  being  closed,  then  the  steam  will  circulate  equally 
in  the  cylinder,  above  and  below  the  piston.  If  the  valve  I  be  open,  then  steam 
will  rush  through  the  tube  T'  into  the  condenser ;  but  this  escape  of  the  steam 
will  be  stopped,  if  the  valve  I  be  closed.  The  valve  H  is  worked  by  the  lever 
H',  and  the  valve  I  by  the  lever  V. 

The  valve  G  is  called  the  upper  steam-valve,  H  the  lower  steam-valve,  I  the 
exhausting  valve,  and  E  the  condensing  valve. 

From  the  bottom  of  the  condenser  D  proceeds  a  tube  leading  to  the  air-pump, 
which  is  also  submerged  in  the  cistern  of  cold  water.  In  this  tube  is  a  valve 
M,  which  opens  outward  from  the  condenser  toward  the  air-pump.  In  the  pis- 
ton of  the  air-pump  N,  is  a  valve  which  opens  upward.  The  piston-rod  Q  of 
the  air-pump  is  attached  to  a  beam  of  wood  called  a  plug-frame,  which  is  con- 
nected with  the  working-beam  by  a  flexible  chain  playing  on  the  small  arch- 
head  immediately  over  the  air-pump.  From  the  top  of  the  air-pump  barrel 
above  the  piston  proceeds  a  pipe  or  passage  leading  to  a  small  cistern  B  called 
the  hot-well.  The  pipe  which  leads  to  this  well  is  supplied  with  a  valve,  K, 
which  opens  outward  from  the  air-pump  barrel  toward  the  well.  From  the 
nature  of  its  construction,  the  valve  M  admits  the  flow  of  water  from  the  con- 
denser toward  the  air-pump,  but  prevents  its  return  ;  and,  in  like  manner,  the 
valve  K  admits  the  flow  of  water  from  the  upper  part  of  the  air-pump  barrel 
into  the  hot-well  B,  but  obstructs  its  return. 

Let  us  now  consider  how  these  valves  should  be  worked  in  order  to  move 
the  piston  upward  and  downward  with  the  necessary  force.  It  is,  in  the  first 
place,  necessary  that  all  the  air  which  fills  the  cylinder,  the  tubes,  and  the  con- 
denser, shall  be  expelled.  To  accomplish  this,  it  is  only  necessary  to  open  at 
once  the  three  valves,  G,  H,  and  I.  The  steam  then  rushing  from  the  boiler 
through  the  steam-pipe  S,  and  the  open  valve  G  will  pass  into  the  cylinder 
above  the  piston,  will  fill  the  tube  T,  pass  through  the  lower  steam-valve  H, 
will  fill  the  cylinder  C  below  the  piston,  and  will  pass  through  the  open  valve 
I  iifto  the  condenser.     If  the  valve  E  be  closed  so  that  no  jet  shall  play  in  the 


THE   STEAM-ENGINE. 


condenser,  the  steam  rushing  into  it  will  be  partially  condensed  by  the  cold 
surfaces  to  which  it  will  be  exposed  ;  but  if  the  boiler  supply  it  through  the 
pipe  S  in  sufficient  abundance,  it  will  rush  with  violence  through  the  cylinder 
and  all  the  passages,  and  its  pressure  in  the  condenser  D,  combined  with  that 
of  the  heated  air  with  which  it  is  mixed,  will  open  the  valve  M,  and  it  will 
rush  through,  mixed  with  the  air,  into  the  air-pump  barrel  N.  It  will  press  the 
valves  in  the  air-pump  piston  upward,  and,  opening  them,  will  rush  through, 
and  will  collect  in  the  air-pump  barrel  above  the  piston.  It  will  then,  by  its 
pressure,  open  the  valve  K,  and  will  escape  into  the  cistern  B. 

Throughout  this  process,  the  steam  which  mixed  with  the  air  fills  the  cylin- 
der, condenser,  and  air-pumps,  will  be  only  partially  condensed  in  the  last  two, 
and  it  will  escape,  mixed  with  the  air,  through  the  valve  K ;  and  this  process 
will  continue  until  all  the  atmospheric  air  which  at  first  filled  the  cylinder, 
tubes,  condenser,  and  air-pump  barrel,  shall  be  expelled  through  the  valve  K, 
and  these  various  spaces  shall  be  filled  with  pure  steam.  When  that  has  hap- 
pened, let  us  suppose  all  the  valves  closed.  In  closing  the  valve  I,  the  flow 
of  steam  to  the  condenser  will  be  stopped,  and  the  steam  contained  in  it  will 
speedily  be  condensed  by  the  cold  surface  of  the  condenser,  so  that  a  vacuum 
will  be  produced  in  the  condenser,  the  condensed  steam  falling  in  the  form  of 
water  to  the  bottom.  In  like  manner,  and  for  like  reasons,  a  vacuum  will  be 
produced  in  the  air-pump.  The  valve  M,  and  the  valves  in  the  air-pump  pis- 
ton, will  be  closed  by  their  own  weight. 

By  this  process,  which  is  called  blowing  through,  the  atmospheric  air,  and 
other  permanent  gases,  which  filled  the  cylinder,  tubes,  condenser,  and  air- 
pump,  are  expelled,  and  these  spaces  will  be  a  vacuum.  The  engine  is  then 
prepared  to  be  started,  which  is  effected  in  the  following  manner  :  The  upper 
steam-valve  G  is  opened,  and  steam  allowed  to  flow  from  the  boiler  through 
the  passage  leading  to  the  top  of  the  cylinder.  This  steam  cannot  pass  to  the 
bottom  of  the  cylinder,  since  the  lower  steam-valve  H  is  closed.  The  space 
in  the  cylinder  below  the  piston  being  therefore  a  vacuum,  and  the  steam  press- 
ing above  it,  the  piston  will  be  pressed  downward  with  a  corresponding  force. 
When  it  has  arrived  at  the  bottom  of  the  cylinder,  the  steam-valve  G  must  be 
closed,  and  at  the  same  time  the  valve  H  opened.  The  valve  I  leading  to  the 
condenser  being  also  closed,  the  steam  which  fills  the  cylinder  above  the  pis- 
ton is  now  admitted  to  circulate  through  the  open  valve  H  below  the  piston,  so 
that  the  piston  is  pressed  equally  upward  and  downward  by  steam,  and  there 
is  no  force  to  resist  its  movement,  save  its  friction  with  the  cylinder.  The 
weight  of  the  pump-rods  on  the  opposite  end  of  the  beam  being  more  than 
equivalent  to  overcome  this,  the  piston  is  drawn  to  the  top  of  the  cylinder,  and 
pushes  before  it  the  steam  which  is  drawn  through  the  tube  T,  and  the  open  \ 
valve  H,  and  passes  into  the  cylinder  C  below  the  piston.  < 

When  the  piston  has  thus  arrived  once  more  at  the  top  of  the  cylinder,  let  ! 
the  valve  H  be  closed,  and  at  the  same  time  the  valves  G  and  I  opened,  and  ' 
the  condensing-cock  E  also  opened,  so  as  to  admit  the  jet  to  play  in  the  con-  < 
denser.  The  steam  which  fills  the  cylinder  C  below  the  piston,  will  now  rush  * 
through  the  open  valve  I  into  the  condenser  which  has  been  hitherto  a  vacuum  < 
and  there  encountering  the  jet,  will  be  instantly  converted  into  water,  and  a 
mixture  of  condensed  steam  and  injected  water  will  collect  in  the  bottom  of  the  < 
condenser.  At  the  same  time,  the  steam  proceeding  from  the  boiler  by  the  j 
steam-pipe  S  to  the  upper  steam-box  A,  will  pass  through  the  open  steam-valve  \ 
G  to  the  top  of  the  piston,  but  cannot  pass  below  it  because  of  the  lower  ) 
steam-valve  H  being  closed.  The  piston,  thus  acted  upon  above  by  the  pres-  \ 
sure  of  the  steam,  and  the  space  in  the  cylinder  below  it  being  a  vacuum,  its  ) 
downward  motion  is  resisted  by  no  force  but  the  friction,  and  it  is  therefore  \ 


432 


THE    STEAM-ENGINE. 


driven  to  the  bottom  of  the  cylinder.  During  its  descent,  the  valves  G,  I,  and 
E,  remain  open.  At  the  moment  it  arrives  at  the  bottom  of  the  cylinder,  all 
these  three  valves  are  closed,  and  the  valve  H  opened.  The  steam  w^hich 
fills  the  cylinder  above  the  piston  is  now  permitted  to  circulate  below  it,  by  the 
open  yalve  H  and  the  piston  being  consequently  pressed  equally  upward  and 
downward,  will  be  drawn  upward  as  before  by  the  preponderance  of  the  pump- 
rods  at  the  opposite  end  of  the  beam.  The  weight  of  these  rods  must  also  be 
sufficiently  great  to  draw  the  air-pump  piston  N  upward.  As  this  piston  rises 
in  the  air-pump,  it  leaves  a  vacuum  below  it,  into  which  the  water  and  air  col- 
lected in  the  condenser  will  be  drawn  through  the  valve  M,  which  opens  out- 
ward. When  the  air-pump  piston  has  arrived  at  the  top  of  the  barrel,  which 
it  will  do  at  the  same  time  that  the  steam-piston  arrives  at  the  top  of  the  cyl- 
inder, the  water  and  the  chief  part  of  the  air  or  other  fluids  which  may  have 
been  in  the  condenser,  will  be  drawn  into  the  barrel  of  the  air-pump,  and  the 
valve  M  being  closed  by  its  own  weight,  assisted  by  the  pressure  of  these  flu- 
ids, they  cannot  return  into  the  condenser.  At  the  moment  the  steam-piston 
arrives  at  the  top  of  the  cylinder,  the  valve  H  is  closed,  and  the  three  valves 
G,  I,  and  E,  are  opened.  The  effect  of  this  change  is  the  same  as  was  al- 
ready described  in  the  former  case,  and  the  piston  will  in  the  same  manner  and 
from  the  same  causes  be  driven  downward.  The  air-pump  piston  will  at  the 
same  time  descend  by  the  force  of  its  own  weight,  aided  by  the  weight  of  the 
plug-frame  attached  to  its  rod.  As  it  descends,  the  air  below  it  will  be  gradu- 
ally compressed  above  the  surface  of  the  water  in  the  bottom  of  the  barrel,  un- 
til its  pressure  becomes  sufficiently  great  to  open  the  valves  in  the  air-pump 
piston.  When  this  happens,  the  valves  in  the  air-pump  piston,  as  represented 
on  a  large  scale  in  fig.  9,  will  be  opened,  and  the  air  will  pass  through  them 


Fig.  9. 


above  the  piston.  When  the  piston  comes  in  contact  with  the  water  in  the 
bottom  of  the  barrel,  this  water  will  likewise  pass  through  the  open  valves. 
When  the  piston  has  arrived  at  the  bottom  of  the  air-pump  barrel,  the  valves  in 
it  will  be  closed  by  the  pressure  of  the  fluids  above  them.  The  next  ascent 
of  the  steam  piston  will  draw  up  the  air-pump  piston,  and  with  it  the  fluids  in 


THE  STEAM-ENGINE. 


the  pump  barrel  above  it.  As  the  air-pump  piston  approaches  the  top  of  its 
barrel,  the  air  and  water  above  it  will  be  drawn  through  the  valve  K  into  the 
hot  cistern  B.  The  air  will  escape  in  bubbles  through  the  water  in  that  cis- 
tern, and  the  warm  water  will  be  deposited  in  it. 

The  magnitude  of  the  opening  in  the  condensing  valve  E,  must  be  regulated 
by  the  quantity  of  steam  admitted  to  the  cylinder.  As  much  water  ought  to  be 
supplied  through  the  injection  valve  as  will  be  sufficient  to  condense  the  steam 
contained  in  the  cylinder,  and  also  to  reduce  the  temperature  of  the  water  itself, 
when  mixed  with  the  steam,  to  a  sufficiently  low  degree  to  prevent  it  from 
producing  vapor  of  a  pressure  which  would  injuriously  affect  the  working  of 
the  piston.  It  has  been  shown,  that  five  and  a  half  cubic  inches  of  ice-cold 
water  mixed  with  one  cubic  inch  of  water  in  the  stale  of  steam  would  produce 
six  and  a  half  cubic  inches  of  water  at  the  boiling  temperature.  If  then  the 
cylinder  contained  one  cubic  inch  of  water  in  the  state  of  steam,  and  only  five 
and  a  half  cubic  inches  of  water  were  admitted  through  the  condensing  jet, 
supposing  this  water,  when  admitted,  to  be  at  the  temperature  of  32°,  then  the 
consequence  would  be  that  six  and  a  half  cubic  inches  of  water  at  the  boiling 
temperature  would  be  produced  in  the  condenser.  Steam  would  immediately 
arise  from  this,  and  at  the  same  time  the  temperature  of  the  remaining  water 
would  be  lowered  by  the  amount  of  the  latent  heat  taken  up  by  the  steam  so 
produced.  This  vapor  would  rise  through  the  open  exhausting  valve  I,  would 
fill  the  cylinder  below  the  piston,  and  would  impair  the  efficiency  of  the  steam 
above  pressing  it  down.  The  result  of  the  inquiries  of  Watt  respecting  the 
pressure  of  steam  at  different  temperatures,  showed,  that  to  give  efficiency  to 
the  steam  acting  upon  the  piston  it  would  always  be  necessary  to  reduce  the 
temperature  of  the  water  in  the  condenser  to  100°. 

Let  us  then  see  what  quantity  of  water  at  the  common  temperature  would  be 
necessary  to  produce  these  effects. 

If  the  latent  heat  of  steam  be  taken  at  1,000°,  a  cubic  inch  of  water  in  the  state 
of  steam  may  be  considered  for  the  purposes  of  this  computation,  as  equivalent 
to  one  cubic  inch  of  water  at  1,212°.  Now  the  question  is,  how  many  cubic 
inches  of  water  at  60°  must  be  mixed  with  this,  in  order  that  the  mixture  may 
have  the  temperature  of  100°  ?  This  will  be  easily  computed.  As  the  cubic 
inch  of  water  at  1,212°  is  to  be  reduced  to  100°,  it  must  be  deprived  of  1,112° 
of  its  temperature.  On  the  other  hand,  as  many  inches  of  water  at  60°  as  are 
to  be  added,  must  be  raised  in  the  same  mixture  to  the  temperature  of  100°, 
and  therefore  each  of  these  must  receive  40°  of  temperature.  The  number  of 
cubic  inches  of  water  necessary  to  be  added  will  therefore  be  determined  by 
finding  how  often  40°  are  contained  in  1,112°.  If  1,112  be  divided  by  40, 
the  quotient  will  be  27  8.  Hence  it  appears,  that  to  reduce  the  water  in  the 
condenser  to  the  ternperature  of  100°,  supposing  the  temperature  of  the  water 
injected  to  be  60°,  it  will  be  necessary  to  supply  by  the  injection  cock  very 
nearly  twenty-eight  times  as  much  water  as  passes  through  the  cylinder  in  the 
state  of  steam  ;  and  therefore  if  it  be  supposed  that  all  the  water  evaporated 
in  the  boiler  passes  through  the  cylinder,  it  follows  that  about  twenty-eight 
times  as  much  water  must  be  thrown  into  the  condenser  as  is  evaporated  in 
the  boiler. 

From  these  circumstances  it  will  be  evident  that  the  cold  cistern  in  which 
the  condenser  and  air-pump  are  submerged,  must  be  supplied  with  a  consider- 
able quantity  of  water.  Independently  of  the  quantity  drawn  from  it  by  the 
injection  valve,  as  just  explained,  the  water  in  the  cistern  itself  must  be  kept 
down  to  a  temperature  of  about  60°.  The  interior  of  the  condenser  and  air- 
pump  being  maintained  by  the  steam  condensed  in  them  at  a  temperature  not 
-less  than  100°;  the  outer  surfaces  of  these  vessels  consequently  impart  heat 

VOIi.  II,  —  38 


THE  STEAM-ENGINE. 


to  the  water  in  the  cold  cistern,  and  have  therefore  a  tendency  to  raise  the 
temperature  of  that  water.  To  prevent  this,  a  pump  called  the  cold  pump, 
represented  at  L  in  fig.  8,  is  provided.  By  this  pump  water  is  raised  from  any 
convenient  reservoir,  and  driven  through  proper  tubes  into  the  cold  cistern. 
This  cold  pump  is  wrought  by  the  engine,  the  rod  being  attached  to  the  beam. 
Water  being,  bulk  for  bulk,  heavier  the  lower  its  temperature,  it  follows  that 
the  water  supplied  by  the  cold  pump  to  the  cistern  will  have  a  tendency  to 
sink  to  the  bottom,  pressing  upward  the  warmer  water  contained  in  it.  A 
waste-pipe  is  provided,  by  which  this  water  is  drained  off,  and  the  cistern 
therefore  maintained  at  the  necessary  temperature. 

From  what  has  been  stated,  it  is  also  evident  that  the  hot  well  B,  into  which 
the  warm  water  is  thrown  by  the  air-pump,  will  receive  considerably  more 
water  than  is  necessary  to  feed  the  boiler.  A  waste-pipe,  to  carry  off  this,  is 
also  provided  ;  and  the  quantity  necessary  to  feed  the  boiler  is  pumped  up  by 
a  small  pump,  O,  the  rod  of  which  is  attached  to  the  beam,  as  represented  in 
fig.  8,  and  which  is  worked  by  the  engine.  The  water  raised  by  this  pump  is 
conducted  to  a  reservoir  from  which  the  boiler  is  fed,  by  means  which  will  be 
hereafter  explained. 

We  shall  now  explain  the  manner  in  which  the  machine  is  made  to  open 
and  close  the  valves  at  the  proper  times.  By  referring  to  the  explanation 
already  given,  it  will  be  perceived  that  at  the  moment  the  piston  reaches  the 
top  of  the  cylinder,  the  upper  steam-valve  G  must  be  open,  to  admit  the  steam 
to  press  it  down  ;  while  the  exhausting  valve  I  must  be  opened,  to  allow  the 
steam  to  pass  to  the  condenser;  and  the  condensing  valve  E  must  be  opened, 
to  let  in  the  water  necessary  for  the  condensation  of  the  steam  ;  and  at  the 
same  time  the  lower  steam-valve  H  must  be  closed,  to  prevent  the  passage  of 
the  steam  which  has  been  admitted  through  G.  The  valves  G,  I,  and  E,must 
be  kept  open,  and  the  valve  H  kept  closed,  until  the  piston  arrives  at  the  bottom 
of  the  cylinder,  when  it  will  be  necessary  to  close  all  the  three  valves,  G,  I, 
and  E,  and  to  open  the  valve  H,  and  the  same  effects  must  be  produced  each 
time  the  piston  arrives  at  the  top  and  bottom  of  the  cylinder.  All  this  is  ac- 
complished by  a  system  of  levers,  which  are  exhibited  in  fig.  8.  The  pivots 
on  which  these  levers  play  are  represented  on  the  framing  of  the  engine,  and 
the  arms  of  the  levers  G',  H',  and  I',  communicating  with  the  corresponding 
valves  G,  H,  and  I,  are  represented  opposite  a  bar  attached  to  the  rod  of  the 
air-pump,  called  the  plug-frame.  This  bar  carries  certain  pegs  and  detents, 
which  act  upon  the  arms  of  the  several  levers  in  such  a  manner  that,  on  the 
arrival  of  the  beam  at  the  extremities  of  its  play  upward  and  downward,  the 
levers  are  so  struck  that  the  valves  are  opened  and  closed  at  the  proper  times. 
It  is  needless  to  explain  all  the  details  of  this  arrangement.  Let  it  be  sufficient, 
as  an  example  of  all,  to  explain  the  method  of  working  the  upper  steam-valve 
G.  When  the  piston  reaches  the  top  of  the  cylinder,  a  pin  strikes  the  arm  of 
the  lever  G',  and  throws  it  upward  :  this,  by  means  of  the  system  of  levers, 
pulls  the  arm  of  the  valve  G  downward,  by  which  the  upper  steam-valve  is 
raised  out  of  its  seat,  and  a  passage  is  opened  from  the  steam-pipe  to  the  cyl- 
inder. The  valve  is  maintained  in  this  state  until  the  piston  reaches  the  bottom 
of  the  cylinder,  when  the  arm  G'  is  pressed  downward,  by  which  the  arm  G 
is  pressed  upward,  and  the  valve  restored  to  its  seat.  By  similar  methods  the 
levers  governing  the  other  three  valves,  H,  I,  and  E,  are  worked. 

The  valves  used  in  these  engines  were  of  the  kind  called  spindle-valves. 
They  consisted  of  a  flat  circular  plate  of  bell  metal,  A  D,  fig.  10,  with  a  round 
spindle  passing  perpendicularly  through  its  centre,  and  projecting  above  and 
below  it.  This  valve,  having  a  conical  form,  was  fitted  very  exactly,  by 
grinding  into  a  corresponding  circular  conical  seat,  A  B  C  D,  fig.  11,  which 


THE  STEAM-ENGINE. 


435 


Fig.  10. 


forms  the  passage  which  it  is  the  office  of  the  valve  to  open  and  close.  When 
the  valve  falls  into  its  seat,  it  tits  the  aperture  like  a  plug,  so  as  entirely  to 
stop  it.  The  spindle  plays  in  sockets  or  holes,  one  above  and  the  other  below 
the  aperture  which  the  valve  stops;  these  holes  keep  the  valve  in  its  proper 
position,  so  as  to  cause  it  to  drop  exactly  into  its  place. 


In  the  experimental  engine  made  by  Mr.  Watt  at  Kinneal,  he  used  cocks, 
and  sometimes  sliding  covers,  like  the  regulator  described  in  the  old  engines  ; 
but  these  he  found  very  soon  to  become  leaky.  He  was,  therefore,  obliged  to 
change  them  for  the  spindle-valves  just  described,  which,  being  truly  ground, 
and  accurately  fitted  in  the  first  instance,  were  not  so  liable  to  go  out  of  order. 
These  valves  are  also  called  puppet-clacks,  or  button-valves. 

In  the  earlier  engines  constructed  by  Watt,  the  condensation  was  produced 
by  the  contact  of  cold  surfaces,  without  injection.  The  reason  of  rejecting 
the  method  of  condensing  by  injection  was,  doubtless,  to  avoid  the  injurious 
effects  of  the  air,  which  would  always  enter  the  condenser,  in  combination 
with  the  water  of  condensation,  and  vitiaie  the  vacuum.  It  was  soon  found, 
however,  that  a  condenser  acting  by  cold  surfaces  without  injection,  being 
necessarily  composed  of 'narrow  pipes  or  passages,  was  liable  to  incrustation 
from  bad  water,  by  which  the  conducting  power  of  the  material  of  the  condenser 
was  diminished  ;  so  that,  while  its  outer  surface  was  kept  cold  by  the  water 
of  the  cold  cistern,  the  inner  surface  might,  nevertheless,  be  so  warm  that  a 
very  imperfect  condensation  would  be  produced. 

At  the  time  that  Watt,  in  conjunction  with  Dr.  Roebuck,  obtained  the  patent 
for  his  improved  engine,  the  idea  occurred  to  him,  that  the  steam  which  had 
impelled  the  piston  in  its  descent  rushed  from  the  cylinder  with  a  mechanical 
force  much  more  than  sufficient  to  overcome  any  resistance  which  it  had  to 
encounter  in  its  passage  to  the  condenser ;  and  that  such  force  might  be  ren- 
dered available  as  a  moving  power,  in  addition  to  that  already  obtained  from 
the  steam  during  the  stroke  of  the   piston.     This  motion  involved  the  whole 


436 


THE   STEAM-ENGINE. 


principle  of  the  expansive  action  of  steam,  which  subsequently  proved  to  be  ) 
of  such  importance  in  the  performance  of  steam-engines.  Watt  was,  howev-  j 
er,  so  much  engrossed  at  that  time,  and  subsequently,  by  the  difficulties  he  had  ) 
to  encounter  in  the  construction  of  his  engines,  that  he  did  not  attempt  to  bring  < 
this  principle  into  operation.  It  was  not  until  after  he  had  organized  that  part  \ 
of  the  establishment  at  Soho  which  was  appropriated  to  the  manufacture  of  < 
steam-engines,  that  he  proceeded  to  apply  the  expansive  principle.  Since  { 
the  date  of  the  patent  which  he  took  out  for  this  (1782)  was  subsequent  to  < 
the  application  of  the  same  principle  by  another  engineer,  named  Hornblower,  < 
it  is  right  to  state  that  the  claim  of  Mr.  Watt  to  this  important  step  in  the  im-  < 
provement  of  the  steam-engine,  is  established  by  a  letter  addressed  by  him  to  ( 

.Dr.  Small,  of  Birmingham,  dated  Glasgow,  May,  1769  : — •  < 

"  I  mentioned  to  you  a  method  of  still  doubling  the  effect  of  the  steam,  and  ( 
that  tolerably  easy,  by  using  the  power  of  steam  rushing   into  a  vacuum,  at  ' 
present  lost.     This  would  do  little  more  than  double  the  effect,  but  it  would  , 
too  much  enlarge  the  vessels  to  use  it  all  :  it  is  peculiarly  applicable  to  wheel-  ' 
engines,  and  may  supply  the  want  of  a  condenser,  where  the  force  of  steam  i 
only  is  used  ;   for  open  one  of  the  steam-valves,  and    admit  steam  until  one  \ 
fourth  of  the  distance  between  it  and  the  next  valve  is  filled  with  steam,  then 
shut  the  valve,  and  the  steam  will  continue  to  expand,  and  to  press  round  the 
wheel,  with  a  diminishing  power,  ending  in   one  fourth  of  its   first  exertion. 
The  sum  of  the   series  you  will   find  greater  than  one  half,  though  only  one 
fourth  of  steam  was  used.     The  power  will  indeed  be  unequal,  but  this  can  be 
remedied  by  a  fly,  or  by  several  other  means." 

In  1776,  the  engine,  which  had  been  then  recently  erected  at  Soho,  was 
adapted  to  act  upon  the  principle  of  expansion.  When  the  piston  had  been 
pressed  down  in  the  cylinder  for  a  certain  portion  of  the  stroke,  the  fur- 
ther supply  of  steam  from  the  boiler  was  cut  off,  by  closing  the  upper  steam- 
valve,  and  the  remainder  of  the  stroke  was  accomplished  by  the  expansive 
power  of  the  steam  which  had  already  been  introduced  into  the  cylinder. 

To  make  this  method  of  applying  the  force  of  steam  intelligible,  some  pre- 
vious explanation  of  mechanical  principles  will  be  necessary. 

If  a  body  which  offers  a  certain  resistance  be  urged  by  a  certain  moving 
force,  the  motion  which  it  will  receive  will  depend  on  the  relation  between 
the  energy  of  the  moving  force  and  the  amount  of  the  resistance  opposed  to  it. 
If  the  moving  force  be  precisely  equal  to  the  resistance,  the  motion  which  the 
body  will  receive  will  be  perfectly  uniform. 

If  the  energy  of  the  moving  force  be  greater  than  the  resistance,  then  its 
surplus  or  excess  above  the  amount  of  resistance  will  be  expended  in  impart- 
ing momentum  to  the  mass  of  the  body  moved,  and  the  latter  will  consequently 
continually  acquire  augmented  speed.  The  motion  of  the  body  will  therefore 
be  in  this  case  accelerated. 

If  the  energy  of  the  moving  force  be  less  in  amount  than  the  resistance, 
then  all  that  portion  of  the  resistance  which  exceeds  the  amount  of  the  moving 
force  will  be  expended  in  depriving  the  mass  of  the  body  of  momentum,  and 
the  body  will  therefore  be  moved  with  continually  diminished  speed  until  it  be 
brought  to  rest. 

Whenever,  therefore,  a  uniform  motion  is  produced  in  a  body,  it  may  be 

i  taken  as  an  indication  of  the  equality  of  the  moving  force  to  the  resistance  ; 

'  and,  on  the  other  hand,  according  as  the  speed  of  the  body  is  augmented  or 

,  diminished,  it  may  be  inferred  that  the  energy  of  the  moving  force  has  been 

'  greater  or  less  than  the  resistance. 

I       It  is  an  error  to  suppose  that  rest  is  the  only  condition  possible  for  a  body 

'  to  assume  when  under  the  operation  of  two  or  more  mechanical  forces  which 


THE   STEAM-ENGINE. 


are  in  equilibrium.  By  the  laws  of  motion  the  state  of  a  body  which  is  not 
under  the  operation  of  any  external  force  must  be  either  in  a  state  of  rest  or 
of  uniform  motion.  Whichever  be  its  state,  it  will  suffer  no  change  if  the 
body  be  brought  under  the  operation  of  two  or  more  forces  which  are  in  equi- 
librium ;  for  to  suppose  such  forces  to  produce  any  change  in  the  state  of  the 
body,  whether  from  rest  to  motion,  or  vice  versa,  or  in  the  velocity  of  the  mo- 
tion which  the  body  may  have  previously  had,  would  be  equivalent  to  a  sup- 
position that  the  forces  applied  to  the  body  being  in  equilibrium  were  capable 
of  producing  a  dynamical  effect,  which  would  be  a  contradiction  in  terms. 
This,  though  not  always  clearly  understood  by  mere  practical  men,  or  by  per- 
sons superficially  informed,  is,  in  fact,  among  the  fundamental  principles  of 
mechanical  science. 

When  the  piston  is  at  the  top  of  the  cylinder,  and  about  to  commence  its 
motion  downward,  the  steam  acting  upon  it  will  have  not  only  to  overcome 
the  resistance  arising  from  the  friction  of  the  various  parts  of  the  engine,  but 
will  also  have  to  put  in  motion  the  whole  mass  of  matter  of  the  piston  pump- 
rods,  pump-pistons,  and  the  column  of  water  in  the  pump-barrels.  Besides 
imparting  to  this  mass  the  momentum  corresponding  to  the  velocity  with  which 
it  will  be  moved,  it  will  also  have  to  encounter  the  resistance  due  to  the  pre- 
ponderance of  the  weight  of  the  water  and  pump-rods  over  that  of  the  steam- 
piston.  The  pressure  of  steam,  therefore,  upon  the  piston  at  the  commence- 
ment of  the  stroke  must,  in  accordance  with  the  mechanical  principles  just 
explained,  have  a  greater  force  than  is  equal  to  all  the  resistances  which  it 
would  have  to  overcome,  supposing  the  mass  to  be  moving  at  a  uniform  ve- 
locity. The  moving  force,  therefore,  being  greater  than  the  resistance,  the 
mass,  when  put  in  motion,  will  necessarily  move  with  a  gradually-augmented 
speed,  and  the  piston  of  the  engine  which  has  been  already  described  would 
necessarily  move  from  the  top  to  the  bottom  of  the  cylinder  with  an  accelera- 
ted motion,  having  at  the  moment  of  its  arrival  at  the  bottom  a  greater  velocitv 
than  at  any  other  part  of  the  stroke.  As  the  piston  and  all  the  matter  which 
it  has  put  in  motion  must  at  this  point  come  to  rest,  the  momentum  of  the  mo- 
ving mass  must  necessarily  expend  itself  on  some  part  of  the  machinery,  and 
would  be  so  much  mechanical  force  lost.  It  is  evident,  therefore,  indepen- 
dently of  any  consideration  of  the  expansive  principle,  to  which  we  shall  pres- 
ently refer,  that  the  action  of  the  moving  power  in  the  descent  of  the  piston 
ought  to  be  suspended  before  the  arrival  of  the  piston  at  the  bottom  of  the  cyl- 
inder, in  order  to  allow  the  momentum  of  the  mass  which  is  in  motion  to  ex- 
pend itself,  and  to  allow  the  piston  to  come  gradually  to  rest  at  the  termination 
of  the  stroke. 

Thus,  if  we  were  to  suppose  that  after  the  piston  had  descended  through 
three  fourths  of  the  whole  length  of  the  cylinder,  and  had  acquired  a  certain 
velocity,  the  steam  above  it  were  suddenly  condensed,  so  as  to  leave  a  vacu- 
um both  above  and  below  il,  the  piston,  being  then  subject  to  no  impelling 
force,  would  still  move  downward,  in  virtue  of  the  momentum  it  had  acquired, 
until  the  resistance  would  deprive  it  of  that  momentum,  and  bring  it  to  rest ;  and 
if  the  remaining  fourth  part  of  the  cylinder  were  necessary  for  the  accomplish- 
ment of  this,  then  it  is  evident  that  that  part  of  the  stroke  would  be  accom- 
plished without  further  expenditure  of  the  moving  power. 

In  fact,  this  part  of  the  stroke  would  be  made  by  the  expenditure  of  that  ex- 
cess of  moving  power,  which,  at  the  commenceinent  of  the  stroke,  had  been 
employed  in  putting  the  machinery  and  its  load  in  motion,  and  in  subsequently 
accelerating  that  motion. 

Although  under  such  circumstances  the  resistance,  during  the  operation  of 
the  moving  power,  shall  not  have  been  at  any  time  equal  to  the  moving  pow- 


438 


THE    STEAM-ENGINE. 


er,  since  while  the  motion  was  accelerated  it  was  less,  and  while  retarded 
greater,  than  that  power,  yet  as  the  whole  moving  power  has  been  expended 
upon  the  resistance,  the  mechanical  effect  which  the  moving  power  has  pro- 
duced under  such  circumstances  will  be  equal  to  the  actual  amount  of  that 
power.  If  in  an  engine  of  this  kind  the  steam  was  not  cut  off  till  the  conclu- 
sion of  the  stroke,  a  part  of  the  moving  power  would  be  lost  upon  those  fixed 
points  in  the  machinery  which  would  sustain  the  shock  produced  by  the  in- 
stantaneous cessation  of  motion  at  the  end  of  the  stroke. 

Independently,  therefore,  of  any  consideration  of  the  expansive  principle,  it 
appears  that,  in  an  engine  of  this  kind,  the  steam  ought  to  be  cut  off  before  the 
completion  of  the  stroke. 

To  render  the  expansive  action  of  steam  intelligible,  let  A  B,  fig.  12,  repre- 

Fisr.12. 


f 

1 

|iijiiniiiiiiiiiiiiiiiiiiiiiiiiii,.' 

A 

i 

1 

! 

ji  j 

fl( 

; 

ill 

t 

n 

]  B 

-ff- 

IP 

Ji- 

'  ! 

1 

ij'ifji 

i  1 1 

III 

P' 

4b- 

it 

yj 

C 

sent  a  cylinder  whose  area  we  will  suppose,  for  the  sake  of  illustration,  to  be 
a  square  foot,  and  whose  length,  A  B,  shall  also  be  a  foot.  If  steam  of  a 
pressure  equal  to  the  atmosphere  be  supplied  to  this  cylinder,  it  will  exert  a 
pressure  of  about  one  ton  on  the  piston  ;  and  if  such  steam  be  uniformly  sup- 
plied from  the  boiler,  the  piston  will  be  moved  from  A  to  B  with  the  force  of 
one  ton,  and  that  motion  will  be  uniform  if  the  piston  be  opposed  throughout 
the  same  space  by  a  resistance  equal  to  a  ton.  When  the  piston  has  arrived 
at  B,  let  us  suppose  that  the  further  supply  of  steam  from  the  boiler  is  stopped 
by  closing  the  upper  steam-valve,  and  let  us  also  suppose  the  cylinder  to  be 
continued  downward  so  that  B  C  shall  be  equal  to  A  B,  and  suppose  that  B  C 
has  been  previously  in  communication  with  the  condenser,  and  is  therefore  a 
vacuurn.  The  piston  at  B  will  then  be  urged  with  a  force  of  one  ton  down- 
ward, and  as  it  descends  the  steam  above  it  will  be  diffused  through  an  in- 
creased volume,  and  will  consequently  acquire  a  diminished  pressure.  We 
shall,  for  the  present,  assume  that  this  diminution  of  pressure  follows  the  law 
of  elastic  fluids  in  general  ;  that  it  will  be  decreased  in  the  same  proportion 
as  the  volume  of  the  steam  is  augmented.  While  the  piston,  therefore,  moves 
from  B  downward,  it  will  be  urged  by  a  continually-decreasing  force.  Let  us 
suppose,  that,  by  some  expedient,  it  is  also  subject  to  a,  continually-decreasing 
resistance,  and  that  this  resistance  decreases  in  the  same  proportion  as  the 
force  which  urges  the  piston.  In  that  case  the  motion  of  the  piston  would 
continue  uniform.  When  the  piston  would  arrive  at  P',  the  middle  of  the 
second  cylinder,  then  the  spaces  occupied  by  the  steam  being  increased  in  the 
proportion  of  2  to  3,  the   pressure  on  the   piston  would  be  diminished  in  the 


THE  STEAM-ENGINE. 


439 


proportion  of  3  to  2,  and  the  pressure  at  B  being  one  ton,  it  would  be  two 
thirds  of  a  ton  at  P'.  In  like  manner  when  the  piston  would  arrive  at  C,  the 
space  occupied  by  the  steam  being  double  that  which  it  occupied  when  the 
piston  was  at  B,  the  pressure  of  the  steam  would  be  half  its  pressure  at  B, 
and  therefore  at  the  termination  of  the  stroke,  the  pressure  on  the  piston  would 
be  half  a  ton. 

If  the  space  from  B  to  C,  through  which  the  steam  is  here  supposed  to  act 
expansively,  be  divided  into  ten  equal  parts,  the  pressure  on  the  piston  at  the 
moment  of  passing  each  of  those  divisions  would  be  calculated  upon  the  same 
principle  as  in  the  cases  now  mentioned.  After  moving  through  the  first  di- 
vision, the  volume  of  the  steam  would  be  increased  in  the  proportion  of  10  to 
11,  and  therefore  its  pressure  would  be  diminished  in  the  proportion  of  11  to 
10.  The  pressure,  therefore,  driving  the  piston  at  the  end  of  the  first  of 
these  ten  divisions  would  be  -i^ths  of  a  ton.  In  like  manner,  its  pressure  at 
the  second  of  the  divisions  would  be  yfths  of  a  ton,  and  the  third  jf ths  of  a 
ton  ;  and  so  on,  as  indicated  in  the  figure. 

Now  if  the  pressure  of  the  steam  through  each  of  these  divisions  were  to 
continue  uniform,  and,  instead  of  gradually  diminishing,  to  suffer  a  sudden 
change  in  passing  from  one  division  to  another,  then  the  mechanical  efTect 
produced  from  B  to  C  would  be  obtained  by  taking  a  mean  or  average  of  the 
several  pressures  throughout  each  of  the  ten  divisions.  In  the  present  case  . 
it  has  been  supposed  that  the  force  on  the  piston  at  B  was  2,240  pounds.  To  j 
obtain  the  pressure  in  pounds  corresponding  to  each  of  the  successive  divis- 
ions, it  will  therefore  only  be  necessary  to  multiply  2,240  by  10,  and  to  divide 
it  successively  by  11,  12,  13,  &c.  The  pressures,  therefore,  in  pounds,  at 
each  of  the  ten  divisions,  will  be  as  follows  : — 

1st 2,036-3 

2d ]  .866-6 

3d 1,723-1 

4th 1,600-0 

.5th 1,493-3 

6th 1,400-0 

7th 1,317-6 

8th 1,244-4 

9th 1,179-0 

10th 1,120-0 


If  the  mean  of  these  be  taken  by  adding  them  together  and  dividing  by  10, 
it  will  be  found  to  be  1,498  pounds.  It  appears,  therefore,  that  the  pressures 
through  each  of  the  ten  divisions  being  supposed  to  be  uniform  (which,  how- 
ever, strictly,  they  are  not),  the  mechanical  efTect  of  the  steam  from  B  to  C 
would  be  the  same  as  if  it  acted  uniformly  throughout  that  space  upon  the  pis- 
ton with  a  force  of  about  1,500  pounds,  being  rather  less  than  three  fourths 
of  its  whole  effect  from  A  to  B. 

But  it  is  evident  that  this  principle  will  be  equally  applicable  if  the  second 
cylinder  had  any  other  proportion  to  the  first.  Thus  it  might  be  twice  the 
length  of  the  first ;  and  in  that  case,  a  further  mechanical  effect  would  be  ob- 
tained from  the  expansion  of  the  steam. 

The  more  accurate  method  of  calculating  the  effect  of  the  expansion  from  B 
to  C,  would  involve  more  advanced  mathematical  principles  than  could  proper- 
ly be  introduced  here  ;  but  the  result  of  such  a  computation  would  be  that  the 
actual  average  effect  of  the  steam  from  B  to  C  would  be  equal  to  a  uniform 
pressure  through  that  space,  amounting  to  one  thousand  five  hundred  and  forty- 
five  pounds,  being  greater  than  the  result  of  the  above  computation,  the  differ- 


440 


THE  STEAM-ENGINE. 


ence  being  due  to  the  expansive  action  through  each  of  the  ten  divisions,  which 
was  omitted  in  the  above  computation. 

It  is  evident  that  the  expansive  principle,  as  here  explained,  involves  the 
condition  of  a  variation  in  the  intensity  of  the  moving  power.  Thus,  if  the 
steam  act  with  a  uniform  energy  on  the  piston  so  long  as  its  supply  from  the 
boiler  continues,  the  moment  that  supply  is  stopped,  by  closing  the  steam-valve, 
the  steam  contained  in  the  cylinder  will  fill  a  gradually-increasing  volume  by 
the  motion  of  the  piston,  and  therefore  will  act  above  the  piston  with  a  gradually- 
decreasing  energy.  If  the  resistance  to  the  moving  power  produced  by  the 
load,  friction,  &c.,  be  not  subject  to  a  variation  corresponding  precisely  to  such 
variation  in  the  moving  power,  then  the  consequence  must  be  that  the  motion 
imparted  to  the  load  will  cease  to  be  uniform.  If  the  energy  of  the  moving 
power  at  any  part  of  the  stroke  be  greater  than  the  resistance,  the  motion  pro- 
duced will  be  accelerated  ;  if  it  be  less,  the  motion  will  be  retarded ;  and  if 
it  be  at  one  time  greater,  and  another  time  less,  as  will  probably  happen,  then 
the  motion  will  be  alternately  accelerated  and  retarded.  This  variation  in  the 
speed  of  the  body  moved  will  not,  however,  affect  the  mechanical  effect  pro- 
duced by  the  power,  provided  that  the  momentum  imparted  to  the  moving  mass 
be  allowed  to  expend  itself  at  the  end  of  the  stroke,  so  that  the  piston  may  be 
brought  to  rest  as  nearly  as  possible  by  the  resistance  of  the  load,  and  not  by 
any  shock  on  any  fixed  points  in  the  machine.  This  is  an  object  which,  con- 
sequently, should  be  aimed  at  with  a  view  to  the  economy  of  power,  independ- 
ently of  other  considerations  connected  with  the  wear  and  tear  of  the  machine- 
ry. So  long  as  the  engine  is  only  applied  to  the  operation  of  pumping  water, 
great  regularity  of  motion  is  not  essential,  and,  therefore,  the  variation  of  speed 
which  appears  to  be  an  almost  inevitable  consequence  of  any  extensive  applica- 
tion of  the  expansive  principle,  is  of  little  importance.  In  the  patent  which  Watt 
took  out  for  the  application  of  the  expansive  principle,  he  specified  several 
methods  of  producing  a  uniform  effect  upon  a  uniform  resistance,  notwithstand- 
ing the  variation  of  the  energy  of  the  power  which  necessarily  attended  the 
expansion  of  the  steam.  This  he  proposed  to  accomplish  by  various  mechani- 
cal means,  some  of  which  had  been  previously  applied  to  the  equalization  of  a 
varying  power.  One  consisted  in  causing  the  piston  to  act  on  a  lever,  which 
should  have  an  arm  of  variable  length,  the  length  increasing  in  the  same  pro- 
portion as  the  energy  of  the  moving  power  diminished.  This  was  an  expedient 
which  had  been  already  applied  in  mechanics  for  the  purpose  of  equalizing  a 
varying  power.  A  well-known  example  of  it  is  presented  in  the  mainspring 
and  fuzee  of  a  watch.  According  as  the  watch  goes  down,  the  mainspring 
becomes  relaxed,  and  its  force  is  diminished  ;  but,  at  the  same  time,  the 
chain  by  which  it  drives  the  fuzee  acts  upon  a  wheel  or  circle,  having  a 
diameter  increased  in  the  same  proportion  as  the  energy  of  the  spring  is 
diminished. 

Another  expedient  consisted  in  causing  the  moving  power,  when  acting  with 
greatest  energy,  to  lift  a  weight  which  should  be  allowed  to  descend  again, 
assisting  the  pibton  when  the  energy  of  the  moving  force  was  diminished. 

Another  method  consisted  in  causing  the  moving  force,  when  acting  with 
greatest  energy,  to  impart  momentum  to  a  mass  of  inert  matter,  which  should 
be  made  to  restore  the  same  force  when  the  moving  power  was  more  enfeebled. 
We  shall  not  more  than  allude  here  to  these  contrivances  proposed  by  Watt, 
since  their  application  has  never  been  found  advantageous  in  cases  where  the 
expansive  principle  is  used. 

The  application  of  the  expansive  principle  in  the  engines  constructed  by 
Boulton  and  Watt,  was  always  very  limited,  by  reason  of  their  confining  them- 
selves to  the  use  of  steam  having  a  pressure  not  much  exceeding  that  of  the 


THE  STEAM-ENGINE. 


441 


atmosphere.  If  the  principle  of  expansion,  as  above  explained,  be  attentively- 
considered,  it  will  be  evident  that  the  extent  of  its  application  will  mainly 
depend  on  the  density  and  pressure  of  the  steam  admitted  from  the  boiler.  If 
the  density  and  pressure  be  not  considerable  when  the  steam  is  cut  off,  the 
extent  of  its  subsequent  expansion  will  be  proportionally  limited.  It  was  in 
consequence  of  this,  that  this  principle  from  which  considerable  economy  of 
power  has  been  derived,  was  applied  with  much  less  advantage  by  Mr.  Watt 
than  it  has  since  been  by  others,  who  have  adopted  the  use  of  steam  of  much 
higher  pressure.  In  the  engines  of  Boulton  and  Watt,  where  the  expansive 
principle  was  applied,  the  steam  was  cut  off  after  the  piston  had  performed 
from  one  half  to  two  thirds  of  the  stroke,  according  to  the  circumstances  under 
which  the  engine  was  worked.  The  decreasing  pressure  produced  by  expan- 
sion was,  in  this  case,  especially  with  the  larger  class  of  engines,  little  more 
than  would  be  necessary  to  allow  the  momentum  of  the  mass  moved  to  spend 
itself,  before  the  arrival  of  the  piston  at  the  end  of  the  stroke. 

Subsequently,  however,  boilers  producing  steam  of  much  higher  pressure 
were  applied,  and  the  steam  was  cut  ofl'  when  the  piston  had  performed  a 
much  smaller  part  of  the  whole  stroke.  The  great  theatre  of  these  experiments 
and  improvements  has  been  the  mining  districts  in  Cornwall,  where,  instead 
of  working  with  steam  of  a  pressure  not  much  exceeding  that  of  the  atmosphere, 
it  has  been  found  advantageous  to  use  steam  whose  pressure  is  at  least  four 
times  as  great  as  that  of  the  atmosphere  ;  and  instead  of  limiting  its  expansion 
to  the  last  half  or  fourth  of  the  stroke,  it  is  cut  off  after  the  piston  has  performed 
one  fourth  part  of  the  stroke  or  less,  all  the  remainder  of  the  stroke  being  ac- 
complished by  the  expansive  power  of  the  steam,  and  by  momentum. 

For  several  years  after  the  extension  of  Watt's  first  patent  had  been  obtained 
from  parliament,  he  was  altogether  engrossed  by  the  labor  of  bringing  to  per- 
fection the  application  of  the  steam-engine  to  the  drainage  of  mines,  and  in 
surmounting  the  numerous  difficulties  which  presented  themselves  to  its  general 
adoption,  even  after  its  manifold  advantages  were  established  and  admitted. 
When,  however,  these  obstacles  had  been  overcome,  and  the  works  for  the 
manufacture  of  engines  for  pumping  water,  at  Soho,  had  been  organized  and 
brought  into  active  operation,  he  was  relieved  from  the  pressure  of  these 
anxieties,  and  was  enabled  to  turn  his  attention  to  the  far  more  extensive  and 
important  uses  of  which  he  had  long  been  impressed  with  the  conviction  that 
the  engine  was  capable.  His  sagacious  mind  enabled  him  to  perceive  that 
the  machine  he  had  created  was  an  infant  force,  which  by  the  fostering  in- 
fluence of  his  own  genius  would  one  day  extend  its  vast  power  over  the  arts 
and  manufactures,  the  commerce  and  the  civilization  of  the  world.  Filled  with 
such  aspirations,  he  addressed  his  attention  about  the  year  1779,  to  the  adapta- 
tion of  the  steam-engine  to  move  machinery,  and  thereby  to  supersede  animal 
power,  and  the  natural  agents,  wind  and  water. 

The  idea  that  steam  was  capable  of  being  applied  extensively  as  a  prime 
mover,  had  prevailed  from  a  very  early  period  ;  and  now  that  we  have  seen 
its  powers  so  extensively  brought  to  bear,  it  will  not  be  uninteresting  to  reA'ert 
to  the  faint  traces  by  which  its  agency  was  sketched  in  the  crude  speculations 
of  the  early  mechanical  inventors. 

Papin,  to  whom  the  credit  of  discovering  the  method  of  producing  a  vacuum 
by  the  condensation  of  steam  is  due,  was  the  earliest  and  most  remarkable  of 
those  projectors.  W^ith  very  limited  powers  of  practical  application,  he  was, 
nevertheless,  peculiarly  happy  in  his  mechanical  conceptions  ;  and  had  his 
experience  and  opportunities  been  proportionate  to  the  clearsighted  character 
of  his  mind,  he  would  doubtless  have  anticipated  some  of  the  most  memorable 
of  his  successors  in  the  progressive  improvement  of  the  steam-engine. 


442 


THE    STEAM-ENGINE. 


In  his  work,  after  describing  his  method  of  imparting  an  ahernate  motion  to 
a  piston  by  the  atmospheric  pressure  acting  against  a  vacuum  produced  by  the 
condensation  of  steam,  he  stated  that  his  invention,  besides  being  applicable  to 
pumping  water,  could  be  available  for  rowing  vessels  against  wind  and  tide, 
which  he  proposed  to  accomplish  in  the  following  manner  : — 

Paddle-wheels,  such  as  have  since  been  brought  into  general  use,  were  to 
be  placed  at  the  sides,  and  attached  to  a  shaft  extending  across  the  vessel. 
Within  the  vessel,  and  under  this  shaft,  he  proposed  to  place  several  cylinders 
supplied  with  pistons,  to  be  worked  by  the  atmospheric  pressure.  On  the  pis- 
ton-rods were  to  be  constructed  racks  furnished  with  teeth  ;  these  teeth  were 
to  work  in  the  teeth  of  wheels  or  pinions,  placed  on  the  shaft  of  the  paddle- 
wheels.  These  pinions  were  not  to  be  fixed  on  the  shaft,  but  to  be  connected 
with  it  by  a  ratchet  ;  so  that  when  they  turned  in  one  direction,  they  would 
revolve  without  causing  the  shaft  to  revolve  ;  but  when  driven  in  the  other 
direction,  the  catch  of  the  ratchet-wheel  would  act  upon  the  shaft  so  as  to  com- 
pel the  shaft  and  paddle-wheels  to  revolve  with  the  motion  of  the  pinion  or 
wheel  upon  it.  By  this  arrangement,  whenever  the  piston  of  any  cylinder  was 
forced  down  by  the  atmospheric  pressure,  the  rack  descending  would  cause  the 
corresponding  pinion  of  the  paddle-shaft  ta  revolve  ;  and  the  catch  of  the 
ratchet-wheel,  being  thus  in  operation,  would  cause  the  paddle-shaft  and  pad- 
dle-wheels also  to  revolve  ;  but  whenever  the  piston  would  rise,  the  rack 
driving  the  pinion  in  the  opposite  direction,  the  catch  of  the  ratchet-wheel 
would  merely  fall  from  tooth  to  tooth,  without  driving  the  paddle-shaft. 

It  is  evident  that  by  such  an  arrangement  a  single  cylinder  and  piston  would 
give  an  intermitting  motion  to  the  paddle-shaft,  the  motion  of  the  wheel  being 
continued  only  during  the  descent  of  the  piston  ;  but  if  several  cylinders  were 
provided,  then  their  motion  might  be  so  managed,  that  when  one  would  be  per- 
forming its  ascending  stroke,  and  therefore  giving  no  motion  to  the  paddle-shaft, 
another  should  be  performing  its  descending  stroke,  and  therefore  driving  the 
paddle-shaft.  As  the  interval  between  the  arrival  of  the  piston  at  the  bottom 
of  the  cylinder  and  the  commencement  of  its  next  descent  would  have  been, 
in  the  imperfect  machine  conceived  by  Papin,  much  longer  than  the  time  of 
the  descent,  it  was  evident  that  more  than  two  cylinders  would  be  necessary  to 
insure  a  constantly-acting  force  on  the  paddle-shaft,  and,  accordingly,  Papin 
proposed  to  use  several  cylinders. 

In  addition  to  this,  Papin  proposed  to  construct  a  boiler  having  a  fireplace 
surrounded  on  every  side  by  water,  so  that  the  heat  might  be  imparted  to  the 
water  with  such  increased  rapidity  as  to  enable  the  piston  to  make  four  strokes 
per  minute.  These  projects  were  promulged  in  1690,  but  it  does  not  appear 
that  they  were  ever  reduced  to  experiment. 

Savery  proposed,  in  his  original  patent,  in  1698,  to  apply  his  steam-engine 
as  a  general  prime  mover  for  all  sorts  of  machinery,  by  causing  it  to  raise  water 
to  make  an  artificial  fall,  by  which  overshot  water-wheels  might  be  driven. 
This  proposal  was  not  acted  on  during  the  lifetime  of  Savery,  but  it  was  at  a 
subsequent  period  partially  carried  into  effect.  Mr.  Joshua  Rigley  erected 
several  steam-engines  on  this  principle  at  Manchester,  and  other  parts  of  Lan- 
cashire, to  impel  the  machinery  of  some  of  the  earliest  manufactories  and 
cotton  mills  in  that  district.  The  engines  usually  raised  the  water  from  sixteen 
to  twenty  feet  high,  whence  it  was  conveyed  to  an  overshot  wheel,  to  which 
it  gave  motion.  The  same  water  was  repeatedly  elevated  by  the  engine,  so 
that  no  other  supply  was  necessary,  save  what  was  sufficient  to  make  good  the 
waste.  These  engines  continued  in  use  for  some  years,  until  superseded  by 
improved  machines.* 

*  Farey,  Treatise  on  the  Steaniv 


THE   STEAM-ENGINE. 


443 


In  1736,  Jonathan  Hulls  obtained  a  patent  for  a  method  of  towing  ships  into  ? 
or  out  of  harbor  against  wind  and  tide.  This  method  was  little  more  than  a  S 
revival  of  that  proposed  by  Papin  in  1690.  The  motion,  however,  was  to  be  } 
communicated  to  the  paddle-shaft  by  a  rope  passing  over  a  pulley  fixed  on  an  S 
axis,  and  was  to  be  maintained  during  the  returning  stroke  of  the  piston  by  the  ? 
descent  of  a  weight  which  was  elevated  during  the  descending  stroke.  There  S 
is  no  record,  however,  of  this  plan,  any  more  than  that  of  Papin,  ever  having  } 
been  reduced  to  experiment.  ; 

During  the  early  part  of  the  last  century  the  manufactures  of  this  country  < 
had  not  attained  to  such  an  extent  as  to  render  the  moving  power  supplied  by  \ 
water  insufficient  or  uncertain  to  any  inconvenient  degree  ;  and   accordingly  < 
mills,  and  other  works  in  which  machinery  required  to  be  driven  by  a  moving  ] 
power,  were  usually  built  along  the  streams  of  rivers.     About  the  year  1750  i 
the  general  extension   of  manufactures,  and  their  establishment  in  localities  ] 
where  water  power  was  not  accessible,  called  the  steam-engine  into  more  ex-  > 
tensive  operation.     In  the  year  1752,  Mr.  Champion,  of  Bristol,  applied  the  ' 
atmospheric  engine  to  raise  water,  by  which  a  number  of  overshot  wheels  were 
driven.     These  were  applied  to  move  extensive  brass-works  in  that  neighbor- 
hood, and  this  application  w;as  continued  for  about  twenty  years,  but  ultimately 
given  up  on  account  of  the  expense  of  fuel  and  the  improved  applications  of  the 
steam-engine. 

About  this  time  Smeaton  applied  himself  with  great  activity  and  success  to 
the  improvement  of  wind  and  water  mills,  and  succeeded  in  augmenting  their 
useful  effect  in  a  twofold  proportion  with  the  same  supply  of  water.  From  the 
year  1750  until  the  year  1780  he  was  engaged  in  the  construction  of  his  im- 
proved water-mills,  which  he  erected  in  various  parts  of  the  country,  and  which 
were  imitated  so  extensively  that  the  improvement  of  such  mills  became  general. 
In  cases  where  a  summer  drought  suspended  the  supply  of  water,  horse  ma- 
chinery Avas  provided,  either  to  work  the  mill  or  to  throw  back  the  water. 
These  improvements  necessarily  obstructed  for  a  time  the  extension  of  steam 
power  to  millwork  ;  but  the  increase  of  manufactures  soon  created  a  demand 
for  power  greatly  exceeding  what  could  be  supplied  by  such  limited  means. 

In  the  manufacture  of  iron,  it  is  of  great  importance  to  keep  the  furnaces 
continually  blown,  so  that  the  heat  may  never  be  abated  by  day  or  night.  In 
the  extensive  iron-works  at  Colebrook  Dale,  several  water-wheels  were  used 
in  the  different  operations  of  the  manufacture  of  iron,  especially  in  driving  the 
blowers  of  the  iron  furnaces.  These  wheels  were  usually  driven  by  the  water 
of  a  river,  but  in  the  summer  months  the  supply  became  so  short  that  it  was 
insufficient  to  work  them  all.  Steam-engines  were  accordingly  erected  to  re- 
turn the  water  for  driving  these  wheels.  This  application  of  the  engine  as  an 
occasional  power  for  the  supply  of  water- wheels  having  been  found  so  effectual, 
returning  engines  were  soon  adopted  as  the  permanent  and  regular  means  of 
supplying  water-wheels.  The  first  attempt  of  this  kind  is  recorded  to  have 
been  made  by  Mr.  Oxley,  in  1762,  who  constructed  a  machine  to  draw  coals 
out  of  a  pit  at  Hartley  colliery,  in  Northumberland.  It  was  originally  intended 
to  turn  the  machine  by  a  continuous  circular  motion  received  from  the  beam  of 
the  engine  ;  but  that  method  not  being  successful,  the  engine  was  applied  to 
raise  water  for  a  wheel  by  which  the  machine  was  worked.  This  engine  was 
continued  in  use  for  several  years,  and  though  it  was  at  length  abandoned,  on 
account  of  its  defective  construction,  it  nevertheless  established  the  practica- 
bility of  using  steam  power  as  a  means  of  driving  water-wheels.* 

In  the  year  1777,  Mr.  John  Stewart  read  a  paper  before  the  royal  society, 
describing  a  method  for  obtaining  a  continued  circular  motion  for  turning  ail 
*  Farey  on  the  Steam-Eugine,  p.  297. 


*  kinds  of  mills  from  the  reciprocating  motion  of  a  steam-engine.  He  proposed 
to  accomplish  this  by  means  of  two  endless  chains  passing  over  pulleys,  which 
should  be  moved  upward  and  downward  by  the  motion  of  the  engine,  in  the 
manner  of  a  window-sash.  The  joint  pins  of  the  links  of  the  two  chains 
worked  in  teeth  at  the  opposite  sides  of  a  cog-wheel,  to  which  they  imparted 
a  circular  motion,  first  by  one  chain,  and  then  by  the  other,  acting  alternately 
on  opposite  sides  of  the  wheel.  One  chain  impelled  it  during  the  descent  of 
the  piston,  and  the  other  during  the  ascent;  but  one  of  these  chains  always 
passed  over  its  pulleys  so  as  to  produce  no  effect  on  one  side  of  the  cog-wheel, 
while  the  other  chain  worked  on  the  opposite  side  to  turn  it  round.  For 
this  purpose  each  chain  was  provided  with  a  catch,  to  prevent  its  circulating 
over  its  pulleys  in  one  direction,  but  to  allow  it  free  motion  in  the  other.  The 
cog-wheel  thus  kept  in  revolution  might  be  applied  to  the  axis  of  any  mill 
which  the  engine  was  required  to  work.  Thus,  if  it  were  applied  to  a  flour- 
mill,  the  millstone  itself  would  perform  the  office  of  a  fly-wheel  to  regulate  the 
intermission  of  the  power,  and  in  other  mills  a  fly-wheel  might  be  added  for 
this  purpose. 

The'  hints  obtained  by  Mr.  Stewart  from  Papin's  contrivance,  before  men- 
tioned, will  not  fail  to  be  perceived.  In  Mr.  Stewart's  paper  he  notices  indi- 
rectly the  method  of  obtaining  a  continued  circular  motion  from  a  reciprocating 
motion  by  means  of  a  crank  or  winch,  which,  he  says,  occurs  naturally  in 
theory,  but  in  practice  would  be  impossible,  from  the  nature  of  the  motion  of 
the  engine,  which  depends  on  the  force  of  the  steam,  and  cannot  be  ascertained 
in  its  length.  Therefore,  on  the  first  variation,  the  machine  would  be  either 
broken  in  pieces  or  turned  back.  Such  an  opinion,  pronounced  by  a  man  of 
considerable  mechanical  knowledge  and  ingenuity,  against  a  contrivance  which, 
as  will  presently  appear,  proved  in  practice,  not  less  than  in  theory,  to  be  the 
most  eff'ectual  means  of  accomplishing  the  end  here  pronounced  to  be  impossi- 
ble, is  sufficiently  remarkable.  It  might  cast  some  doubt  on  the  extent  of  Mr. 
Stewart's  practical  knowledge,  if  it  did  not  happen  to  be  in  accordance  with  a 
judgment  so  generally  unimpeachable  as  that  of  Mr.  Smeaton.  This  paper  of 
Mr.  Stewart's  was  referred  by  the  council  of  the  royal  society  to  Mr.  Smeaton, 
who  remarked  upon  the  difficulty  arising  from  the  absolute  stopping  of  the 
whole  mass  of  moving  power,  whenever  the  direction  of  the  motion  is  changed  ; 
and  observed,  that  although  a  fly-wheel  might  be  applied  to  regulate  the  motion, 
it  must  be  such  a  large  one  as  would  not  be  readily  controlled  by  the  engine 
itself ;  and  he  considered  that  the  use  of  such  a  fly-wheel  would  be  a  greater 
incumbrance  to  a  mill  than  a  water-wheel  to  be  supplied  by  water  pumped  up 
by  the  engine.  This  engineer,  illustrious  as  he  was,  not  only  fell  into  the 
error  of  Mr.  Stewart  in  respect  of  the  crank,  but  committed  the  further  blunder 
of  condemning  the  very  expedient  which  has  since  rendered  the  crank  effectual. 
It  will  presently  appear  that  the  combination  of  the  crank  and  fly-wheel  have 
been  the  chief  means  of  establishing  the  dominion  of  the  steam-engine  over 
manufactures. 

In  1779,  Mr.  Matthew  Wasbrough,  an  engineer  at  Bristol,  took  out  a  patent 
for  the  application  of  a  steam-engine  to  produce  a  continuous  circular  motion 
by  means  of  ratchet-wheels,  similar  to  those  previously  used  by  Mr.  Oxley, 
at  Hartley  colliery  ;  to  which,  however,  Mr.  Wasbrough  added  a  fly-wheel  to 
maintain  and  regulate  the  motion.  Several  machines  were  constructed  under 
this  patent ;  and  among  others,  one  was  erected  at  Mr.  Taylor's  saw-mills  a,nd 
block  manufactory  at  Southampton.  In  1780,  one  was  erected  at  Birmingham, 
where  the  ratchet-work  was  found  to  be  subject  to  such  objections,  that  one 
of  the  persons  about  the  works  substituted  for  it  the  simple  crank,  which  has 
since  been  invariably  used.     A  patent  was  taken  out  for  this  application  of  the 


crank  in  the  same  year,  by  Mr.  James  Pickard,  of  Birmingham.  It  will 
presently  appear,  however,  that  the  suggestion  of  this  application  of  the  crank 
Avas  derived  Irom  the  proceedings  of  Walt,  who  was  at  the  same  time  engaged 
in  similar  experiments. 

The  single-acting  steam-engine,  as  constructed  by  Watt,  was  not  adapted 
to  produce  continuous  uniform  motion  of  rotation,  for  the  following  reasons  : — 

First.  The  effect  required  was  that  of  a  uniformly-acting  force.  The 
steam-engine,  on  the  other  hand,  supplied  an  intermitting  force.  Its  operation 
was  continued  during  the  descending  motion  of  the  piston,  but  it  was  suspended 
during  the  ascent  of  the  piston.  To  produce  the  continued  effect  now  required, 
either  its  principle  of  operation  should  be  altered,  or  some  expedient  should  be 
devised  for  maintaining  the  motion  of  the  revolving  shaft  during  the  ascent  of 
the  piston,  and  the  consequent  suspension  of  the  moving  power. 

Secondly.  The  action  of  the  steam-engine  was  rectilinear.  It  was  a  power 
which  acted  in  a  straight  line,  viz.,  in  the  direction  of  the  cylinder.  The 
motion,  however,  required  to  be  produced,  was  a  circular  motion — a  motion  of 
rotation  around  the  axis  or  shaft  of  the  mill. 

The  steps  by  which  Watt  proceeded  to  accomplish  these  objects  have  been 
recorded  by  himself  as  follows,  in  his  notes  upon  Dr.  Robinson's  article  on  the 
steam-engine  :- — 

"  I  had  very  early  turned  my  mind  to  the  producing  of  continued  motion 
round  an  axis  ;  and  it  will  be  seen,  by  reference  to  my  first  specification  in 
1769,  that  I  there  described  a  steam-wheel,  moved  by  the  force  of  steam,  acting 
in  a  circular  channel  against  a  valve  on  one  side,  and  against  a  column  of 
mercury,  or  some  other  fluid  melal,  on  the  other  side.  This  was  executed 
upon  a  scale  of  about  six  feet  diameter  at  Soho,  and  worked  repeatedly,  but 
was  given  up,  as  several  practical  objections  were  found  to  operate  against  it  ; 
similar  objections  lay  against  other  rotative  engines,  which  had  been  contrived 
by  myself  and  others,  as  well  as  to  the  engines  producing  rotatory  motions  by 
means  of  ratchet-wheels. 

"  Having  made  my  single  reciprocating  engines  very  regular  in  their  move- 
ments, I  considered  how  to  produce  rotative  motions  from  them  in  the  best 
manner  ;  and  among  various  schemes  which  were  subjected  to  trial,  or  which 
passed  through  my  mind,  none  appeared  so  likely  to  answer  the  purpose  as  the 
application  of  the  crank,  in  the  manner  of  the  common  turning  lathe  ;  but  as 
the  rotative  motion  is  produced  in  that  machine  by  impulse  given  to  the  crank 
in  the  descent  of  the  foot  only,  it  requires  to  be  continued  in  its  ascent  by  the 
energy  of  the  wheel,  which  acts  as  a  fly  ;  being  unwilling  to  load  my  engines 
with  a  fly-wheel  heavy  enough  to  continue  the  motion  during  the  ascent  of  the 
piston  (or  with  a  fly-wheel  heavy  enough  to  equalize  the  motion,  even  if  a 
counterweight  were  employed  to  act  during  that  ascent),  I  proposed  to  employ 
two  engines,  acting  upon  two  cranks  fixed  on  the  same  axis,  at  an  angle  of 
120°  to  one  another,  and  a  weight  placed  upon  the  circumference  of  the  fly- 
wheel at  the  same  angle  to  each  of  the  cranks,  by  which  means  the  motion 
might  be  rendered  nearly  equal,  and  only  a  very  light  fly-wheel  would  be 
requisite. 

"  This  had  occurred  to  me  very  early  ;  but  my  attention  being  fully  employed 
in  making  and  erecting  engines  for  raising  water,  it  remained  in  petto  until 
about  the  year  1778  or  1779,  when  Mr.  Wasbrough  erected  one  of  his  ratchet- 
wheel  engines  at  Birmingham,  the  frequent  breakages  and  irregulariiies  of 
which  recalled  the  subject  to  my  mind,  and  I  proceeded  to  make  a  model  of 
my  method,  which  answered  my  expectations ;  but  having  neglected  to  take 
out  a  patent,  the  invention  was  communicated  by  a  workman  employed  to 
make  the  model,  to  some  of  the  people  about  Mr.  Wasbrough's  engine,  and  a 


446 


THE  STEAM-ENGINE. 


patent  was  taken  out  by  tliem  for  the  application  of  the  crank  to  steam-engines.  ) 
This  fact  the  said  workman  confessed,  and  the  engineer  who  directed  the  works  ( 
acknowledged  it ;  but  said,  nevertheless,  that  the  same  idea  had  occurred  to  ) 
him  prior  to  his  hearing  of  mine,  and  that  he  had  even  made  a  model  of  it  be-  ( 
fore  that  time  ;  which  might  be  a  fact,  as  the  appUcation  to  a  single  crank  was  ) 
sufficiently  obvious.  j 

"  In  these  circumstances,  I  thought  it  better  to  endeavor  to  accomplish  the 
same  end  by  other  means,  than  to  enter  into  litigation  ;  and  if  successful,  by 
demolishing  the  patent,  to  lay  the  matter  open  to  everybody.  Accordingly,  \ 
in  1781,  I  invented  and  took  out  a  patent  for  several  methods  of  producing  i 
rotative  motions  from  reciprocating  ones  ;  among  which  was  the  method  of  the  , 
sun-and-planet  v^heels.  This  contrivance  was  applied  to  many  engines,  and  < 
possesses  the  great  advantage  of  giving  a  double  velocity  to  the  fly-wheel  ;  but  , 
is  perhaps  more  subject  to  wear,  and  to  be  broken  under  great  strains,  than  a  < 
simple  crank,  which  is  now  more  commonly  used,  although  it  requires  a  fly-  , 
wheel  of  four  times  the  weight,  if  fixed  upon  the  first  axis  ;  my  application  of  ' 
the  double  engine  to  these  rotative  machines  rendered  the  counterweight  un-  , 
necessary,  and  produced  a  more  regular  motion."  J 

Watt's  second  patent  here  referred  to,  was  dated  25th  of  October,  1781,  and  \ 
was  entitled,  "  A  patent  for  certain  new  methods  of  applying  the  vibrating  or  | 
reciprocating  motions  of  steam  or  fire  engines  to  produce  a  continued  rotative  ! 
or  circular  motion  round  an  axis  or  centre,  and  thereby  to  give  motion  to  the 
Avheels  of  mills  and  other  machines." 

All  the  methods  specified  in  this  patent  were  intended  to  be  worked  by  the 
single-acting  engine,  already  described,  a  counterweight  being  applied  to  impel 
the  machinery  during  the  returning  stroke  of  the  engine,  which  weight  would 
be  elevated  during  the  descent  of  the  piston.  There  were  five  different  ex- 
pedients proposed  in  the  specification  for  producing  a  rotatory  motion  ;  but,  of 
these  five,  two  only  were  ever  applied  in  practice. 

Suppose  a  rod  or  bar  attached  by  a  pin  or  joint  at  the  upper  extremity  to  the 
working  end  of  the  beam  of  the  engine,  and  by  a  similar  pin  or  joint  at  the 
lower  extremity  to  an  iron  wheel  fixed  on  the  extremity  of  the  axis  of  the  fly- 
wheel.    One   half  of  this  wheel  is  formed  of  a  solid  semicircle  of  cast  iron, 
while  the  other  half  is  constructed  of  open  spokes,  so  as  to  be  as  light  as  is 
consistent  with  strength.     The  position  of  the  wheel  on  the  axis  is  such  that 
during  the  returning  stroke  of  the  piston,  when  the  operation  of  the  steam  is 
suspended,  the  heavy  semicircle  of  the  wheel  w-ill  be  descending,  and  by  its 
weight  will  draw  down  the  connecting  bar,  and  thereby  draw  down  the  working 
end   of  the   beam,  and  draw  up  the  piston  in  the  cylinder.     When  the  piston 
'  descends  and  is  driven  by  the  power  of  the  steam,  the  heavy  semicircle  of  the 
I  above-mentioned  wheel  will  be  drawn  upward,  and  in  the  same  way  the  motion 
[  will  be  continued. 

»  The  second  method  of  producing  a  rotatory  motion,  which  was  subsequently 
!  continued  for  many  years  in  practical  operation,  was  that  which  was  called 
)  the  sun-and-pla7iet  wheels.  A  toothed  wheel,  A,  fig.  13,  called  the  sun- wheel, 
I  was  fixed  on  the  axle  of  the  fly-wheel,  to  which  rotation  was  to  be  imparled. 

>  The  wheel  B,  called  the  planet-wheel,  having  an  equal  diameter,  was  fastened 
(  on  the  end  I  of  the  connecting  rod  H  I,  so  as  to  be  incapable  of  revolving. 
)  During  the  descent  of  the  piston,  the  working  end  of  the  beam  was  drawn 
(  upward,  and  the  end  I  of  the  connecting  rod  travelled  from  C  to  D,  through 
)  the  dotted  semicircle  C  I  D.  The  wheel  B  not  being  capable  of  revolving  on 
(  the  centre  I,  would,  during  this  motion,  drive  the  sun-wheel  A.     During  the 

>  ascent  of  the  steam-piston,  the  working  end  of  the  beam  would  descend,  and 
I  the  centre  I  of  the  planet-wheel   B  would  be  driven  downward  from  D  to  C, 


through  the  other  clotted  semicircle,  and  would  consequently  continue  to  drive 
the  sun-wheel  round  in  the  same  direction. 

This  contrivance,  although  in  the  main  inferior  to  the  more  simple  one  of 
the  crank,  is  not  without  some  advantages  ;  among  others,  it  gives  to  the  sun- 
wheel  double  the  velocity  which  would  be  communicated  by  the  crank  ;  for  in 
the  crank  one  revolution  only  on  the  axle  is  produced  by  one  revolution  of  the 
crank,  but  in  the  sun-and-planet  wheels,  two  revolutions  of  the  sun-wheel  are 
produced  by  one  of  the  planet-wheel ;  thus  a  double  velocity  is  obtained  from 
the  same  motion  of  the  beam.  This  will  be  evident  from  considering  that 
when  the  planet-wheel  is  in  its  highest  position,  its  lowest  tooth  is  engaged 
with  the  highest  tooth  of  the  sun-wheel  ;  as  the  planet-wheel  passes  from  the 
highest  position,  its  teeth  drive  those  of  the  sun-wheel  before  them,  and  when 
it  comes  into  the  lowest  position,  the  highest  tooth  of  the  planet-wheel  is  en- 
gaged with  the  lowest  of  the  sun-wheel  :  but  then  half  of  the  sun-wheel  has 
rolled  off  the  planet-wheel,  and,  therefore,  the  tooth  which  was  engaged  with 
it  in  its  highest  position,  must  now  be  distant  from  it  by  half  the  circumference 
of  the  wheel,  and  must,  therefore,  be  again  in  the  highest  position  ;  so  that 
while  the  planet-wheel  has  been  carried  from  the  top  to  the  bottom,  the  sun- 
wheel  has  made  a  complete  revolution. 

This  advantage  of  giving  an  increased  velocity  may  be  obtained  also  by  the 
crank,  by  placing  toothed  wheels  on  its  axle.  Independently  of  the  greater 
expense  attending  the  construction  of  the  sun-and-planet  wheel,  its  liability  to 
go  out  of  order,  and  the  rapid  wear  of  the  teeth,  and  other  objections,  rendered 
it  inferior  to  the  crank,  which  has  entirely  superseded  it. 

Although  by  these  contrivances  Watt  succeeded  in  obtaining  a  continuous 
circular  motion  from  the  reciprocating  motion  of  the  steam-engine,  the  machine 
was  still  one  of  intermitting,  instead  of  continuous  action.  The  expedient  of 
a  counterweight,  elevated  during  the  descending  stroke,  and  giving  back  the 
power  expended  on  it  in  the  interval  of  the  returning  stroke,  did  not  satisfy 
the  fastidious  mechanical  taste  of  Watt.  He  soon  perceived  that  all  which  he 
proposed  to  accomplish  by  the  application  of  two  cylinders  and  pistons  work- 
ing alternately,  could  be  attained  with  greater  simplicity  and  effect  by  a  single 
cylinder,  if  he  could  devise  means  by  which  the  piston  might  be  impelled  by 
steam  upward  as  well  as  downward.  To  accomplish  this,  it  was  only  neces- 
sary to  throw  the  lower  end  of  the  cylinder  into  alternate  communication  with 
the  boiler,  while  the  upper  end  would  be  put  into  communication  with  the 
condenser.  If,  for  example,  during  the  descent  of  the  piston,  the  upper  end 
{  of  the    cylinder   communicated  with  the  boiler,  and   the  lower  end  with  the 


448 


THE  STEAM-ENGINE. 


condenser  ;  and,  on  the  other  hand,  during  the  ascent  of  the  piston,  the  lower 
end  commimicated  with  the  boiler,  and  the  upper  end  with  the  condenser  ; 
then  the  piston  would  be  driven  continually,  whether  upward  or  downward,  by 
the  power  of  steam  acting  against  a  A'aCuum.  Watt  obtained  his  third  patent 
for  this  contrivance,  on  the  r2th  of  March,  1782. 

This  change  in  the  principle  of  the  machine  involved  several  other  changes 
in  the  details  of  its  mechanism. 

It  was  necessary,  in  the  first  place,  to  provide  means  for  admitting  and  with- 
drawing the  steam  at  either  end  of  the  cylinder.  For  this  purpose  let  B  and 
B^  fig.  14,  be  two  steam-boxes,  B  the  upper,  and  B'  the  lower,  communicating 

Fiff.  14. 


respectively  with  the  top  and  bottom  of  the  cylinder  by  proper  passages  D  D'. 
Let  two  valves  be  placed  in  B,  one,  S,  above  the  passage  D,  and  the  other,  C, 
below  it ;  and  in  like  manner  two  other  valves  in  the  lower  valve-box,  B',  one, 
S',  above  the  passage  D',  and  the  other,  C,  below  it.  Above  the  valve  S  in 
the  upper  steam-box  is  an  opening  at  which  the  steam-pipe  from  the  boiler 
enters,  and  below  the  valve  C  is  another  opening,  at  which  enters  the  ex- 
hausting-pipe leading,  to  the  condenser.  In  like  manner,  above  the  v^lve  S^ 
in  the  lower  steam-box  enters  a  steam-pipe  leading  from  the  boiler,  and  below 
the  valve  C  enters  an  exhausting-pipe  leading  to  the  condenser.  It  is  evident, 
therefore,  that  steam  can  always  be  admitted  above  the  piston  by  opening  the 
valve  S,  and  below  it  by  opening  the  valve  S' ;  and,  in  like  manner,  steam  can 
be  withdrawn  from  the  cylinder  above  the  piston,  and  allowed  to  pass  to  the 
condenser,  by  opening  the  valve  C,  and  from  below  it  by  opening  the  valve  C 
Supposing  the  piston  P  to  be  at  the  top  of  the  cylinder,  and  the  cylinder 
below  the  piston  to  be  filled  with  pure  steam,  let  the  valves  S  and  C''  be  open- 
ed, the  valves  C  and  S'  being  closed  as  represented  in  fig.  15.  Steam  i'rom 
the  boiler  will,  therefore,  flow  in  through  the  open  valve  S,  and  will  press  the 
piston  downward,  while  the  steam  that  has  filled  the  cylinder  below  the  piston 
will,  pass  through  the  open  valve  C  into  the  exhausting-pipe  leading  to  the 
condenser,  and  being  condensed  will  leave  the  cylinder  below  the  piston  a 
vacuum;  The  piston  will,  therefore,  be  pressed  downward  by  the  action  of 
the  steam  above  it,  as  in  the  single-acting  engine.  Having  arrived  at  the 
bottom  of  the  cylinder,  let  the  valves  S  and  C  be  both  closed,  and  the  valves 
S'  and  C  be  opened,  as  represented  in  fig.  15.  Steam  will  now  be  admitted 
through  the  open  valve  S'  and  through  the  passage  D'  below  the  piston,  while 
the  steam  which  has  just  driven  the  piston  downward,  filling  the  cylinder  above 
the  piston,  will  be  drawn  ofl'  through  the  "open  valve  C,  and  the  exhausting- 
pipe,   into  the   condenser,  leaving  the   cylinder  above  the  piston  a  vacuum. 


THE  STEAM-ENGINE. 


449 


Fig.  15. 


The  piston  will,  therefore,  be  pressed  upward  by  the  action  of  the  steam  below 
it,  against  the  vacuum  above  it,  and  will  ascend  with  the  same  force  as  that 
with  which  it  had  descended. 

This  alternate  action  of  the  piston  upward  and  downward  may  evidently  be 
continued  by  opening  and  closing  the  valves  alternately  in  pairs.  Whenever 
the  piston  is  at  the  top  of  the  cylinder,  as  represented  in  fig.  14,  the  valves  S 
and  C,  that  is,  the  upper  steam-valve  and  the  lower  exhausting-valve,  are 
opened,  and  the  valves  C  and  S',  that  is,  the  upper  exhausting-valve  and  the 
lower  steam-valve,  are  closed  ;  and  when  the  piston  has  arrived  at  the  bottom 
of  the  cylinder,  as  represented  in  fig.  15,  the  valves  C  and  S',  that  is,  the  upper 
exhausting-valve  and  the  lower  steam-valve,  are  opened,  and  the  valves  S  and 
C,  that  is,  the  upper  steam-valve  and  the  lower  exhausting-valve,  are  closed. 

If  these  valves,  as  has  been  here  supposed,  be  opened  and  closed  at  the 
moments  at  which  the  piston  reaches  the  top  and  bottom  of  the  cylinder,  it  is 
evident  that  they  may  be  all  worked  by  a  single  lever  connected  with  them 
by  proper  mechanism.  When  the  piston  arrives  at  the  top  of  the  cylinder,  this 
lever  would  be  made  to  open  the  valves  S  and  C,  and  at  the  same  time  to 
close  the  valves  S'  and.  C  ;  and  when  it  arrives  at  the  bottom  of  the  cylin- 
der, it  would  be  made  to  close  the  valves  S  and  C,  and  to  open  the  valves 
S'  and  C. 

If,  however,  it  be  desired  to  cut  off"  the  steam  before  the  arrival  of  the  piston 
at  the  termination  of  its  stroke,  whether  upward  or  downward,  then  the  steam- 
valves  must  be  closed  before  the  arrival  of  the  piston  at  the  end  of  its  stroke  ; 
and  as  the  exhausting-valve  ought  to  be  left  open  until  the  stroke  is  completed, 
these  valves  ought  to  be  moved  at  different  times.  In  that  case  separate  levers 
should  be  provided  for  the  diff'erent  valves.  We  shall,  however,  return  again 
to  the  subject  of  the  valves  which  regulate  the  admission  of  steam  to  the  cyl- 
inder and  its  escape  to  the  condenser. 

It  will  be  remembered  that  in  the  single-acting  engine  the  process  of  con- 
densation was  suspended  while  the  piston  ascended  in  the  cylinder,  and  there- 
fore the  play  of  the  jet  of  cold  water  in  ihe  condenser  was  stopped  during  this 
interval.  In  the  double-acting  engine,  however,  the  flow  of  steam  from  the 
cylinder  to  the  condenser  is  continued,  whether  the  piston  ascend  or  descend, 
and  therefore  a  constant  condensation  of  steam  must  be  produced.  The  con- 
densing jet,  therefore,  does  not  in  this  case,  as  in  the  former,  play  with  inter- 
vals of  intermission.  A  constant  jet  of  cold  water  must  be  maintained  in  the 
condenser. 

It  will  presently  appear  that  in  the  double-acting  engine  applied  to  manu- 

VOL.  II.— 39 


450 


THE  STEAM-ENGINE. 


factures,  the  motion  of  the  piston  was  subject  to  more  or  less  variation  of 
speed,  and  the  quantity  of  steam  admitted  to  the  cylinder  was  subject  to  a 
corresponding  change.  The  quantity  of  steam,  therefore,  drawn  into  the  con- 
denser was  subject  to  variation,  and  required  a  considerable  change  in  the 
quantity  of  cold  water  admitted  through  the  jet  to  condense  it.  To  regulate 
this,  the  valve  or  cock  by  which  the  water  was  admitted  into  the  condenser 
was  worked  in  the  double-acting  engine  by  a  lever  furnished  with  an  index, 
by  which  the  quantity  of  condensing  water  admitted  into  the  condenser  could 
be  regulated.  This  index  played  upon  a  graduated  arch,  by  which  the  engine- 
man  was  enabled  to  regulate  the  supply. 


THE    STEAM-EIGOE. 


(THIRD    LECTURE.) 


Methods  of  Connecting  the  Piston-Rod  and  Beam  in  the  Double-acting  Engine. — Rack  and  Sector. 
—Parallel  Motion. — Connexion  of  Piston-Rod  and  Beam. — Connecting  Rod  and  Crank. — Fly- 
wheel.— Throttle-Valve. — Governor. — Construction  and  Operation  of  the  Double-acting  Engine. 
— Eccentric. — Cocks  and  Valves. — Single-Clack  Valves. — Double-Clack  Valves. — Conical  Valves. 
— Slide  Valves. — Murray's  SHdes. — The  D  Valves. — Seaward's  Slides. — Single  Cock. — Two- 
way  Cock. — Four-way  Cock. — Pistons. — Common  Hemp-packed  Piston. — Woolf  s  Piston. — Me- 
tallic Pistons. — Cartwright's  Engine. — Cartwright's  Piston. — Barton's  Piston. 


r- 


THE  STEAM-ENGINE. 


453 


THE   STEAM-ENGINE 


(THIRD    LECTURE.) 


In  the  single-acting  engine,  the  force  of  the  piston  acted  on  the  beam  only 
during  its  descent ;  and  this  force  was  transmitted  from  the  piston  to  the  beam, 
as  we  have  seen,  by  a  flexible  chain,  extended  from  the  end  of  the  piston-rod, 
and  playing  upon  the  arch  head  of  the  beam.  In  the  double-acting  engine, 
however,  the  force  of  the  steam  pressing  the  piston  upward  must  likewise  be 
transmitted  to  the  beam,  so  as  to  drive  the  latter  upward  while  the  piston  as- 
cends. This  action  could  not  be  accomplished  by  a  chain  connecting  the 
piston  with  the  arch  head  of  the  beam. 

Where  the  mechanical  action  to  be  transmitted  is  a  pull,  and  not  a  push,  a 
flexible  chain,  cord,  or  strap,  is  sufficient ;  but  if  a  push  or  thrust  is  required 
to  be  transmitted,  then  the  flexibility  of  the  medium  of  mechanical  communica- 
tion afforded  by  a  chain  renders  it  inapplicable.  In  the  double-acting  engine, 
during  the  descent,  the  piston-rod  still  pulls  the  beam  down  ;  and  so  far  a  chain 
connecting  the  piston-rod  with  the  beam  would  be  sufficient  to  transmit  the 
action  of  the  one  to  the  other  ;  but  in  the  ascent  the  beam  no  longer  pulls  up 
the  piston-rod,  but  is  pushed  up  by  it.  A  chain  from  the  piston-rod  to  the 
arch  head,  as  described  in  the  single-acting  engine,  would  fail  to  transmit  this 
force.  If  such  a  chain  were  used  with  the  double  engine,  where  there  is  no 
counter-weight  on  the  opposite  end  of  the  beam,  the  consequence  would  be, 
that  in  the  ascent  of  the -piston  the  chain  would  slacken,  and  the  beam  would 
still  remain  depressed.  It  is  therefore  necessary  that  some  other  mechanical 
connexion  be  contrived  between  the  piston-rod  and  the  beam,  of  such  a  nature 
that  in  the  descent  the  piston-rod  may  pull  the  beam  down,  and  may  push  it  up 
in  the  ascent. 

Watt  first  proposed  to  effect  this  by  attaching  to  the  end  of  the  piston-rod  a 
straight  rack,  faced  with  teeth,  which  should  work  in  corresponding  teeth 
raised  on  the  arch  head  of  the  beam,  aS  represented  in  fig.  16.  If  his  im- 
proved steam-engines  required  no  further  precision  of  operation  and  construe- 


454 


THE  STEAM-ENGINE. 


Pis.  16. 


tion  than  the  atmospheric  engines,  this  might  have  been  sufficient ;  but  in 
these  engines  it  was  indispensably  necessary  that  the  piston-rod  should  be 
guided  with  a  smooth  and  even  motion  through  the  stuffing-box  in  the  top  of 
the  cylinder,  otherwise  any  shake  or  irregularity  would  cause  it  to  work  loose 
in  the  stuffing-box,  and  either  to  admit  the  air,  or  to  let  the  steam  escape. 
Under  these  circumstances,  the  motion  of  the  rack  and  toothed  arch  head  were 
inadmissible,  since  it  was  impossible  by  such  means  to  impart  to  the  piston- 
rod  that  smooth  and  equable  motion  which  was  requisite.  Another  contrivance 
which  occurred  to  Watt  was,  to  attach  to  the  top  of  the  piston-rod  a  bar,  which 
should  extend  above  the  beam,  and  to  use  two  chains  or  straps,  one  extending 
from  the  top  of  the  bar  to  the  lower  end  of  the  arch  head,  and  the  other  from 
the  bottom  of  the  bar  to  the  upper  end  of  the  arch  head.  By  such  means  the 
latter  strap  would  pull  the  beam  down  when  the  piston  would  descend,  and  the 
former  would  pull  the  beam  up  when  the  piston  would  ascend.  These  con- 
trivances, however,  were  superseded  by  the  celebrated  mechanism  since  called 
the  Parallel  Motion,  one  of  the  most  ingenious  mechanical  combinations  con- 
nected with  the  history  of  the  steam-engine. 

It  will  be  observed  that  the  object  was  to  connect  by  some  inflexible  means 
the  end  of  the  piston-rod  with  the  extremity  of  the  beam,  and  so  to  contrive 
the  mechanism,  that  while  the  end  of  the  beam  would  move  alternately  up  and 
down  in  part  of  a  circle,  the  end  of  the  piston-rod  connected  with  the  beam 
should  move  up  and  down  in  a  straight  line.  If  the  end  of  the  piston-rod  were 
fastened  upon  the  end  of  the  beam  by  a  pivot  without  any  other  connexion,  it 
is  evident  that,  being  moved  up  and  down  in  the  arch  of  a  circle,  it  would  be 
drawn  to  the  left  and  the  right  alternately,  and  would  consequently  either  be 
broken  or  bent,  or  would  work  loose  in  the  stuffing-box.  Instead  of  connect- 
ing the  end  of  the  rod  immediately  with  the  end  of  the  beam  by  a  pivot.  Watt 
proposed  to  connect  them  by  certain  moveable  rods,  so  arranged  that,  as  the 
end  of  the  beam  would  move  up  and  down  in  the  circular  arch,  the  rods  would 
so  accommodate  themselves  to  that  motion,  that  the  end  connected  with  the 
piston-rod  should  not  be  disturbed  from  its  rectilinear  course. 

To  explain  the  principle  of  the  mechanism  called  the  parallel  motion,  let  us 
suppose  that  0  P,  fig.  17,  is  a  rod  or  lever  moveable  on  a  centre  O,  and  that 
the  end  P  of  this  rod  shall  move  through  a  circular  arch  P  P'  P"  V"  in  a 
vertical  plane,  and  let  its  play  be  limited  by  two  stops  S,  which  shall  prevent 
its  ascent  above  the  point  P,  and  its  descent  below  the  point  V" .  Let  the 
position  of  the  rod  and  the  limitation  of  its  play  be  such  that  the  straight  line 
A  B  drawn  through  P  V",  the  extreme  positions  of  the  lever  O  P,  shall  be  a 
vertical  line. 

Let  0  be  a  point  on  the  other  side  of  the  vertical  line  A  B,  and  let  the  dis- 
tance of  0  to  the  right  of  A  B  be  the  same  as  the  distance  of  a  to  the  left  of 
A  B.     Let  0  j5  be  a  rod  equal  in  length  to  O  P,  moving  like  O  P  on  the  cen-  - 
tre  0,  so  that  its  extremity  p  shall  play  upward  and  downward  through  the  arch  \ 
p  p'  p"  p'",  its  play  being  limited  in  like  manner  by  stops  s.  i 


THE   STEAM-ENGINE. 


455 


p//» 


a;' 


x" 


Now,  let  us  suppose  that  the  ends  P  p  of  these  two  rods  are  joined  by  a 
link  P  p,  the  connexion  being  made  by  a  pivot,  so  that  the  angles  formed  by 
the  link  and  the  rods  shall  be  capable  of  changing  their  magnitude.  This 
link  will  make  the  motion  of  one  rod  depend  on  that  of  the  other,  since  it  will 
preserve  their  extremities  P  p  always  at  the  same  distance  from  each  other. 
If,  therefore,  we  suppose  the  rod  O  P  to  be  moved  to  the  position  0  V",  its 
extremity  P  tracing  the  arch  P  P'  P"  P'",  the  link  connecting  the  rods  will 
at  the  same  time  drive  the  extremity  p  of  the  rod  op  through  the  arch,  p p' p" p'", 
so  that  when  the  extremity  of  the  one  rod  arrives  at  P"',  the  extremity  of  the 
other  rod  will  irrive  at  p'".  By  this  arrangement,  in  the  simultaneous  motion 
of  the  rods,  whether  upward  or  downward,  through  the  circular  arches  to 
which  their  play  is  limited,  the  extremities  of  the  link  joining  them  will  devi- 
ate from  the  vertical  line  A  B  in  opposite  directions.     At  the  limits  of  their 


456 


THE   STEAM-ENGINE. 


play,  the  extremities  of  the  link  will  always  be  in  the  line  A  B  ;  but  in  all  in-  i 
terraediate  positions,  the  lower  extremity  of  the  link  will  be  to  the  right  of  ' 
A  B,  and  its  upper  extremity  to  the  left  of  A  B.  So  far  as  the  derangement  i 
of  the  lower  extremity  of  the  link  is  concerned,  the  matter  composing  the  link  ' 
would  be  transferred  to  the  right  of  A  B  ;  and  so  far  as  the  upper  extremity  of  - 
the  link  is  concerned,  the  matter  composing  it  would  be  transferred  to  the  left 
of  A  B. 

By  the  combined  effects  of  these  contrary  derangements  of  the  extremities 
of  the  link  from  the  vertical  line,  it  might  be  expected  that  a  point  would  ex- 
ist, in  the  middle  of  the  link,  where  the  two  contrary  derangements  would 
neutralize  each  other,  and  which  point  would  therefore  be  expected  to  be  dis- 
turbed neither  to  the  right  nor  to  the  left,  but  to  be  moved  upward  and  down- 
ward in  the  vertical  line  A  B.  Such  is  the  principle  of  the  parallel  motion  ; 
and  in  fact  the  middle  point  of  the  link  will  move  for  all  practical  purposes  ac- 
curately in  the  vertical  line  A  B,  provided  that  the  angular  play  of  the  levers 
0  P  and  op  does  not  exceed  a  certain  limit,  within  which,  in  practice,  their 
motion  may  always  be  restrained. 

To  trace  the  motion  of  the  middle  point  of  the  link  more  minutely,  let 
p  p/  p//  p///  ]jg  ^Qyj.  positions  of  the  lever  0  P,  and  let  p  p'  p"  p'"  be  the  four 
corresponding  positions  of  the  lever  o  p.  In  the  positions  0  P  o  jo,  the  link 
will  take  the  position  P  />  in  which  the  entire  link  will  be  vertical,  and  its 
middle  point  x  will  therefore  be  in  the  vertical  line  A  B. 

When  the  one  rod  takes  the  position  O  P^,  the  other  rod  will  have  the  po- 
sition 0  p' ;  and  the  link  will  have  the  position  P'  p' .  The  middle  point  of 
the  link  will  be  at  a:^,  which  will  be  found  to  be  on  the  vertical  line  A  B. 
Thus  one  half  of  the  link  P'  x'  will  be  to  the  left  of  the  vertical  line  A  B  ; 
while  the  other  half,  p'  x',  will  be  to  the  right  of  the  vertical  line  ;  the  de- 
rangement from  the  vertical  line  affecting  each  half  of  the  link  in  contrary 
directions. 

Again,  taking  the  one  rod  in  the  position  0  P^^,  the  corresponding  position 
of  the  other  rod  will  be  o  p",  and  the  position  of  the  link  will  be  P"  p".  If 
the  middle  point  of  the  link  in  this  position  be  taken,  it  will  be  found  to  be  at 
x",  on  the  vertical  line  A  B  ;  and,  as  before,  one  half  of  the  link  P"  x"  will  be 
thrown  to  the  left  of  the  vertical  line,  while  the  other  half,  p"  x",  will  be 
thrown  to  the  right  of  the  vertical  line. 

Finally,  let  the  one  rod  be  in  its  lowest  position,  O  P'",  while  the  other  rod 
shall  take  the  corresponding  position,  o  p'" .  The  direction  of  the  link  Y'" p'" 
will  now  coincide  with  the  vertical  line  ;  and  its  middle  point  x'"  will  there- 
fore be  upon  that  line.  The  previous  derangement  of  the  extremities  of  the 
rod,  to  the  right  and  to  the  left,  are  now  redressed,  and  all  the  parts  of  the 
rod  have  assumed  the  vertical  position. 

It  is  plain,  therefore,  that  by  such  means  the  alternate  motion  of  a  point 
such  as  P  or  p,  upward  and  downward  in  a  circular  arch,  may  be  made  to 
produce  the  alternate  motions  of  another  point  a;,  upward  and  downward  in  a 
straight  line. 

Although  the  guidance  of  the  air-pump  rod  in  a  true  vertical  line  is  not  so 
necessary  as  that  of  the  steam-piston,  and  as  the  air-pump  piston  is  always 
brought  down  by  its  own  weight  and  that  of  its  rod,  the  connexion  of  the  air- 
pump  piston-rod  with  the  beam,  by  any  contrivance  of  the  kind  now  described, 
was  not  so  necessary.  Nevertheless,  by  a  slight  addition  to  the  mechanical 
contrivance  which  has  been  just  described,  Watt  obtained  the  means  of  at  once 
preserving  the  true  rectilinear  motion  of  both  piston-rods. 

Let  the  lever  represented  by  0  P,  in  fig.  17,  be  conceived  to  be  prolonged 
to  twice  its  length,  as  represented  in  fig.  18,  so  that  O  P'  shall  be  twice  O  P. 


THE   STEAM-ENGINE. 


457 


Fig.  18. 


Let  the  points  P  p  be  connected  by  a  link,  as  before.  Let  a  link  P^  x',  equal 
in  length  to  the  link  P  p,  be  attached  to  the  point  P'',  and  let  the  extremity  x' 
of  this  link  be  connected  with  the  point  p  by  another  link,  equal  in  length  to 
P  P',  by  pivots  at  x'  and  p,  so  that  the  figure  P  P^  x'  p  shall  be  a  jointed  par- 
allelogram, the  angles  of  which  will  be  capable  of  altering  their  magnitude 
Avith  every  change  of  position  of  the  rods  op  and  0  P.  Thus,  when  the  rod 
0  P  descends,  the  angles  of  the  parallelogram  at  P  and  x^  will  be  diminished 
in  magnitude,  while  the  angles  at  P'  and  p  will  be  increased  in  magnitude. 
Now,  let  a  line  be  conceived  to  be  drawn  from  0  to  x'.  It  is  evident  that 
that  line  will  pass  through  the  middle  point  of  the  link  p  P,  for  the  triangle 
0  P  a:  is  in  all  respects  similar  to  the  greater  triangle  O  p"  x'  only  on  half  the 
scale,  so  that  every  side  of  the  one  is  half  the  corresponding'  side  of  the  other. 
Therefore  P  a;  is  half  the  length  of  P'  x' ;  but  P'  x'  was  made  equal  to  P  p, 
and  therefore  p  x  is  half  of  P  p ;  that  is  to  say,  x  is  the  middle  point  of  P  p. 

It  has  been  already  shown,  that  in  the  alternate  motion  of  the  rods  op,  O  P, 
in  ascending  and  descending,  the  point  x  is  moved  upward  and  downward  in 
a  true  vertical  Une.  Now  since  the  triangle  O  P  a;  is  in  all  respects  similar 
to  0  P^  x',  and  subject  to  a  similar  motion  during  the  ascent  and  descent  of 
the  rods,  it  is  apparent  that  the  point  x'  must  be  subject  to  a  motion  in  all  re- 
spects similar  to  that  which  affects  the  points  x,  except  that  the  point  x'  will 
move  through  double  the  space.  In  fact,  the  principle  of  the  mechanism  is 
precisely  similar  to  that  of  the  common  pantograph,  where  two  rods  are  so 
connected  as  that  the  motion  of  the  one  governs  the  motion  of  the  other  so 
that  whatever  line  or  figure  may  be  described  by  one,  a  similar  line  or  figure 
must  be  described  by  the  other.  Since,  then,  the  point  x  is  moved  upward 
and  downward  in  a  vertical  straight  line,  the  point  x'  will  also  be  moved  in  a 
vertical  straight  line  of  double  the  length. 

If  such  an  arrangement  of  mechanism  as  has  been  here  described  can  be 
connected  with  the  beam  of  the  steam-engine,  so  that  while  the  point  x'  is  at- 
tached to  the  top  of  the  steam-piston,  and  the  space  through  which  it  ascends 
and  descends  shtll  be  equal  to  the  length  of  the  stroke  of  that  piston,  the  point 
X  shall  be  attached  to  the  rod  of  the  air-pump  piston,  the  stroke  of  the  latter 
being  half  that  of  the  steam-piston,  then  the  points  x'  and  x  wiU  guide  the 
motion  of  the  two  pistons  so  as  to  preserve  them  in  true  vertical  straight  lines. 

The  manner  in  which  these  ideas  are  reduced  to  practice  admits  of  easy  ex- 


THE  STEAM-ENGINE. 


planation  :  let  the  point  o  be  the  centre  of  the  great  working  beam,  and  let 
O  P'  be  the  arm  of  the  beam  on  the  side  of  the  steam-cylinder.  Let  P  be  a 
pivot  upon  the  beam,  at  the  middle  point  between  its  centre  0  and  its  extremi- 
ty P'  ;  and  let  the  links  P  p,  P'  x' ,  and  P  p,  be  joined  together,  as  already  de- 
scribed. Let  the  point  or  pivot  o  be  attached  to  some  part  of  the  fixed  framing 
of  the  engine  or  engine-house,  and  let  the  rod  o  p,  equal  to  half  the  arm  of  the 
beam,  be  attached  by  a  pivot  to  the  corner  of  the  parallelogram  at  p.  Let  the 
end  of  the  steam  piston-rod  be  attached  to  the  corner  of  the  parallelogram  x' , 
and  let  the  end  of  the  air-pump  be  attached  to  the  middle  point  x  of  the  link 
V  p ;  by  which  arrangement  it  is  evident  that  the  rectilinear  motion  of  the  two 
piston-rods  will  be  rendered  compatible  with  the  alternate  circular  motions  of 
the  points  P'  and  P  on  the  beam. 

Among  the  many  mechanical  inventions  produced  by  the  fertile  genius  of 
Watt,  there  is  none  which  has  excited  such  universal,  such  unqualified,  and 
such  merited  admiration,  as  that  of  the  parallel  motion.  It  is  indeed  impossi- 
ble, even  for  an  eye  unaccustomed  to  view  mechanical  combinations,  to  behold 
the  beam  of  a  steam-engine  moving  the  pistons,  through  the  instrumentality  of 
the  parallel  motion,  without  an  instinctive  feeling  of  pleasure  at  the  unexpect- 
ed fulfilment  of  an  end  by  means  having  so  little  apparent  connexion  with  it. 
When  this  feeling  was  expressed  to  Watt  himself,  by  those  who  first  beheld 
the  performance  of  this  exquisite  mechanism,  he  exclaimed,  with  his  usual 
vivacity,  that  he  himself,  when  he  first  beheld  his  own  contrivance  in  action, 
was  affected  by  the  same  sense  of  pleasure  and  surprise  at  its  regularity  and 
precision.  He  said  that  he  received  from  it  the  same  species  of  enjoyment 
that  usually  accompanies  the  first  view  of  the  successful  invention  of  another 
person. 

"  Among  the  parts  composing  the  steam-engine,  you  have  doubtless,"  says 
M.  Arago,  "  observed  a  certain  articulated  parallelogram.  At  each  ascent  and 
descent  of  the  piston,  its  angles  open  and  close  with  the  sweetness — I  had  al- 
most said  with  the  grace — which  charms  you  in  the  gestures  of  a  consummate 
actor.  Follow  with  your  eye  alternately  the  progress  of  its  successive  changes, 
and  you  will  find  them  subject  to  the  most  curious  geometrical  conditions. 
You  will  see,  that  of  the  four  angles  of  the  jointed  parallelograms,  three  de- 
scribe circular  arches,  but  the  fourth,  which  holds  the  piston-rod,  is  moved 
nearly  in  a  straight  line.  The  immense  utility  of  this  result  strikes  mechani- 
cians with  even  less  force  than  the  simplicity  of  the  means  by  which  Watt  has 
attained  it." 

The  parallel  motion,  of  which  there  are  several  other  varieties — depending, 
however,  generally  upon  the  same  principle — formed  part  of  a  patent  which 
Mr.  Watt  obtained  in  the  year  1784,  another  part  of  which  patent  was  for  a 
locomotive-engine,  by  which  a  carriage  was  to  be  propelled  on  a  road.  In  a 
letter  to  Mr.  Sraeaton,  dated  22d  October,  in  the  same  year,  Watt  says  : — 

"  I  have  lately  contrived  several  methods  of  getting  entirely  rid  of  all  the 
chains  and  circular  arches  about  the  great  levers  of  steam-engines,  and  never- 
j  theless  making  the  piston-rods  ascend  and  descend  perpendicularly,  without 
any  sliding  motions  or  right-lined  guides,  merely  by  combinations  of  motions 
about  centres  ;  and  with  this  further  advantage,  that  they  answer  equally  well 
to  push  upward  as  to  pull  downward,  so  that  this  method  is  applicable  to  our 
double  engines,  which  act  both  in  the  ascent  and  descent  of  their  pistons. 

"  A  rotative-engine  of  this  species  with  the  new  motion  which  is  now  at 
work  in  our  manufactory  (but  must  be  sent  away  very  soon)  answers  admira- 
bly. It  has  cost  much  brain-work  to  contrive  proper  working-gear  for  these 
double  engines,  but  I  have  at  last  done  it  tolerably  well,  by  means  of  the  cir- 
cular valves,  placed  in  an  inverted  position,  so  as  to  be  opened  by  the  force  of 


THE  STEAM-ENGINE. 


459 


the  steam  ;  and  they  are  kept  shut  by  the  working-g^ar.     We  have  erected  an 
engine  at  Messrs.  Goodwyne  and  Co.'s  brewery,  East  Smithfield,  London." 

By  the  contrivance  which  has  been  explained  above,  the  force  of  the  piston 
in  ascending  and  descending  would  be  conveyed  to  the  working  end  of  the 
beam  ;  and  the  next  problem  which  Watt  had  to  solve  was,  to  produce  by  the 
force  exerted  by  the  working  end  of  the  beam  in  ascending  and  descending  a 
continuous  motion  of  rotation.  In  the  first  instance  he  proposed  to  accomplish 
this  by  a  crank  placed  upon  the  axle  to  which  rotation  was  to  be  imparted,  and 
driven  by  a  rod  connecting  it  with  the  working  end  of  the  beam.     Let  K,  fig.  19, 


Piar.  19. 


be  the  centre,  to  which  motion  is  to  be  imparted  by  the  working  end  H  of 
the  beam.  On  the  axle  K  suppose  a  short  lever  K  I  to  be  fixed,  so  that  when 
K  I  is  turned  round  the  centre  K,  the  axle  must  turn  with  it.  Let  an  iron  rod, 
the  weight  of  which  shall  balance  the  piston  and  piston-rod  at  the  other  end 
of  the  beam,  be  connected  by  joints  with  the  working  end  H  of  the  beam,  and 
the  extremity  I  of  the  lever  K  L  As  the  end  H  of  the  beam  is  moved  upward 
and  downward,  the  lever  K  I  will  be  turned  round  the  centre  K,  taking  suc- 
ceesively  the  positions  represented  by  faint  lines  in  the  figure  ;  and  thus  a  mo- 
tion of  continued  rotation  will  be  imparted  to  the  axle  K. 


This  simple  and  effectual  expedient  of  producing  a  continued  rotatory  mo- 
tion by  a  crank  was  abandoned  by  Watt,  as  already  explained,  by  reason  of  a 
patent  having  been  obtained  upon  information  of  his  experiments  surreptitious- 
ly procured.  To  avoid  litigation,  he  therefore  substituted  for  the  crank  the  sun- 
and-planet  wheel  already  described  ;  but  at  the  expiration  of  the  patent,  which 
restricted  the  use  of  the  crank,  the  sun-and-planet  wheel  was  discontinued  in 
Watt's  engine,  and  the  crank  restored. 

Whether  the  crank  or  the  sun-and-planet  wheel  be  used,  there  is  still  a  dif- 
ficulty in  the  maintenance  of  a  regular  motion  of  rotation.  In  the  various  po- 
sitions which  the  crank  and  connecting-rod  assume  throughout  a  complete 
revolution,  there  are  two  in  which  the  moving  power  loses  all  influence  in  im- 
pelling the  crank.  These  positions  are  those  which  the  crank  assumes  when 
the  piston  is  at  the  top  and  bottom  of  the  cylinder,  and  is  just  about  to  change 
the  direction  of  its  motion.  When  the  piston  is  at  the  bottom  of  the  cylinder, 
the  pivot  I,  fig.  19,  by  which  the  connecting-rod  H  I  is  attached  to  the  end  of 
the  crank,  is  immediately  over  the  axle  K  of  the  crank,  and  under  the  pivot  H, 
which  joins  the  upper  end  of  the  connecting-rod  with  the  beam.  In  fact,  in 
this  position  the  connecting-rod  and  crank  are  in  the  same  straight  line,  ex- 
tending from  the  end  of  the  beam  to  the  axle  of  the  crank.  The  steam,  on 
entering  the  cylinder  below  the  piston,  and  pressing  it  upward,  would  produce 
a  corresponding  downward  force  on  the  connecting  rod  at  H,  which  would  be 
continued  along  the  connecting-rod  and  crank  to  the  axle  K.  It  is  evident 
th^t  such  a  force  could  have  no  tendency  to  turn  the  crank  round,  but  would 
expend  its  whole  energy  in  pressing  the  axle  K  downward. 

The  other  position  in  which  the  power  loses  its  effect  upon  the  crank  is 
when  the  piston  is  at  the  top  of  the  cylinder.  In  this  case,  the  working  end 
of  the  beam  will  be  at  the  lowest  point  of  its  play,  and  the  crank-pin  I  will  be 
immediately  below  the  axle  K  ;  so  that  K  will  be  placed  immediately  between 
H  and  I.  When  the  steam  presses  on  the  top  of  the  piston,  it  will  expend  its 
force  in  drawing  the  end  H  of  the  connecting-rod  upward,  by  which  the  crank- 
pin  I  will  likewise  be  drawn  upward.  It  is  evident  that  this  force  can  have  no 
effect  in  turning  the  crank  round,  but  will  expend  its  whole  energy  in  pro- 
ducing an  upward  strain  on  the  axle  K. 

If  the  crank  were  absolutely  at  rest  in  either  of  the  positions  above  de- 
scribed, it  is  apparent  that  the  engine  could  not  be  put  in  motion  by  the 
st^am  ;  but  if  the  engine  has  been  previously  in  motion,  then  the  mass  of 
matter  forming  the  crank,  and  the  axle  on  which  the  crank  is  formed,  having 
already  had  a  motion  of  rotation,  will  have  a  tendency  to  preserve  the  mo- 
mentum it  has  received,  and  this  tendency  will  be  sufficient  to  throw  the 
crank  K  I  out  of  either  of  those  critical  positions  which  have  been  described. 
Having  once  escaped  these  dead  points,  then  the  connecting-rod  forming  an 
angle,  however  obtuse  or  acute,  with  the  crank,  the  pressure  or  pull  upon  the 
former  will  have  a  tendency  to  produce  rotation  in  the  latter.  As  the  crank 
revolves,  however,  the  influence  of  the  connecting-rod  upon  it  will  vary  accord- 
ing to  the  angle  formed  by  the  connecting-rod  and  crank.  When  that  angle  is 
a  right  angle,  then  the  effect  of  the  connecting-rod  on  the  crank  is  greatest, 
since  the  force  upon  it  has  the  advantage  of  the  whole  leverage  of  the  crank  ; 
but  according  as  the  angle  formed  by  the  crank  and  connecting-rod  becomes 
more  or  less  acute  or  obtuse  in  the  successive  attitudes  which  they  assume  in 
the  revolution  of  the  crank,  the  influence  of  the  connecting-rod  over  the  crank 
varies,  changing  from  nothing  at  the  two  dead  points  already  described,  to  the 
full  effect  produced  in  the  two  positions  where  they  are  at  right  angles.  In 
consequence  of  this  varying  leverage,  by  which  the  force  with  which  the  con- 
necting-rod is  driven  by  the  steam  is  transmitted  to  the  axle  on  which  the  crank 


THE   STEAM-ENGINE. 


461 


)  reA^olves,  a  corresponding  variation  of  speed  would  necessarily  be  ptoduced  in  < 
s  the  motion  imparted  to  the  crank.  The  speed  at  the  dead  points  would  be  ' 
?  least,  being  due  altogether  to  the  momentum  already  imparted  to  the  revolving  ' 
i  mass  of  the  crank  and  axle  ;  and  it  would  gradually  increase  and  be  greatest  at  [ 
)  the  points  where  the  effect  of  the  crank  on  the  connecting-rod  is  greatest.  Al- 
i  though  this  change  of  speed  would  not  affect  the  actual  mechanical  efficacy  of 

<  the  machine,  and  although  the  same  quantity  of  steam  would  perform  the  same 
5  work  at  the  varying  velocity  as  it  would  do  if  the  velocity  were  regulated,  yet 
?  this  variation  of  speed  would  be  incompatible  with  the  purposes  to  which  it 
S  was  now  proposed  that  the  steam-engine  should  be  applied  in  manufactures. 
(  In  these  a  regular  uniform  motion  should  be  imparted  to  the  main  axle. 

)  One  of  the  expedients  which  Watt  proposed  for  the  attainment  of  this  end 
(  was,  by  placing  two  cranks  on  the  same  axle,  in  different  positions,  to  be 
)  worked  by  different  cylinders,  so  that  while  one  crank  should  be  at  its  dead 
}  points,  the  other  should  be  in  the  attitude  most  favorable  for  its  action.  This 
)  expedient  has  since,  as  we  shall  see,  been  carried  into  effect  in  steam-vessels  ; 
?  but  one  more  simple  and  efficient  presented  itself  in  the  use  of  b,  fly -wheel. 
)  On  the  main  axle  driven  by  the  crank  Watt  placed  a  large  wheel  of  metal, 
(  called  di  Jly-wheel.  This  wheel  being  well  constructed,  and  nicely  balanced 
;  on  its  axle,  was  subject  to  very  little  resistance  from  friction  ;  any  moving 
\  force  which  it  would  receive  it  would  therefore  retain,  and  would  be  ready  to 
>  impart  such  moving  force  to  the  main  axle  whenever  that  axle  ceased  to   be 

<  driven  by  the  power.  When  the  crank,  therefore,  is  in  those  positions  in 
)  which  the  action  of  the  power  upon  it  is  most  efficient,  a  portion  of  the  energy 
I  of  the  power  is  expended  in  increasing  the  velocity  of  the  mass  of  matter 
)  composing  the  fly-wheel.     As  the  crank  approaches  the  dead  points,  the  effect 

<  of  the  moving  power  upon  the  axle  and  upon  the  crank  is  generally  enfeebled, 
;  and  at  these  points  vanishes  altogether.     The  momentum  which  has  been  im- 

<  parted  to  the  fly-wheel  then  comes  into  play,  and  carries  forward  the  axle  and 
5  crank  out  of  the  dead  points  with  a  velocity  very  little  less  than  that  which  it 
s  had  when  the  crank  was  in  the  most  favorable  position  for  receiving  the  action 
)  of  the  moving  power. 

\  By  this  expedient,  the  motion  of  revolution  received  by  the  axle  from  the 
)  steam-piston  is  subject  to  no  other  variation  than  just  the  amount  of  change  of 
s  momentum  in  the  great  mass  of  the  fly-wheel,  which  is  sufficient  to  extricate 
)  the  crank  twice  in  every  revolution  from  the  mechanical  dilemma  to  which  its 
s  peculiar  form  exposes  it ;  and  this  change  of  velocity  may  be  reduced  to  as 
;  small  an  amount  as  can  be  requisite  by  giving  the  necessary  weight  and  magni- 
\  tude  to  the  fly-wheel. 

)  By  such  arrangements  the  motion  imparted  to  the  main  axle  K  would  be 
J  uniform,  provided  that  the  moving  power  of  the  engine  be  always  proportionate 
5  to  the  load  which  it  drives.  But  in  the  general  application  of  the  steam-engine 
S  to  manufactures  it  was  evident  that  the  amount  of  the  resistance  to  which  any 
)  given  machine  would  be  subject  must  be  liable  to  variation.  If,  for  example, 
S  the  engine  drive  a  cotton-mill,  it  will  have  to  impart  motion  to  all  the  spinnino- 
)  frames  in  that  mill.  The  operation  of  one  or  more  of  these  may  from  time  to 
S  time  be  suspended,  and  the  moving  power  would  be  relieved  from  a  cor- 
)  responding  amount  of  resistance.  If,  under  such  circumstances,  the  energy 
S  of  the.  moving  power  remained  the  same,  the  velocity  with  which  the  machines 
/  would  be  driven  would  be  subject  to  variation,  being  increased  whenever  the 
S  operation  of  any  portion  of  the  machines  usually  driven  by  it  is  suspended  ; 
(  and,  on  the  other  hand,  diminished  when  any  increased  number  of  machines 
)  are  brought  into  operation.     In  fine,  the  speed  would  vary  nearly  in  the  inverse 

<  proportion  of  the  load  driven,  increasing  as  the  load  is  diminished,  and  vice  versa.  , 


THE  STEAM-ENGINE. 


On  the  other  hand,  supposing  that  no  change  took  place  in  the  amount  of  the 
load  driven  by  the  engine,  and  that  the  same  number  of  machines  of  whatever 
kind  would  have  to  be  continually  driven,  the  motion  imparted  to  the  main  axle 
would  still  be  subject  to  variation  by  the  changes  inevitable  to  the  moving 
power.  The  piston  of  the  engine  being  subject  to  an  unvaried  resistance,  a 
uniform  motion  could  only  be  imparted  to  it,  by  maintaining  a  corresponding 
uniformity  in  the  impelling  power.  This  would  require  a  uniform  supply  of 
steam  from  the  boiler,  which  would  further  imply  a  uniform  rate  of  evaporation 
in  the  boiler,  unless  means  were  provided  in  the  admission  of  steam  from  the 
boiler  to  the  cylinder  to  prevent  any  excess  of  steam  which  might  be  produced 
in  the  boiler  from  reaching  the  cylinder. 

This  end  was  attained  by  a  contrivance  afterward  called  the  throttle-valve. 
An  axis  A  B  figs.  20,  21,  was  placed  across  the  steam-pipe  in  a  ring  of  cast- 


Fig.  20. 


iron  D  E,  of  proper  thickness.  On  this  axis  was  fastened  a  thin  circular 
plate  T,  of  nearly  the  same  diameter  as  the  steam-pipe.  On  the  outer  end  B 
of  this  axle  was  placed  a  short  lever  or  handle  B  C,  by  which  it  could  be 
turned.  When  the  circular  plate  T  was  turned  into  such  a  position  as  to  be 
at  right  angles  to  the  length  of  the  tube,  it  stopped  the  passage  within  the 
tube  altogether,  so  that  no  steam  could  pass  from  the  boiler  to  the  engine. 
On  the  other  hand,  when  the  handle  was  turned  through  a  fourth  of  a  revolu- 
tion from  this  position,  then  the  circular  plate  T  had  its  plane  in  the  direction 
of  the  length  of  the  tube,  so  that  its  edge  would  be  presented  toward  the  cur- 
rent of  steam  flowing  from  the  boiler  to  the  cylinder.  In  that  position  the 
passage  within  the  tube  would  be  necessarily  unobstructed  by  the  throttle-valve. 
In  intermediate  positions  of  the  valve,  as  that  represented  in  figs.  20,  21,  the 
passage  might  be  left  more  or  less  opened,  so  that  steam  from  the  boiler  might 
be  admitted  to  the  cylinder  in  any  regulated  quantity  according  to  the  position 
given  to  the  lever  B  C. 

A  view  of  the  throttle-valve  taken  by  a  section  across  the  steam-pipe  is  exhibit- 
ed in  fig.  21,  and  a  section  of  it  through  the  axis  of  the  steam-pipe  is  represented 
in  fig.  20.  The  form  of  the  valve  is  such,  that,  if  accurately  constructed,  the 
steam  in  passing  from  the  boiler  would  have  no  effect  by  its  pressure  to  alter 
any  position  which  might  be  given  to  the  valve ;  and  any  slight  inaccuracy 
of  form  which  might  give  a  tendency  to  the  steam  to  alter  the  position  would 
be  easily  counteracted  by  the  friction  of  the  valve  upon  its  axle.  The  latter 
might  be  regulated  at  pleasure. 


THE  STEAM-ENGINE. 


463 


By  this  expedient,  however  the  evaporation  of  w^ater  in  the  boiler  might  S 
vary  within  practical  limits,  the   supply  of  steam  to  the   cylinder  would  be  < 
rendered  regular  and  uniform.     If  the  boiler  became  too  active,  and  produced  ) 
more   steam  than   was  necessary  to  move  the  engine  with  its  load  at  the  re-  ( 
quisite  speed,  then  the  throttle-valve  was  shifted  so  as  to  contract  the  passage  | 
and  limit  the  supply  of  steam.     If,  on  the  other  hand,  the  process  of  evapora- 
tion in  the  boiler  was  relaxed,  then  the  throttle-valve  was  placed  with  its  edge  j 
more  directed  toward  the  steam.     Independently  of  the  boiler,  if  the  load  on  < 
the  engine  was  lightened,  then  the  same  supply  of  steam  to  the  cylinder  would  , 
unduly  accelerate  the  motion.     In  this  case,  likewise,  the  partial  closing  of  the  < 
throttle -valve   would  limit  the  supply  of  steam  and  regulate  the  motion ;  and  , 
if,  on  the  other  hand,  the  increase  of  load  upon  the  engine  rendered  necessary  ' 
an   increased   supply  of  steam,  then  the  opening  of  the  throttle-valve   would  ' 
accomplish  the  purpose.     By  these  means,  therefore,  a  uniform  motion  might  ' 
be  maintained,  provided  the  vigilance  of  the  engine-man  was  sufficient  for  the 
due  management  of  the  lever  B   C,  and  provided  that  the  furnace  under  the 
boiler  was  kept  in  sufficient  activity  to  supply  the  greatest  amount  of  steam 
which  would  be  necessary  for  the  maintenance  of  a  uniform  motion  with  the 
throttle-valve  fully  opened. 

Watt,  however,  soon  perceived  that  the  proper  manipulation  of  the  lever  B 
C  would  be  impracticable  with  any  degree  of  vigilance  and  skill  which  could 
be  obtained  from  the  persons  employed  to  attend  the  engine.  He,  therefore, 
adapted  to  this  purpose  a  beautiful  application  of  a  piece  of  mechanism,  which 
had  been  previously  used  in  the  regulation  of  mill-work,  and  which  has  since 
been  well  known  by  the  name  of  the  governor,  and  has  always  been  deservedly 
a  subject  of  much  admiration. 

The  governor  is  an  apparatus  by  which  the  axle  of  the  fly-wheel  is  made  to 
regulate  the  throttle-valve,  so  that  the  moment  that  the  axle  begins  to  increase 
its  velocity,  it  shifts  the  position  of  the  throttle-valve,  so  as  to  limit  the  supply 
of  steam  from  the  boiler,  and  thereby  to  check  the  increase  of  speed.  And  on 
the  other  hand,  whenever  the  velocity  of  the  axle  is  diminished,  the  lever  B  C 
is  moved  in  the  contrary  direction,  so  as  to  open  more  fully  the  passage  for 
the  steam,  and  accelerate  the  motion  of  the  engine. 

A  small  grooved  wheel  A  B  fig.  22,  is  attached  to  a  vertical  spindle  sup- 
ported in  pivots  or  sockets  C  and  D,  in  which  it  is  capable  of  revolving.  An 
endless  cord  works  in  the  groove  A  B,  and  is  carried  over  proper  pulleys  to 
the  axle  of  the  fly-wheel,  where  it  likewise  works  in  a  groove.  When  this 
cord  is  properly  tightened  the  motion  of  the  fly-wheel  will  give  motion  to  the 
wheel  A  B,  so  that  the  velocity  of  the  one  will  be  subject  to  all  the  changes 
incidental  to  the  velocity  of  the  other.  By  this  means  the  speed  of  the  grooved 
wheel  A  B  may  be  considered  as  representing  the  speed  of  the  fly-wheel,  and 
of  the  machinery  which  the  axle  of  the  fly-wheel  drives. 

It  is  evident  that  the  same  end  might  be  attained  by  substituting  for  the 
grooved  wheel  A  B  a  toothed  wheel,  which  might  be  connected  by  other 
toothed  wheels,  and  proper  shafts,  and  axles  with  the  axle  of  the  fly-wheel. 

A  ring  or  collar  E  is  placed  on  the  upright  spindle,  so  as  to  be  capable  of 
moving  freely  upward  and  downward.  To  this  ring  are  attached  by  pivots 
two  short  levers,  E  F,  the  pivots  or  joints  at  E  allowing  these  levers  to  play 
upon  them.  At  F  these  levers  are  joined  by  pivots  to  other  levers  F  G, 
which  cross  each  other  at  H,  where  an  axle  or  pin  passes  through  them,  and 
attaches  them  to  the  upright  spindle  C  D.  These  intersecting  levers  are 
capable,  however,  of  playing  on  this  axle  or  pin  H.  To  the  ends  G  of  these 
levers  are  attached  two  heavy  balls  of  metal  I.  The  levers  F  G  pass  through 
slits  in  a  metallic  arch  attached  to  the  upright  spindle,  so  as  to  be  capable  of 


464 


THE  STEAM-ENGINE. 


revolving  upon  it.  If  the  balls  I  are  drawn  outward  from  the  vertical  axis,  it 
is  evident  that  the  ends  F  of  the  levers  will  be  drawn  down,  and  therefore  the 
pivots  E  likewise  drawn  down.  In  fact,  the  angles  E  F  H  will  become  more 
acute,  and  the  angle  F  E  F  more  obtuse.  By  these  means  the  sliding  ring  E 
will  be  drawn  down.  To  this  sliding  ring  E,  and  immediately  above  it,  is  at- 
tached a  grooved  collar,  which  slides  on  the  vertical  spindle  upward  and  down- 
ward with  the  ring  E.  In  the  grooved  collar  are  inserted  the  prongs  of  a  fork 
K,  formed  at  the  end  of  the  lever  K  L,  the  fulcrum  or  pivot  of  the  lever  being 
at  L.  By  this  arrangement,  when  the  divergence  of  the  balls  I  causes  the 
collar  E  to  be  drawn  down,  the  fork  K,  whose  prongs  are  inserted  in  the  groove 
of  that  collar,  is  likewise  drawn  down  ;  and,  on  the  other  hand,  when,  by 
reason  of  the  balls  I  falling  toward  the  vertical  spindle,  the  collar  E  is  raised, 
the  fork  K  is  likewise  raised. 

The  ascent  and  descent  of  the  fork  K  necessarily  produce  a  contrary  motion 
in  the  other  end  N  of  the  lever.  This  end  is  connected  by  a  rod,  or  system 
of  rods,  with  the  end  M  of  the  short  lever  which  works  the  throttle-valve  T. 
By  such  means  the  motion  of  the  balls  I,  toward  or  from  the  vertical  spindle, 
produces  in  the  throttle-valve  a  corresponding  motion  ;  and  they  are  so  con- 
nected that  the  divergence  of  the  balls  I  will  cause  the  throttle-valve  to  close, 
while  their  descent  toward  the  vertical  spindle  will  cause  it  to  open. 

These  arrangements  being  comprehended,  let  us  suppose  that,  either  by 
reason  of  a  diminished  load  upon  the  engine  or  an  increased  activity  of  the 
boiler,  the  speed  has  a  tendency  to  increase.  This  would  impart  increased 
velocity  to  the  grooved  wheel  A  B,  which  would  cause  the  balls  I  to  revolve 
with  an  accelerated  speed.  The  centrifugal  force  which  attends  their  motion 
Avould  therefore  give  them  a  tendency  to  move  from  the  axle,  or  to  diverge. 
This  would  cause,  by  the  means  already  explained,  the  throttle-valve  T  to  be 
partially  closed,  by  which  the  supply  of  steam  from  the  boiler  to  the  cylinder 
would  be  diminished,  and  the  energy  of  the  moving  power,  therefore,  mitigated. 
The  undue  increase  of  speed  would  thereby  be  prevented. 

If,  on  the  other  hand,  either  by  an  increase  of  the  load,  or  a  diminished 
activity  in  the  boiler,  the  speed  of  the  machine  was  lessened,  a  corresponding 
diminution  of  velocity  would  take  place  in  the  grooved  wheel  A  B.     This 


THE  STEAM-ENGINE. 


would  cause  the  balls  I  to  revolve  with  less  speed,  and  the  centrifugal  force 
produced  by  their  circular  motion  would  be  diminished.  This  force  being 
thus  no  longer  able  fully  to  counteract  their  gravity,  they  would  fall  toward  the 
spindle,  which  would  cause  as,  already  explained,  the  throttle-valve  to  be 
more  fully  opened.  This  would  produce  a  more  ample  supply  of  steam  to 
the  cylinder,  by  which  the  velocity  of  the  machine  would  be  restored  to  its 
proper  amount. 

The  principle  which  renders  the  governor  so  perfect  a  regulator  of  the 
velocity  of  the  machine  is  difficult  to  be  explained  without  having  recourse  to 
the  aid  of  the  technical  language  of  mathematical  physics.  As,  however,  this 
instrument  is  of  such  great  practical  importance,  and  has  attracted  such  general 
admiration,  it  may  be  worth  while  here  to  attempt  to  render  intelligible  the 
mechanical  principles   which   govern   its   operation.     Let   S   fig.  23,  be   the 

Fig.  23. 


-B 


point  of  suspension  of  a  common  pendulum  S  P,  and  let  P  0  P'  be  the  arch 
of  its  vibration,  so  that  the  ball  P  shall  swing  or  vibrate  alternately  to  the  east 
and  to  the  west  of  the  lowest  point  O,  through  the  arches  O  P'  and  0  P.  It 
is  a  property  of  such  an  instrument  that,  provided  the  arch  in  which  it  vibrates 
be  not  considerable  in  magnitude,  the  time  of  its  vibration  will  be  the  same 
whether  the  arch  be  long  or  short.  Thus,  for  example,  if  the  pendulum,  in- 
stead of  vibrating  in  the  arch  P  P',  vibrated  in  the  arch  p  p',  the  time  which  it 
would  take  to  perform  its  vibrations  would  be  the  same.  If,  however,  the 
magnitude  of  the  arch  of  vibration  be  increased,  then  a  variation  will  take  place 
in  the  time  of  vibration  ;  but  unless  the  arch  of  vibration  be  considerably  in- 
creased, this  variation  will  not  be  great. 

Now  let  it  be  supposed  that  while  the  pendulum  P  P'  continues  to  vibrate 
east  and  west  through  the  arch  P  P',  it  shall  receive  such  an  impulse  from 
north  and  south  as  would,  if  it  were  not  in  a  state  of  previous  vibration,  cause 
it  to  vibrate  between  north  and  south,  in  an  arch  similar  to  the  arch  P  P'. 
This  second  vibration  between  north  and  south  would  not  prevent  the  continu- 
ance of  the  other  vibration  between  east  and  west ;  but  the  ball  P  would  be  at 
the  same  time  affected  by  both  vibrations.  While,  in  virtue  of  the  vibration 
from  east  to  west,  the  ball  would  swing  from  P  to  V\  it  would,  in  virtue  of  the 
other  vibration,  extend  its  motion  toward  the  north  to  a  distance  from  the  line 
W  E  equal  to  half  a  vibration,  and  will  return  from  that  distance  again  to  the 
position  P'.  While  returning  from  P^  to  P,  its  second  vibration  will  carry  it 
toward  the  south  to  an  equal  distance  on  the  southern  side  of  W  E,  and  it  will 
return  again  to  the  position  P.  If  the  combination  of  these  two  motions  or 
VOL.  II.— 30 


THE  STEAM-ENGINE. 


vibrations  be  attentively  considered,  it  will  be  perceived  that  the  effect  on  the 
ball  -will  be  a  circular  motion,  precisely  similar  to  the  circular  motion  of  the 
balls  of  the  governor  already  described. 

Now  the  time  of  vibration  of  the  pendulum  S  P  between  east  and  west  will 
not  in  any  way  be  affected  by  the  second  vibration,  which  it  is  supposed  to 
receive  between  north  and  south,  and  therefore  the  time  the  pendulum  takes 
in  moving  from  P  to  P'  and  back  again  from  P'  to  P  will  be  the  same  whether 
it  shall  have  simultaneously  or  not  the  other  vibration  between  north  and  south. 
Hence  it  follows  that  the  time  of  revolution  of  the  circular  pendulum  will  be 
equal  to  the  time  of  similar  vibrations  of  the  same  pendulum,  if,  instead  of 
having  a  circular  motion,  it  were  allowed  to  vibrate  in  the  manner  of  a  common 
pendulum. 

If  this  point  be  understood,  and  if  it  also  be  remembered  that  the  time  of 
vibration  of  a  common  pendulum  is  necessarily  the  same  whether  the  arch  of 
vibration  be  small  or  great,  it  will  be  easily  perceived  that  the  revolving 
pendulum  or  governor  will  have  nearly  the  same  time  of  revolution  whether  it 
revolve  in  a  large  circle  or  a  small  one  :  in  other  words,  whether  the  balls 
revolve  at  a  greater  or  a  less  distance  from  the  central  spindle  or  axis.  This, 
however,  is  to  be  understood  only  approximately.  When  the  angle  of  diverg- 
ence of  the  balls  is  as  considerable  as  it.  usually  is  in  governors,  the  time  of 
revolution  at  different  distances  from  the  axis  will  therefore  be  subject  to  some 
variation,  but  to  a  very  small  one. 

The  centrifugal  force  (which  is  the  name  given  in  mechanics  to  that  influence 
which  makes  a  body  revolving  in  a  circle  fly  from  the  centre)  depends  con- 
jointly on  the  velocity  of  revolution,  and  on  the  distance  of  the  revolving  body 
from  the  centre  of  the  circle.  If  the  velocity  of  revolution  be  the  same,  then 
the  centrifugal  force  will  increase  in  the  same  proportion  as  the  distance  of 
the  revolving  body  from  the  centre.  If,  on  the  other  hand,  the  distance  of  the 
revolving  body  from  the  centre  remain  the  same,  the  centrifugal  force  will  in- 
crease in  the  same  proportion  as  the  square  of  the  time  of  vibration  diminishes, 
or,  in  other  words,  it  will  increase  in  the  same  proportion  as  the  square  of  the 
number  of  revolutions  per  minute.  It  follows  from  this,  therefore,  that  the 
greater  is  the  divergence  of  the  balls  of  the  governor,  and  the  more  rapidly 
they  revolve,  the  greater  will  be  their  centrifugal  force.  Now  this  centrifugal 
force,  if  it  were  not  counterbalanced,  would  give  the  balls  a  constant  tendency 
to  recede  from  the  centre  ;  but  from  the  construction  of  the  apparatus,  the 
further  they  are  removed  from  the  centre  the  greater  will  be  the  effect  of  their 
gravitation  in  resisting  the  centrifugal  force. 

It  is  evident  that  the  ball  at  P  will  have  a  greater  tendency  to  fall  by  gravita- 
tion toward  0  than  it  would  have  at  p,  because  the  acclivity  of  the  arch  descend- 
ing toward  0  at  P  is  greater  than  its  acclivity  at  p.     The   gravitation,  there- 
fore, or  tendency  of  the  ball  to  fall  toward  the  central  axis  being  greater  at  P 
than  at  p  it  will  be  able  to  resist  a  greater  centrifugal  force.     This  increased 
centrifugal  force,  which  the  ball  would  have  revolving  at  the  distance  P  above 
I  what  it  would  have  at  the  distance  p,  is  produced  partly  by  the  greater  distance 
'  of  the  ball  from  the   central  axis,  and  partly  by   the   greater  velocity  of  its 
I  motion.     But  it  will  be  evident  that  the  time  of  its  revolution  may  neverthe- 
'  less  be  the  same,  or  nearly  the. same,  at  both  distances.     If  it  should  appear 
,  that  the   actual  velocity  of  its  motion  of  revolution  at  P  be  greater  than  its 
•  velocity  at  p,  in  the  same  proportion  as  the  circles  in  which  they  revolve,  then 
)  it  is  evident  that  the  time  of  revolution  would  be  as  much  increased  by  the 
'  greater  space  which  P  will  have  to  travel  over,  as  it  will  have  to  be  diminish- 
\  ed   by  the  greater  speed  with  which  that  space  is  traversed.     The   time  of 
[  revolution,  therefore,  may  be  the  same,  or  nearly  the  same,  in  both  cases. 


THE   STEAM-ENGINE, 


•  If  this  explanation  be  comprehended,  it  will  not  be  difficult  to  apply  it  to  the 
[  actual  case  of  the  governor.     If  a  sudden  increase  of  the  energy  of  the  moving 

•  power,  or  a  diminution  of  the  load,  should  give  the  machine  an  increased  ve- 
\  locity,  then  the  increased  speed  of  the  balls  of  the  governor  will  give  them  an 
'  increased  centrifugal  force,  which  for  the  moment  will  be  greater  than  the  ten- 
[  dency  of  their  gravitation  to   make  them  fall  toward  the  vertical   axis.     This 

centrifugal  force,  therefore,  prevailing,  the  balls  will  recede  from  the  axis  ;  but 
as  they  recede,  their  gravitation  toward  the  vertical  axis  will,  as  has  been  al- 
ready explained,  be  increased,  and  will  become  equal  to  the  centrifugal  force 
produced  by  the  increased  velocity,  provided  that  velocity  do  not  exceed  a 
certain  limit.  When  the  balls,  by  diverging,  get  such  increased  gravitation  as 
to  balance  the  centrifugal  force,  then  they  will  continue  to  revolve  at  a  fixed 
distance  from  the  vertical  axis.  When  this  happens,  the  time  of  the  revolu- 
tion must  be  nearly  the  same  as  it  was  before  their  increased  divergence  ;  in 
other  words,  the  proportion  of  the  moving  power  to  the  load  will  be  so  restored 
by  the  action  of  the  levers  of  the  governor  on  the  throttle-valve  that  the  machine 
will  move  at  its  former  velocity,  or  nearly  so. 

The  principle  on  which  the  governor  acts,  as  just  explained,  necessarily 
supposes  temporary  disarrangements  of  the  speed.  In  fact,  the  governor,  strictly 
speaking,  does  not  maintain  a  uniform  velocity,  but  restores  it  after  it  has  been 
disturbed.  When  a  sudden  change  of  motion  of  the  engine  lakes  place,  the 
governor,  being  immediately  affected,  will  cause  a  corresponding  alteration  in 
the  throttle-valve  ;  and  this  will  not  merely  correct  the  change  of  motion,  but  it 
will,  as  it  were,  overdo  it,  and  will  cause  a  derangement  of  speed  of  the  oppo- 
site kind.  Thus  if  the  speed  be  suddenly  increased  to  an  undue  amount,  then 
the  governor  being  affected  will  first  close  the  throttle-valve  too  much,  so  as  to 
reduce  the  speed  below  the  proper  limit.  This  second  error  will  again  affect 
the  governor  in  the  contrary  way,  and  the  speed  will  again  be  increased  rather 
too  much.  In  this  way  a  succession  of  alterations  of  effect  will  ensue  until 
the  governor  settles  down  into  that  position  in  which  it  will  maintain  the  en- 
gine at  the  proper  speed. 

To  prevent  the  inconvenience  which  would  attend  any  excess  of  such  varia- 
tions, the  governor  is  made  to  act  with  great  delicacy  on  the  throttle-valve,  so 
that  even  a  considerable  change  in  the  divergence  of  the  balls  shall  not  pro- 
duce too  much  alteration  in  the  opening  of  that  valve  :  the  steam  in  the  boiler 
should  have  at  least  two  pounds  per  square  inch  pressure  more  than  is  gener- 
ally required  in  the  cylinder.  This  excess  is  necessary  to  afford  scope  for 
that  extent  of  variation  of  the  power  which  it  is  the  duty  of  the  throttle-valve 
to  regulate. 

The  governor  is  usually  so  adjusted  as  to  make  thirty-six  revolutions  per 
minute,  when  in  uniform  motion  ;  but  if  the  motion  is  increased  to  the  rate  of  ' 
thirty-nine  revolutions,  the  balls  will  fly  to  the  utmost  extent  allowed  them,  be-  i 
ing  the  limitation  of  the  grooves  in  which  their  rods  move  ;  and  if,  on  the  other  ' 
hand,  the  speed  be  diminished  to  thirty-four  revolutions  per  minute,  they  will  , 
collapse  to  the  lowest  extent  of  their  play.  The  duty  of  the  governor,  there-  ' 
fore,  is  to  correct  smaller  casual  derangements  of  the  velocity  ;  but  if  any  per-  i 
manent  change  to  a  considerable  extent  be  made  either  in  the  load  driven  by  ' 
the  machine  or  in  the  moving  power  supplied  to  it  from  the  boiler,  then  a  per-  ( 
•manent  change  is  necessary  to  be  made  in  the  connexion  between  the  governor  ' 
and  the  throttle-valve,  so  as  to  render  the  governor  capable  of  regulating  those  ( 
smaller  changes  to  which  the  speed  of  the  machine  is  liable.  ] 

Having  thus  explained  the  principal  mechanical  contrivances  provided,  by  < 
Watt  for  the  maintenance  and  regulation  of  the  rotatory  motion  to  be  produced  J 
by  his  double-acting  steam-engine,  let  us  now  consider  the  machine  as  a  whole,  i 


THE   STEAM-ENGINE. 


and  investigate  the  process  of  its  operation.     Asection  of  this  engine  is  rep- 
resented in  fig.  24. 


Fig.  24. 


Steam  is  supplied  from  the  boiler  to  the  cylinder  by  the  steam-pipe  S.  The 
throttle-valve  T  in  that  pipe,  near  the  cylinder,  is  regulated  by  a  system  of 
levers  connected  with  the  governor.  The  piston  P  is  accurately  fitted  in  the 
steam-cylinder  C  by  packing,  as  already  described  in  the  single-acting  engine. 
This  piston,  as  it  moves,  divides  the  cylinder  into  two  compartments,  between 
which  there  is  no  communication  by  which  steam  or  any  other  elastic  fluid 
can  pass.  The  upper  steam-box  B  is  divided  into  three  compartments  by  the 
two  valves.  Above  the  upper  steam-valve  V  is  a  compartment  communicating 
with  the  steam-pipe  ;  below  the  upper  exhausting-valve  E  is  another  compart- 
ment communicating  with  the  eduction-pipe  which  leads  to  the  condenser. 
By  the  valves  V  and  E  a  communication  may  be  opened  or  closed  between 
the  boiler  on  the  one  hand,  or  the  condenser  on  the  other,  and  the  top  of  the 
cylinder.  The  continuation  S'  of  the  steam-pipe  leads  to  the  lower  box  B', 
which,  like  the  upper,  is  divided  into  three  compartments  by  two  valves  V 
and  E'.  The  upper  compartment  communicates  with  the  steam-pipe,  and 
thereby  with  the  boiler ;  and  the  lower  con)partment  communicates  with  the 
eduction-pipe,  and  thereby  with  the  condenser.  By  means  of  the  two  valves 
V  and  E',  a  communication  maybe  opened  or  closed  between  the  steam-pipe 


on  the  one  hand,  or  the  exhaiisting-pipe  on  the  other,  and  the  lower  part  of 
the  cylinder.  The  four  valves  V,  E,  V,  and  E',  are  connected  by  a  system 
of  levers  with  a  handle  or  spanner  m,  which,  being  driven  downward  or  up- 
ward, is  capable  of  opening  or  closing  the  valves  in  pairs,  in  the  manner  al- 
ready described.  The  condensers,  the  air-pump,  and  the  hot-water  pump,  are 
in  all  respects  similar  to  those  already  described  in  the  single-acting  engine, 
except  that  the  condensing-jet  is  governed  by  a  lever  I,  by  which  it  is  allowed 
to  play  continually  in  the  condenser,  and  by  which  the  quantity  of  water  ad- 
mitted through  it  is  regulated.  The  cold-water  pump  N  is  worked  by  the  en- 
gine as  already  described  in  the  single-acting  engine,  and  supplies  the  cistern  in 
which  the  air-pump  and  condenser  are  submerged,  so  as  to  keep  down  its  temper- 
ature to  the  proper  limit.  On  the  air-pump  rod  R  are  two  pins  properly  placed, 
so  as  to  strike  the  spanner  m,  upward  and  downward,  at  the  proper  times,  when 
the  piston  approaches  the  termination  of  the  stroke  at  the  top  or  bottom  of  the 
cylinder.  The  pump  L  conducts  the  warm  water  drawn  by  the  air-pump  from 
the  condenser  to  a  proper  reservoir  for  feeding  the  boiler.  The  vertical  mo- 
tion of  the  piston-rod  in  a  straight  line  is  rendered  compatible  with  the  circular 
motion  of  the  end  of  the  beam  by  the  parallel  motion  already  described.  The 
point  b,  on  the  beam,  moves  upward  and  downward  in  a  circular  arch,  of  which 
the  axis  of  the  beam  is  the  centre.  In  like  manner,  the  point  d  of  the  rod  d  c 
moves  upward  and  downward  in  a  similar  arch,  of  which  the  fixed  pivot  c  is  the 
centre.  The  joint  or  bar  d  b,  which  joins  these  two  pivots,  will  be  moved  so 
that  its  middle  point  e  will  ascend  and  descend  nearly  in  a  straight  line,  as  has 
been  already  explained  ;  opposite  this  point  e  is  attached  the  piston-rod  of  the 
air-pump,  which  is  accordingly  guided  upward  and  downward  by  this  means. 
The  jointed  parallelogram  b  d  g  f  is  attached  to  the  beam  by  pivots  ;  and,  as 
has  been  explained,  the  point  g  will  be  moved  upward  and  downward  in  a 
straight  line,  through  twice  the  space  through  which  the  point  e  is  moved.  To 
the  point  g  the  rod  of  the  steam-piston  is  attached.  Thus,  the  rods  of  the 
steam-piston  and  air-pump  are  moved  by  the  same  system  of  jointed  bars, 
and  moved  through  spaces  which  are  in  the  proportion  of  two  to  one. 

Although  this  system  of  jointed  rods  forming  the  parallel  motion,  appears  in 
the  ligure  to  consist  only  of  one  parallelogram  b  d  g  f,  and  one  rod  c  d,  called 
the  radius  rod,  it  is,  in  fact,  double,  a  similar  parallelogram  and  radius  rod  be- 
ing attached  to  corresponding  points,  and  in  the  same  manner  on  the  other  side 
of  the  beam  ;  but  from  the  view  given  in  the  cut,  the  one  set  of  rods  hides  the 
other.  The  two  systems  of  rods  thus  attached  to  opposite  sides  of  the  beam 
at  several  inches  asunder,  are  connected  by  cross  rods,  the  ends  of  which  form 
the  pivots  or  joints,  and  extend  between  the  parallelograms.  The  ends  of  these 
rods  are  only  visible  in  the  figure.  It  is  to  the  middle  of  one  of  these  rods,  the 
end  of  which  is  represented  at  e,  that  the  air-pump  piston-rod  is  attached  ;  and 
it  is  to  the  middle  of  another,  the  end  of  which  is  represented  at  g,  that  the 
^team  piston-rod  is  attached.  These  two  piston-rods,  therefore,  are  driven,  not 
immediately  by  either  of  the  parallelograms  forming  the  parallel  motion,  but  by 
the  bars  extending  between  them. 

To  the  working  end  of  the  beam  H  is  attached  a  rod  of  cast-iron  0,  called 
the  connecting-rod,  the  lower  end  of  which  is  attached  to  the  crank  by  a  pivot. 
The  weight  of  the  connecting-rod  is  so  made,  that  it  shall  balance  the  weight 
of  the  piston-rods  of  the  air-pump  and  cylinder  on  the  other  side  of  the  beam  ; 
and  the  weight  of  the  piston-rod  of  the  cold-water  pump  N  nearly  balances  the 
weight  of  the  piston-rod  of  the  hot-water  pump  L.  Thus,  so  far  as  the  weights 
of  the  machinery  are  concerned,  the  engine  is  in  equilibrium,  and  the  piston 
would  rest  in  any  position  indifferently  in  the  cylinder. 

The  axis  of  the  fly-wheel  on  which  the  crank  is  formed  is  square  in  the 


470 


THE   STEAM-ENGINE. 


I  middle  part,  where  th«  fly-wheel  is  attached  to  it,  but  has  cylindrical  necks  at 
'  each  end,  which  rest  in  sockets  or  bearings  supported  by  the  Iramino-  of  the 
I  machine,  in  which  sockets  the  axis  revolves  freely.  On  the  axle  of  the  crank 
'  is  placed  the  fly-wheel,  and  connected  with  its  axle  is  the  governor  Q,  which 
!  regulates  the  throttle-valve  T  in  the  manner  already  described. 

Let  us  now  suppose  the  engine  to  be  in  full  operation.  The  piston  bein^  at 
the  top  of  the  cylinder,  the  spanner  m  will  be  raised  by  the  lower  pin  on  the 
air-pump  rod,  and  the  upper  steam-valve  V,  and  the  lower  exhausting-valve  E', 
will  be  opened,  while  the  upper  exhausting-valve  E  and  the  low^er  steam-valve 
Y'  are  closed.  Steam  will,  therefore,  be  admitted  above  the  piston,  and  the 
steam  which  filled  the  cylinder  below  it  will  be  drawn  ofT  to  the  condenser, 
where  it  will  be  converted  into  water.  The  piston  wil],  therefore,  be  urged 
by  the  pressure  of  the  steam  above  it  to  the  bottom  of  the  cylinder.  As  it  ap- 
proaches that  limit,  the  spanner  m  will  be  struck  downward  by  the  upper  pin 
on  the  air-pump  rod,  and  the  valves  V  and  E'  will  be  closed,  and  at  the  same 
time  the  lower  steam-valve  V  and  the  upper  exhausting-valve  E  will  be  opened. 
Steam  will,  therefore,  be  admitted  below  the  piston,  while  the  steam  above  it 
will  be  drawn  off"  into  the  condenser,  and  converted  into  water.  The  pressure 
of  the  steam,  therefore,  below  the  piston  will  urge  it  upward,  and  in  the  same 
manner  the  motion  will  be  continued. 

While  this  process  is  going  on  in  the  cylinder  and  the  condenser,  the  water 
formed  iri  the  condenser  will  b'e  gradually  drawn  off"  by  the  operation  of  the 
air-pump  piston,  in  the  same  manner  as  explained  in  the  single-acting  en- 
gine ;  and  at  the  same  time  the  hot  water  thrown  into  the  hot  well  by  the  air- 
pump  piston  will  be  carried  off  by  the  hot-water  pump  L. 

Such  are  the  chief  circumstances  attending  the  continuance  of  the  opera- 
tion of  the  double-acting  engine.  It  is  only  necessary  here  to  recall  what 
has  been  already  explained  respecting  the  operation  of  the  fly-wheel.  The 
commencement  of  the  motion  of  the  piston  from  the  top  and  bottom  of  the 
cylinder  is  produced,  not  by  the  pressure  of  the  steam  upon  it  upward  or 
downward,  which  must,  for  the  reasons  already  explained,  be  entirely  in- 
efficient ;  but  by  the  momentum  of  the  fly-wheel,  which  extricates  the  crank 
from  those  positions  in  which  the  moving  power  can  not  affect  it. 

The  manner  in  which  the  motion  of  the  crank  affects  the  connecting-rod  at 
the  dead  points  produces  an  effect  of  great  importance  in  the  operation  of  the 
engine.  When  the  crank-pin  is  approaching  the  lowest  point  of  its  play,  and 
therefore  the  piston  approaching  the  top  of  the  cylinder,  the  motion  of  the 
crank-pin  becomes  nearly  horizontal,  and  consequently  its  effect  in  drawing 
the  connecting-rod  and  the  working  end  of  the  beam  downward  and  the  piston 
upward,  is  extremely  small.  The  consequence  of  this  is,  that  as  the  piston 
approaches  the  top  of  the  cylinder,  its  motion  becomes  very  rapidly  retarded  ; 
and  as  the  motion  of  the  crank-pin  at  its  lowest  point  is  actually  horizontal,  the 
piston  is  brought  to  a  state  of  rest  by  this  gradually-retarded  motion  at  the  top 
of  the  cylinder.  In  like  manner,  when  the  crank-pin  moves  from  its  dead 
point  upward,  its  motion  at  first  is  very  nearly  horizontal,  and  consequently  its 
effect  in  driving  the  working  end  of  the  beam  upward,  and  the  piston  down- 
ward, is  at  first  very  small,  but  gradually  accelerated.  The  effect  of  this  upon 
the  piston  is,  that  it  arrives  at  and  departs  from  the  top  of  the  stroke  with  a 
very  slow  motion,  being  absolutely  brought  to  rest  at  that  point. 

The  same  effect  is  produced  when  the  piston  arrives  at  the  bottom  of  the 
cylinder.  This  retardation  and  suspension  of  the  motion  of  the  piston  at  the 
termination  of  the  stroke  affords  time  for  the  process  of  condensation  to  be  ef- 
fected, so  that  when  the  moving  power  of  the  steam  upon  the  piston  can  come 
into  action,  the  condensation  shall  be  sufficiently  complete.     As  the  piston 


THE    STEAM-ENGINE. 


471 


'  approaches  the  top  of  the  cylinder,  and  its  motion  becomes  slow,  the  working- 
,  gear  is  made  to  open  the  lower  exhausting-valve  ;    the  steam  enclosed  in  the 
'  cylinder  below  the  piston,  and  which  has  just  driven  the  piston  upward,  pres- 
,  ses  with  an  elastic  force  of  seventeen  pounds  per  square  inch  on  every  part  of 
'  the  interior  of  the  cylinder,  while  the  uncondensed  vapor  in  the   condenser 
presses  with  a  force  of  about  two  pounds  per  square  inch.     The  steam,  there- 
'  fore,  will  have  a  tendency  to  rush  from  the  cylinder  to  the  condenser  through 
the  open  exhausting-valve,  with  an  excess   of  pressure  amounting  to  fifteen 
pounds  per  square  inch,  while  the  piston  pauses  at  the  top  of  the  cylinder. 
This  process  goes  on,  and  when  the  piston  has  descended   by  the   motion  of 
the  fly-wheel,  a  sufficient  distance  from  the  top  of  the  cylinder  to  call  the  mov- 
ing force  of  the  steam  into   action,  the  exhaustion  will  be  complete,  and  the 
pressure  of  the  uncondensed  vapor  in  the  cylinder  will  become  the  same  as  in 
the  condenser. 

The  pressure  of  steam  in  the  cylinder,  and  of  uncondensed  vapor  in  the  con- 
denser, varies,  within  certain  limits,  in  different  engines,  and  therefore  the 
amount  here  assigned  them  must  be  taken  merely  as  an  example. 

The  size  of  the  valves  by  which  the  steam  is  allowed  to  pass  from  the  cyl- 
inder to  the^condenser  should  be  such  as  to  cause  the  condensation  to  take 
place  in  a  sufficiently  short  time,  to  be  completed  when  the  steam  impelling 
the  piston  is  called  into  action. 

Watt,  in  the  construction  of  his  engines,  made  the  exhaustion-valves  with  a 
diameter  which  was  one  fifth  of  the  diameter  of  the  cylinder,  and  therefore  the 
actual  magnitude  of  the  aperture  for  the  escape  of  the  steam  was  one  twenty- 
fifth  of  the  magnitude  of  the  cylinder  ;  but  the  spindle  of  the  valve  diminished 
this  so  that  the  available  space  for  the  escape  of  steam  did  not  exceed  one  twenty- 
seventh  of  the  magnitude  of  the  cylinder.  This  was  found  to  produce  a  suffi- 
ciently rapid  condensation. 

It  was  usual  to  make  the  steam-valves  of  the  same  magnitude  as  the  ex- 
hausting-valves, but  the  flow  of  steam  through  the  former  was  resisted  by  the 
throttle-valve,  while  no  obstruction  was  opposed  to  its  passage t-^'t^^^frh  the 
latter.  •  ■  j        ' 

The  rapidity  with  which  the  cylinder  must  be  exhausted  by  the  condenser 
will,  however,  depend  upon  the  velocity  with  which  the  piston  is  moved  in  it. 
The  magnitude,  therefore,  of  the  exhausting-valves  which  would  be  sufficient 
for  an  engine  which  acts  with  a  slow  motion  would  be  too  small  where  a  rapid 
motion  is  required. 

In  the  single-acting  steam-engine,  where  the  moving  force  always  acted 
downward  on  the  piston,  the  pressure  upon  all  the  joints  of  the  machinery  by 
Avhich  the  force  of  the  piston  was  conveyed  to  the  working  parts,  always  took 
■place  in  the  same  direction,  and  consequently  whatever  might  be  the  mechani- 
cal connexion  by  which  the  several  joints  were  formed,  the  pins  by  which  they 
were  connected,  must  always  come  to  a  bearing  in  their  respective  sockets, 
however  loosely  they  may  have  been  fitted.  For  the  same  reason,  however, 
that  the  arch  head  and  chain  were  abandoned  as  a  means  of  connecting  the 
steam-piston  with  the  beam,  and  the  parallel  motion  substituted,  it  was  also 
necessary  in  the  double-acting  engine,  where  all  joints  whatever  were  driven 
alternately  in  opposite  directions,  to  fit  the  connecting-pins  with  the  greatest 
accuracy  in  their  sockets,  and  to  abandon  all  connexion  of  the  parts  by  chains. 
If  any  sensible  looseness  was  left  in  the  joints,  a  violent  jerk  would  be  pro- 
duced every  time  the  motion  of  the  piston  was  reversed.  Any  looseness  either 
in  the  pivots  or  joints  of  the  parallel  motion  of  the  working  beam,  the  connect- 
ing rod,  or  crank,  would,  at  every  change  of  stroke,  be  so  accumulated  as  to 
produce  upon  the  machinery  the  effects  of  percussion,  and  would  consequently 


be  attended  with  the  danger  of  straining  and  breaking  the  moveable  parts  of 
the  mechanism. 

To  secure,  therefore,  the  necessary  accuracy  of  the  joints,  Watt  contrived 
that  every  joint  in  the  engine  should  admit  of  the  size  of  the  socket  being  ex- 
actly adapted  to  the  size  of  the  pin,  so  as  always  to  make  a  good  fitting  by 
closing  the  socket  upon  the  pin,  when  any  looseness  would  be  produced  by 
wear.  With  this  view,  all  the  joints  were  fitted  with  sockets  made  of  brass 
or  gun-metal,  capable  of  adjustment.  Each  socket  was  composed  of  two 
pieces,  accurately  fitted  into  a  cell  or  groove,  in  which  one  of  the  brasses  can 
be  moved  toward  the  other  by  means  of  a  wedge  or  screw.  Each  brass  has 
in  it  a  semi-cylindrical  cavity,  and  the  two  cavities  being  opposed  to  each 
other,  form  a  socket  for  the  joiut-pin.  One  of  the  two  brasses  can  always  be 
tightened  round  that  pin,  so  as  to  enclose  it  tight  between  the  two  semi-cylin- 
drical cavities,  and  to  prevent  any  looseness  taking  place.  The  brasses,  and 
other  parts  of  such  a  joint,  are  represented  in  fig.  25.  These  joints  still  con- 
tinue to  be  used  in  the  engines  as  now  constructed 

Fig.  25. 


The  motion  of  the  working  beam,  and  the  pump-rods  which  it  drives,  and 
of  the  connecting  rod,  ought,  if  the  whole  were  constructed  with  perfect  pre- 
cision, to  take  place  in  the  same  or  parallel  vertical  planes  ;  but  this  supposes 
a  perfection  of  execution  which  could  hardly  have  been  expected  in  the  early 
manufacture  of  such  engines,  whatever  may  have  been  attained  by  improve- 
ments y^varr"-  ^^■'^6  been  since  made.  In  the  details  of  construction.  Watt 
saw  thaiitiere  would  be  a  liability  to  lateral  strain,  owing  to  the  planes  of  the 
different  motions  not  being  truly  vertical  and  truly  parallel,  and  that  if  a 
provision  were  not  made  for  such  laternal  motion,  the  machinery  would  be 
subject  to  constant  strain  in  its  joints  and  rapid  wear.  He  provided  against 
this  by  constructing  the  main  joints  by  which  the  great  working  lever  was 
connected  with  the  pistons  and  connecting  rod,  so  as  to  form  universal  joints, 
giving  freedom  of  motion  laterally  as  well  as  vertically. 

The  great  lever,  or  working  beam,  was  so  called  from  being  originally  made 
from  a  beam  of  oak.  It  is  now,  however,  universally  constructed  of  cast-iron. 
The  connecting  rod  is  also  made  of  cast-iron,  and  attached  to  the  beam  and  to 
the  crank  by  axles  or  pivots. 

The  mechanism  by  which  the  four  valves  are  opened  and  closed,  is  subject 
to  considerable  variation  in  different  engines.  They  have  been  described 
above  as  being  opened  and  closed  simultaneously  by  a  single  lever.  Some- 
times, however,  they  are  opened  alternately  in  pairs  by  two  distinct  levers 
driven  by  two  pins  attached  to  the  air-pump  rod.  One  pin  strikes  the  lever, 
which  opens  and  closes  the  upper  steam-valve,  and  lower  exhausting-valve  ; 
the  other  strikes  that  which  opens  and  closes  the  lower  steam-valve  and  upper 
exhausting-valve. 

Since  the  date  of  the  earlier  double-acting  engines,  constructed  by  Boulton 
and  Watt,  a  great  variety  of  mechanical  expedients  have  been  practised  for 
working  the  valves,  by  which  the  steam  is  admitted  to  and  withdrawn  from  the 
cylinder.     We  shall  here  describe  a  few  of  these  methods  : — 


THE  STEAM-ENGINE. 


473 


The  method  of  working  the  valves  by  pins  on  the  air-pump  rod  driving  levers 
connected  with  the  valves  has  been,  in  almost  all  modern  double-acting  machines, 
superseded  by  an  apparatus  called  an  eccentric,  by  which  the  motion  of  the 
axle  of  the  fly-wheel  is  made  to  open  and  close  the  valves  at  the  proper  times. 

An  eccentric  is  a  metallic  circle  attached  to  a  revolving  axle,  so  that  the 
centre  of  the  circle  shall  not  coincide  with  the  centre  round  which  the  axle 
revolves.     Let  us  suppose  that  G,  iig.  26,  is  a  square  revolving  shaft.     Let  a 

Fig.  26. 


circular  plate  of  metal  B  D,  having  its  centre  at  C,  have  a  square  hole  cut  in 
it,  corresponding  to  the  shaft  G,  and  let  the  shaft  G  pass  through  this  square 
aperture,  so  that  the  circular  plate  B  D  shall  be  fastened  upon  the  shaft,  and 
capable  of  revolving  with  it  as  the  shaft  revolves.  The  centre  C  of  the  circular 
plate  B  D  will  be  carried  round  the  centre  G  of  the  revolving  shaft,  and  will 
describe  round  it  a  circle,  the  radius  of  which  will  be  the  distance  of  the 
centre  C  of  the  circular  plate  from  the  centre  of  the  shaft.  Such  circular  plate 
so  placed  upon  a  shaft,  and  revolving  with  it,  is  an  eccentric. 

Let  E  F  be  a  metallic  ring,  formed  of  two  semicircles  of  metal  screwed 
together  at  H,  so  as  to  be  capable,  by  the  adjustment  of  the  screws,  of  having 
the  circular  aperture  formed  by  the  ring  enlarged  and  diminished  within  certain 
small  limits.  Let  this  circular  aperture  be  supposed  to  be  equal  to  the  magni- 
tude of  the  eccentric  B  D.  To  the  circular  ring  E  F  let  an  arm  L  M  be  at- 
tached. If  the  ring  E  F  be  placed  around  the  eccentric  B  D,  and  that  the 
screws  H  be  so  adjusted  as  to  allow  the  eccentric  B  D  to  revolve  within  the 
ring  E  F,  then  while  the  eccentric  revolves,  the  ring  not  partaking  of  its 
revolution,  the  arm  L  M  will  be  alternately  driven  to  the  right  and  to  the  left, 
by  the  motion  of  the  centre  C  of  the  eccentric  as  it  revolves  round  the  centre 
G  of  the  axle.  When  the  centre  C  of  the  eccentric  is  in  the  same  horizontal 
line  with  the  centre  G,<and  to  the  left  of  it,  then  the  position  of  L  M  will  be 
that  which  is  represented  in  fig.  26  ;  but  when,  after  half  a  revolution  of  the 
main  axle,  the  centre  C  of  the  eccentric  is  thrown  on  the  other  side  of  the 
centre  G,  then  the  point  M  will  be  transferred  to  the  right,  to  a  distance  equal 
to  twice  the  distance  C  G.  Thus  as  the  eccentric  B  D  revolves  within  the 
ring  E  F,  that  ring,  together  with  the  arm  L  M,  will  be  alternately  driven, 
right  and  left,  through  a  space  equal  to  twice  the  distance  between  the  centre 
of  the  eccentric  and  the  centre  of  the  revolving  shaft. 

If  we  suppose  a  notch  formed  at  the  extremity  of  the  arm  L  M,  which  is  t 
capable  of  embracing  a  lever  N   M,  moveable  on  a  pivot  at  N,  the  motion  of 
the  eccentric  would  give  to  such  a  lever  an  alternate  motion  from  right  to  left. 


474 


THE  STEAM-ENGINE. 


and  vice  versa.  If  we  suppose  another  lever  N  O  connected  with  N  M,  and 
at  right  angles  to  it,  forming  what  is  called  a  bell-crank,  then  the  alternate 
motion  received  by  M,  from  right  to  left,  would  give  a  corresponding  motion  to 
the  extremity  O  of  the  lever  N  O,  upward  and  downward.  If  this  last  point 
0  were  attached  to  a  vertical  arm  or  shaft,  it  would  impart  to  such  arm  or 
shaft  an  alternate  motion  upward  and  downward,  the  extent  of  which  would  be 
regulated  by  the  length  of  the  levers  respectively. 

By  such  a  contrivance  the  revolution  of  the  fly-wheel  shaft  is  made  to  give 
an  alternate  vertical  motion  of  any  required  extent  to  a  vertical  shaft  placed 
near  the  cylinder,  which  may  be  so  connected  with  the  valves  as  to  open  and 
close  them.  Since  the  upward  and  downward  motion  of  this  vertical  shaft  is 
governed  by  the  alternate  motion  of  the  centre  C  to  the  right  and  to  the  left 
of  the  centre  G,  it  is  evident  that  by  the  adjustment  of  the  eccentric  upon  the 
fly-wheel  shaft,  the  valves  may  be  opened  and  closed  at  any  required  position 
of  the  fly-wheel  and  crank,  and  therefore  at  any  required  position  of  the  piston 
in  the  cylinder. 

Such  is  the  contrivance  by  which  the  valves,' whatever  form  may  be  given 
to  them,  are  now  almost  universally  worked  in  double-acting  steam-engines. 

Having  described  the  general  structure  and  operation  of  the  steam-engine  as 
improved  by  Watt,  we  shall  now  explain,  in  a  more  detailed  manner,  some 
parts  of  its  machinery  which  have  been  variously  constructed,  and  in  which 
more  or  less  improvements  have  been  made. 

OF    THE    COCKS    AND    VALVES. 

In  the  steam-engine,  as  well  as  in  every  other  machine  in  which  fluids  act, 
it  is  necessary  to  open  or  close,  occasionally,  the  tubes  or  passages  through 
which  these  fluids  move.  The  instruments  by  which  this  is  accomplished  are 
called  cocks  or  valves. 

Cocks  or  valves  may  be  classified  by  the  manner  in  which  they  are  opened : 
1st,  they  may  be  opened  by  a  motion  similar  to  the  lid  of  a  box  upon  its 
hinges  ;  2d,  they  may  be  opened  by  being  raised  directly  upward,  in  the  same 
manner  as  the  lid  of  a  pot  or  kettle  ;  3d,  they  may  be  opened  by  a  sliding  mo- 
tion, like  that  of  the  sash  of  a  window  or  the  lid  of  a  box  which  slides  in 
grooves  ;  4th,  they  may  be  opened  by  a  motion  of  revolution,  in  the  same 
manner  as  the  cock  of  a  beer-barrel  is  opened  or  closed.  The  term  valve  is 
more  properly  applied  to  the  first  and  second  of  these  classes  ;  the  third  class 
are  usually  called  slides,  and  the  fourth  cocks. 

The  single  clack  valve  is  the  most  simple  example  of  the  first  class.  It  is 
usually  constructed  by  attaching  to  a  plate  of  metal  larger  than  the  aperture 
which  the  valve  is  intended  to  stop,  a  piece  of  leather,  and  to  the  under  side 
of  this  leather  another  piece  of  metal  smaller  than  the  aperture.  The  leather 
extending  on  one  side  beyond  the  larger  metallic  plate,  and  being  flexible, 
forms  the  hinge  on  which  the  valve  plays.  Such  a  valve  is  usually  closed  by 
its  own  weight,  and  opened  by  the  pressure  of  the  fluid  which  passes  through 
it.  It  is  also  held  closed  more  firmly  by  the  pressure  of  the  fluid  whose  re- 
turn it  is  intended  to  obstruct.  An  example  of  this  valve  occurs  in  the  steam- 
engine,  in  the  passage  between  the  condenser  and  the  air-pump.  The  aperture 
which  it  stops  is  there  a  seat  inclined  at  an  angle  whose  inclination  is  such  as 
to  render  the  weight  of  the  valve  sufiicient  to  close  it.  In  cases  where  the 
valve  is  exposed  to  heat,  as  in  the  example  just  mentioned,  where  it  is  con- 
tinually in  contact  with  the  hot  water  flowing  from  the  condenser  to  the  air- 
pump,  the  use  of  leather  is  inadmissible,  and  in  that  case  the  metallic  surface 
of  the  valve  is  ground  smooth  to  fit  its  seat. 

The  extent  to  which  such  a  valve  should  be  capable  of  opening,  ought  to 


THE  STEAM-ENGINE. 


475 


be  such  that  the  aperture  produced  by  it  shall  be  equal  to  the  aperture  which 
it  stops.  This  will  be  effected  if  the  angle  through  which  it  rises  be  about  30°. 
The  valve  by  which  the  air  and  water  collected  in  the  bottom  of  the  air- 
pump  are  admitted^  to  pass  through  the  air-pump  piston  is  a  double  clack,  con- 
sisting of  two  semicircular  plates,  having  the  hinges  on  the  diameters  of  these 
semicircles,  as  represented  in  fig.  27. 


Fig.  27. 


Of  the  valves  which  are  opened  by  a  motion  perpendicular  to  their  seat,  the 
most  simple  is  a  flat  metallic  plate,  made  larger  than  the  orifice  which  it  is 
intended  to  stop,  and  ground  so  as  to  rest  in  steam-tight  contact  with  the  sur- 
face surrounding  the  aperture.  Such  a  A^alve  is  usually  guided  in  its  perpen- 
dicular motion  by  a  spindle  passing  through  its  centre,  and  sliding  in  holes 
made  in  cross  bars  extending  above  and  below  the  seat  of  the  valve. 

The  conical  steam-valves,  which  have  been  already  described,  usually  called 
spindle-valves,  are  the  most  common  of  this  class.  The  best  angle  to  be  given 
to  the  conical  seat  is  found  in  practice  to  be  45°.  With  a  less  inclination  the 
valve  has  a  tendency  to  be  fastened  in  its  seat,  and  a  greater  inclination  would 
cause  the  top  of  the  valve  to  occupy  unnecessary  space  in  the  valve-box.  The 
area,  or  transverse  section  of  the  valve-box,  should  be  rather  more  than  double 
the  magnitude  of  the  upper  surface  of  the  valve,  in  order  to  allow  a  sufficiently 
free  passage  for  the  steam,  and  the  play  of  the  valve  should  be  such  as  to  allow 
it  to  rise  from  its  seat  to  a  height  not  less  than  one  fourth  of  the  diameter  of  its 
upper  surface. 

The  valves  coming  under  this  class  are  sometimes  formed  as  spheres  or 
hemispheres  resting  in  a  conical  seat,  and  in  such  cases  they  are  generally 
closed  by  their  own  weight,  and  opened  by  the  pressure  of  the  fluid  which 
passes  through  them. 

One  of  the  advantages  attending  the  use  of  slides,  compared  with  the  other 
form  of  valves,  is  the  simplicity  with  which  the  same  slide  may  be  made  to 
govern  several  passages,  so  that  a  single  motion  with  a  slide  may  perform  the 
office  of  two  or  more  motions  imparted  to  independent  valves. 

In  most  modern  engines  the  passage  of  the  steam  to  and  from  the  cylinder 
is  governed  by  slides  of  various  forms,  some  of  which  we  shall  now  explain. 

In  figs.  28  and  29,  is  represented  a  slide-valve  contrived  by  Mr.  Murray  of 
Leeds.  A  B  is  a  steam-tight  case  attached  to  the  side  of  the  cylinder  ;  E  F 
is  a  rod,  which  receives  an  alternate  motion,  upward  and  downward,  from  the 
eccentric,  or  from  whatever  other  part  of  the  engine  is  intended  to  move  the 
slide.  This  rod,  passing  through  a  stuffing-box,  moves  the  slide  G  upward 
and  downward.  S  is  the  mouth  of  the  steam-pipe  coming  from  the  boiler  ;  T 
is  the  mouth  of  a  tube  or  pipe  leading  to  the  condenser  ;  H  is  a  passage 
leading  to  the  top,  and  I  to  the  bottom,  of  the  cylinder.  In  the  position  of  the 
slide  represented  in  fig.  28,  the  steam  coming  from  the  boiler  through  S  passes 
through  the  space  H  to  the  top  of  the  cylinder,  while  the  steam  from  the  bottom 
of  the  cylinder  passes  through  the  space  I  into  the  tube  T,  and  goes  to  the 
condenser.  When  the  rod  E  F  is  raised  to  the  position  represented  in  fig.  29, 
then  the  passage  H  is  thrown  into  communication  with  the  tube  T,  while  the 


^ 


passage  I  is  made  to  communicate  with  the  tube  S.  Steam,  therefore,  passes 
from  the  boiler  through  I  below  the  piston,  while  the  steam  which  was  above  the 
piston,  passing  through  H  into  T,  goes  to  the  condenser.  Thus  the  single 
slide  G  performs  the  office  of  the  four  valves  described  in  page  448. 

The  slide  G  has  always  steam  of  a  full  pressure  behind  it,  while  the  steam 
in  front  of  it  escaping  to  the  condenser,  exerts  but  little  pressure  upon  it.  It  is 
therefore  always  forcibly  pressed  against  the  surfaces  in  contact  with  which  it 
moves,  and  is  thereby  maintained  steam-tight.  Indeed  this  pressure  would 
rapidly  wear  the  rubbing  surfaces,  unless  they  were  made  sufficiently  extensive, 
and  hardened  so  as  to  resist  the  effects  of  the  friction.  Where  fresh  water  is 
used,  as  in  land  boilers,  the  slide  may  be  made  of  hardened  steel  ;  and  in  the 
case  of  marine  boilers,  it  may  be  constructed  of  gun-metal.  In  this  and  all 
other  contrivances  in  which  the  apertures  by  which  the  steam  is  admitted  to 
and  withdrawn  from  the  piston  are  removed  to  any  considerable  distance  from 
the  top  and  bottom,  of  the  cylinder,  there  is  a  waste  of  steam,  for  the  steam 
consumed  at  each  stroke  of  the  piston  is  not  only  that  which  would  fill  the 
capacity  of  the  cylinder,  but  also  the  steam  which  fills  the  passage  between 
the  slide  G  and  the  top  or  bottom  of  the  cylinder.  Any  arrangement  which 
would  throw  the  passages  H  and  1  on  the  other  side  of  the  slide  G,  that  is, 
between  S  and  G,  instead  of  being,  as  they  are,  between  G  and  the  top  and 
bottom  of  the  cylinder,  would  remove  this  defect.  This  is  accomplished  by  a 
slide,  which  is  usually  called  the  D  valve,  because,  being  semi-cylindrical  in 
its  form,  and  hollow,  its  cross  section  resembles  the  letter  D.  This  slide, 
which  is  that  which  at  present  is  in  the  most  general  use,  is  represented  in 
figs.  30,  31  ;  E  is  the  rod  by  which  the  slide  is  moved,  passing  through  a 
stuffing-box  F  ;  G  G  is  the  slide  represented  by  a  vertical  section,  a  a  being  a 
passage  in  it  extending  from  the  top  to  the  bottom  ;  S  is  the  mouth  of  the 
great  steam-pipe  coming  from  the  boiler  ;  P  is  the  pipe  leading  to  the  conden- 
ser ;  T  H  is  a  hollow  space  formed  in  the  slide  always  in  communication  with 
the  steam-pipe  S,  and  consequently  always  filled  with  steam  from  the  boiler. 
A  transverse  section  of  the  slide  and  cylinder  is  represented  in  fig.  32,  where 
a  represents  the  top  of  the  passage  marked  a  in  fig.  30.  In  the  position  of 
the  slide  represented  in  fig.  30,  the  steam  filling  the  space  T  H  has  access  to 
the  top  of  the  cylinder,  but  is  excluded  from  the  bottom.     The  steam  which 


THE  STEAM-ENGINE. 


477 


Fig.  30. 


Fig.  31. 


Fig.  32. 


was  below  the  piston,  passing  up  the  passage  a,  escapes  through  the  tube  P  to 
the  condenser.  When  the  piston  has  descended,  the  rod  E  moves  the  slide 
downward,  so  as  to  give  it  the  position  represented  in  fig.  31.  The  steam  in 
T  H  has  now  access  to  the  bottom  of  the  cylinder,  while  the  steam  above  the 
piston  passing  through  P  escapes  to  the  condenser.  In  this  way  the  operation 
of  the  piston  is  continued  and  the  steam  consumed  at  each  stroke  only  exceeds 
the  capacity  of  the  cylinder  by  what  is  necessary  to  fill  the  passages  between 
the  slide  and  the  cylinder. 

In  a  slide  constructed  in  this  manner,  the  steam  filling  the  space  T  H  has  a 
tendency  to  press  the  slide  back,  so  as  to  break  the  contact  of  the  rubbing 
surfaces,  and  thereby  to  cause  the  steam  to  leak  from  the  space  T  H  to  the 
back  of  the  slide.  This  is  counteracted  by  the  packing  x,  at  the  back  of 
the  slide. 

In  engine*  of  very  long  stroke,  the  extent  of  the  rubbing  surfaces  of  slides 
of  this  kind  renders  it  difficult  to  keep  them  in  steam-tight  contact  and  to  in- 
sure their  uniform  wear.  In  such  cases,  therefore,  separate  slides,  upon  the 
same  principle,  are  provided  at  the  top  and  bottom  of  the  cylinder,  moved, 
however,  by  a  single  rod  of  communication. 

In  slides,  as  we  have  here  described  them,  the  same  motion  which  admits 
steam  to  either  end  of  the  cylinder,  withdraws  it  from  the  other  end.  Such 
an  arrangement  is  only  compatible  with  the  operation  of  a  cylinder  which 
works  without  expansion  ;  for  in  such  a  cylinder  the  full  flow  of  steam  to  the 
piston  is  only  interrupted  for  a  moment  during  the  change  of  position  of  the 
slide.  But  if  the  steam  act  expansively,  it  would  be  necessary  to  move  the 
slide,  so  as  to  stop  its  flow  to  one  end  of  the  cylinder,  without  at  the  same  time 
obstructing  the  escape  of  steam  from  the  other  end  to  the  condenser.  It  would 
therefore  be  necessary  that  the  slide  should  close  the  passage  leading  to  the 
cylinder  at  one  end,  without  at  the  same  time  obstructing  the  communication 
between  the  passage  from  the  cylinder  to  the  condenser  at  the  other  end.  On 
the  arrival  of  the  piston,  however,  at  the  bottom  of  the  cylinder,  it  would  be 
necessary  immediately  to  put  the  lower  passage  to  the  cylinder  in  communica- 
tion with  the  steam-pipe,  and  the  upper  passage  in  communication  with  the 
condenser.  This  would  necessarily  suppose  two  motions  of  the  slide  as  well 
as  some  modifications  in  its  length.  Let  the  length  of  the  slide  be  such  that 
when  the  passage  to  the  top  of  the  cylinder  is  stopped,  the  lower  part  of  the 
slide  shall  not  reach  the  passage  to  the  lower  part  of  the  cylinder  ;  and  let  i 
such  a  provision  be  made  in  the  mechanism  by  which  the  rod  E  governino-  ' 
the  slide  is  driven  that  it  shall  receive  two  motions  during  the  descent  of  the  ( 
piston,  the  first  to  be  imparted  to  it  at  the  moment  the  steam  is  to  be  cut  off,  ! 


478 


THE  STEAM-ENGINE. 


Fig.  33. 


Fig.  35. 


and  the  second  just  before  the  termination  of  the  stroke.  Let  the  position  of 
the  slide,  at  the  commencement  of  the  stroke,  be  represented  in  fig.  33,  and 
let  it  be  required  that  the  steam  shall  be  cut  off  at  one  half  of  the  stroke.  When 
the  piston  has  made  half  the  stroke,  the  rod  governing  the  slide  is  moved 
downward,  so  as  to  throw  the  slide  into  the  position  represented  in  fig.  34. 
The  passage  between  the  steam-pipe  and  the  cylinder  is  now  stopped  at  both 
ends  ;  but  the  passage  from  the  bottom  of  the  cylinder  to  the  condenser  re- 
mains open.  During  the  remainder  of  the  stroke,  therefore,  the  steam  in  the 
cylinder  works  expansively.  As  the  piston  approaches  the  bottom  of  the  cyl- 
inder, another  motion  is  imparted  to  the  rod  governing  the  slide,  by  which  the 
latter  is  thrown  into  the  position  represented  in  fig.  35.  Steam  now  flows  be- 
low the  piston  while  the  steam  above  it  passes  to  the  condenser.  In  a  similar 
manner,  by  two  motions  successively  imparted  to  the  slide  during  the  ascent 
of  the  piston,  the  steam  may  be  cut  off"  at  half-stroke  ;  and  it  is  evident  that  by 
regulating  the  lime  at  which  these  motions  are  given  to  the  slide,  the  steam 
may  be  worked  expansively,  to  any  required  extent. 

It  is  easy  to  conceive  various  mechanical  means  by  which,  in  the  same 
engine,  the  point  at  which  the  steam  is  cut  off  may  be  regulated  at  pleasure. 

In  cases  where  the  motion  of  the  piston  is  very  rapid,  as  in  locomotive 
engines,  it  is  desirable  that  the  passages  to  and  from  the  cylinder  should  be 
opened  very  suddenly.  This  is  diflicult  to  be  accomplished  with  any  form  of 
slide  consisting  of  a  single  aperture  ;  but  if,  instead  of  admitting  the  steam  to 
the  cylinder  by  a  single  aperture,  the  same  magnitude  of  opening  were  divided 
among  several  apertures,  then  a  proportionally  less  extent  of  motion  in  the 
slide  would  clear  the  passage  for  the  steam,  and  consequently  greater  sud- 
deimess  of  opening  would  be  effected. 

The  great  advantages  in  the  economy  of  fuel  resulting  from  the  application 
of  the  expansive  principle  have,  of  late  years,  forced  themselves  on  the  atten- 
tion of  engineers,  and  considerable  improvements  have  been  made  in  its  ap- 
plication, especially  in  the  case  of  marine  engines  used  for  long  voyages,  in 
which  the  economy  of  fuel  has  become  an  object  of  the  last  importance.  The 
mechanism  by  which  expansive  slides  are  moved,  is  made  capable  of  adjust-  ) 
ment,  so  that  the  part  of  the  stroke  at  which  the  steam  is  cut  off,  can  be  altered 
at  pleasure.  The  working  power  of  the  engine,  therefore,  instead  of  being 
controlled  by  the  throttle-valve,  is  regulated  by  the  greater  or  less  extent  to 
which  the  expansive  principle  is  applied.     Steam  of  the  same  pressure  is  ad- 


THE  STEAM-ENGINE. 


479 


mitted  to  the  cylinder  in  all  cases ;  but  it  is  cut  off  at  a  greater  or  less  portion 
of  the  stroke,  according  to  the  power  which  the  engine  is  required  to  exert. 

The  last  degree  of  perfection  has  been  conferred  on  this  principle  by  con- 
necting the  governor  with  the  mechanism  by  which  the  slide  is  moved,  so  that 
the  governor,  instead  of  acting  on  the  throttle-valve,  is  made  to  act  upon  the 
slide.  By  this  means,  when,  by  reason  of  any  diminution  of  the  resistance, 
the  motion  of  the  engine  is  accelerated,  the  balls  of  the  governor  diverging, 
shift  the  cam  or  lever  which  governs  the  slide,  so  that  the  steam  is  cut  off 
after  a  shorter  portion  of  the  stroke,  the  expansive  principle  is  brought  into 
greater  play,  and  the  quantity  of  steam  admitted  to  the  cylinder  at  each  stroke 
is  diminished.  If,  on  the  other  hand,  the  resistance  to  the  machine  be  in- 
creased, so  as  to  diminish  the  velocity  of  the  engine,  then  the  balls  collapsing, 
the  levers  of  the  governor  shift  the  cam  which  moves  the  slides,  so  as  to  in- 
crease the  portion  of  the  stroke  made  by  the  piston  before  the  steam  is  cut  off, 
and  thereby  to  increase  the  amount  of  mechanical  power  developed  in  the  cyl- 
inder at  each  stroke.  The  extent  to  which  the  expansive  principle  is  capable 
of  being  applied,  more  especially  in  marine-engines,  has  been  hitherto  limited 
by  the  necessity  of  using  steam  of  very  high  pressure,  whenever  the  steam  is 
cut  off  after  the  piston  has  performed  only  a  small  part  of  the  stroke.  A  method, 
however,  is  now  (March,  1840)  under  experimental  trial,  by  Messrs.  Maudsley 
and  Field,  by  which  the  expansive  principle  may  be  applied  to  any  required 
extent  without  raising  the  steam  in  the  boiler  above  the  usual  pressure  of  from 
three  to  five  pounds  per  square  inch.  This  method  consists  in  the  use  of  a 
piston  of  great  magnitude.  The  force  urging  the  piston  is  thus  obtained,  not 
by  an  excessive  pressure  on  a  limited  surface,  but  by  a  moderate  pressure  dif- 
fused over  a  large  surface.  The  entire  moving  force  acting  on  the  piston 
before  the  steam  is  cut  off,  is  considerably  greater  than  the  resistance  ;  but 
during  the  remainder  of  the  stroke  this  force  is  gradually  enfeebled  until  the 
piston  is  brought  to  the  extremity  of  its  play.- 

Mr.  Samuel  Seaward,  of  the  firm  of  Messrs.  Seawards,  engineers,  has  con- 
trived an  improved  system  of  slides,  for  which  he  has  obtained  a  patent.  A 
section  of  Seaward's  slides  is  represented  in  fig.  36.     The  steam-pipe  pro- 


Fig.  36. 


ceeding  from  the  boiler  to  the  cylinder  is  represented  al  A  A,  and  it  commu- 
nicates with  passages  S  and  S'  leading  to  the  top  and  bottom  of  the  cylinder. 


480 


THE   STEAM-ENGINE. 


These  passages  are  formed  in  nozzles  of  iron  or  other  hard  metal  cast  upon 
the  side  of  the  cylinder.  These  nozzles  present  a  smooth  face  outward,  upon 
which  the  slides  B  B',  also  formed  with  smooth  faces,  play.  The  slides  B  B' 
are  attached  by  knuckle-joints  to  rods  E  E',  which  move  through  stuffing-boxes, 
and  the  connexion  of  these  rods  with  the  slides  is  such  that  the  slides  have 
play  so  as  to  detach  their  surfaces  easily  from  the  smooth  surfaces  of  the  noz- 
zles when  not  pressed  against  these  surfaces.  The  steam  hi  the  steam-pipe 
A  A  will  press  against  the  backs  of  the  slides  B  B',  and  keep  their  faces  in 
steam-tight  contact  with  the  smooth  surfaces  of  the  nozzles.  These  slides 
may  be  opened  or  closed  by  proper  mechanism  at  any  point  of  the  stroke. 
When  steam  is  to  be  admitted  to  the  top  of  the  cylinder,  the  upper  slide  is 
raised  and  the  passage  S  opened  ;  and  when  it  is  to  be  admitted  to  the  bottom 
of  the  cylinder,  the  lower  slide  is  raised  and  the  passage  S'  opened  ;  and  its 
communication  to  the  top  or  bottom  of  the  cylinder  is  stopped  by  the  lowering 
of  these  slides  respectively.  On  the  other  side  of  the  cylinder  are  provided 
two  passages  C  C  leading  to  a  pipe  G,  which  is  continued  to  the  condenser. 
On  this  pipe  are  cast  nozzles  of  iron  or  other  metal,  presenting  smooth  faces 
toward  the  cylinder,  and  having  passages  D  D^  communicating  between  the 
top  and  bottom  of  the  cylinder  respectively  and  the  pipe  G  G  leading  to  the 
condenser.  Twp  slides  b  b',  having  smooth  faces  turned  from  the  cylinder, 
and  pressing  upon  the  faces  of  the  nozzles  D  D',  are  governed  by  rods  playing 
through  stuffing-boxes,  in  the  same  manner  as  already  described.  The  faces 
of  these  slides  being  turned  from  the  cylinder,  the  steam  in  the  cylinder  having 
free  communication  with  them,  has  a  tendency  to  keep  them  by  its  pressure  in 
steam-tight  contact  with  the  surfaces  in  which  the  apertures  leading  to  the  con- 
denser are  formed.  These  two  slides  may  be  opened  or  closed  whenever  it 
is  necessary. 

When  the  piston  commences  its  descent,  the  upper  steam-slide  is  raised,  so 
as  to  open  the  passage  S,  and  admit  steam  above  the  piston  ;  and  the  lower 
exhausting-slide  b'  is  also  raised,  so  as  to  allow  the  steam  below  the  piston  to 
escape  through  G  to  the  condenser,  other  two  passages  S'  and  C  being  closed 
by  their  respective  slides.  The  slide  which  governs  S  is  lowered  at  that  part 
of  the  stroke  at  which  the  steam  is  intended  to  be  cut  off,  the  other  slides  re- 
maining unchanged  ;  and  when  the  piston  has  reached  the  bottom  of  the  cyl- 
'inder,  the  lower  steam-slide  opens  the  passage  S',  and  the  upper  exhausting- 
slide  opens  the  passage  C  ;  and  at  the  same  time  the  lovv^er  exhausting-slide 
closes  the  passage  C^.  Steam  being  admitted  below  the  piston  through  S', 
and  at  the  same  time  the  st&am  above  it  being  drawn  away  to  the  condenser 
through  the  open  passage  C  and  the  tube  G,  the  piston  ascends.  When  it  has 
reached  that  point  at  which  the  steam  is  intended  to  be  cut  off,  the  slide  which 
governs  S'  is  lowered,  the  other  slides  remaining  unaltered,  and  the  upward 
stroke  is  completed  in  the  same  manner  as  the  downward. 

These  four  slides  may  be  governed  by  a  single  lever,  or  they  may  be  moved 
by  separate  means.     From  the  small  spaces  between  the   several  slides  and 
the  body  of  the  cylinder,  it  will  be  evident  that  the  waste  of  steam  by  this  con- 
I  trivance  will  be  very  small. 

'  In  the  slide-valves  commonly  used,  the  packing  of  hemp  at  the  back  of  the 
I  slide,  by  which  the  pressure  necessary  to  keep  the  slide  in  steam-tight  contact 
'  is  obiamed,  requires  constant  attention  from  the  engine-man  while  the  engine 
I  is  at  work.  Any  neglect  of  this  will  produce  a  corresponding  loss  in  the  power 
'  of  the  engine  ;  and  accordingly  it  is  found  that  in  many  cases  where  engines 
I  work  inefficiently,  the  defect  is  owing  either  to  ignorance  or  want  of  attention 
*  on  the  part  of  the  engine-man  in  the  packing  of  the  slides.  In  Sea  ward's 
)  slides  no  hemp-packing  is  used,  nor  is  any  attention  on  the  part  of  the  engine- 


man  required  after  the  slides  are  first  adjusted.  The  slides  receive  the  pres- 
sure necessary  to  keep  them  in  steam-tight  contact  with  the  surfaces  of  the 
nozzles  from  the  steam  itself,  which  acts  behind  them. 

The  eduction  and  steam  slides  being  independent  of  each  other,  they  may 
be  adjusted  so  that  the  engine  shall  work  expansively  in  any  required  degree  ; 
and  this  may  be  accomplished  either  by  working  the  slides  by  separate 
mechanism,  or  by  a  single  eccentric. 

One  of  the  advantages  claimed  by  the  patentees  for  these  slides  is,  that  the 
engines  are  secured  from  the  accidents  which  arise  from  the  accumulation  of 
water  within  the  steam-cylinder.  If  such  a  circumstance  should  occur,  the 
action  of  the  piston  will  press  the  water  against  the  faces  of  the  steam-slides, 
and  the  play  allowed  to  them  by  their  connexion  with  the  rods  which  move 
them  permits  their  faces  to  be  raised  from  the  surfaces  of  the  nozzles,  so  that 
the  water  collected  in  the  cylinder  shall  be  driven  into  the  steam-pipe,  and  sent 
back  thence  to  the  boiler. 

Of  the  cocks  or  valves  which  are  opened  and  closed  by  the  motion  of  an 
axis  passing  through  their  centre,  the  throttle-valve,  whether  worked  by  hand 
or  by  the  governor,  is  an  example.  But  the  most  common  form  for  cocks  is 
that  of  a  cylindrical  or  slightly  conical  plug,  fig.  37,  inserted  in  an  aperture  of 

Fig.  37.  • 


corresponding  magnitude  passing  across  the  pipe  or  passage  which  the  cock  is 
intended  to  open  or  close.  One  or  more  holes  are  pierced  transversely  in  the 
cock,  and  when  the  cock  is  turned  so  that  these  holes  run  in  the  direction  of 
the  tube,  the  passage  through  the  tube  is  opened  ;  but  when  the  passage  through 
the  cock  is  placed  at  right  angles  to  the  tube,  then  the  sides  of  the  tube  stop 
the  ends  of  the  passage  in  the  cock,  and  the  passage  through  the  tube  is  ob- 
structed. The  simple  cock  is  designed  to  open  or  close  the  passage  through 
a  single  tube.  When  the  cock  is  turned,  as  in  fig.  38,  so  that  the  passage 
through  the  cock  shall  be  at  right  angles  to  the  length  of  the  tube,  then  the 
passage  through  the  tube  is  stopped  ;  but  when  the  cock  is  turned  from  that 
position  through  a  quarter  of  a  revolution,  as  in  fig.  39,  then  the  passage Jhrough 

Fig.  39. 


the  cock  takes  the  direction  of  the  passage  through  the  tube,  and  the  cock  is 
opened,  and  the  passage  through  the  tube  unobstructed.     In  such  a  cock  the 
passage  may  be  more  or  less  throttled  by  adjusting  the  position  of  the  cock, 
so  that  a  part  of  the  opening  in  it  shall  be  covered  by  the  side  of  the  tube. 
VOIi.  II.— 31 


482 


THE   STEAM-ENGINE. 


It  is  sometimes  required  to  put  one  tube  or  passage  alternately  in  communica- 
tion with  two  others.  This  is  accomplished  by  a  two-way  cock.  In  this  cock  the 
passage  is  curved,  opening  usually  at  points  on  the  surface  of  the  cock,  at  right 
angles  to  each  other. 

When  it  is  required  to  put  four  passages  alternately  in  communication  by 
pairs,  a  four-way  cock  is  used.     Such  a  cock  has  two  curved  passages  (fig.  40), 


Fig.  41. 


each  similar  to  the  curved  passage  in  the  two-way  cock.  Let  S  C  B  T  be  the 
four  tubes  which  it  is  required  to  throw  alternately  into  communication  by  pairs. 
When  the  cock  is  in  the  position  fig.  40,  the  tube  S  communicates  with  T,  and 
the  tube  C  with  B.  By  turning  the  cock  through  a  quarter  of  a  revolution,  as 
in  fig.  41,  the  tube  S  is  made  to  communicate  with  B,  and  the  tube  C  with  T  ; 
and  if  the  cock  continue  to  be  turned  at  intervals  through  a  quarter  of  a  revo- 
lution, these  changes  of  communication  will  continue  to  be  alternately  made. 
It  is  evident  that  this  may  be  accomplished  by  turning  the  cock  continually  in 
the  same  direction. 

The  four-way  cock  is  sometimes  used  as  a  substitute  for  the  valves  or  slides 
in  a  double-acting  steam-engine  to  conduct  the  steam  to  and  from  the  cylinder. 
If  S  represent  a  pipe  conducting  steam  from  the  boiler,  C  that  which  leads  to 
the  condenser,  T  the  tube  which  leads  to  the  top  of  the  cylinder,  and  B  that 
which  leads  to  the  bottom,  then  when  the  cock  is  in  the  position  fig.  40,  steam 
would  flow  from  the  boiler  to  the  top  of  the  piston,  while  the  steam  below  it 
would  be  drawn  off  to  the  condenser  ;  and  in  the  position  fig.  41,  steam  would 
flow  from  the  boiler  to  the  bottom  of  the  piston,  while  the  steam  above  it  would 
be  drawn  off  to  the  condenser.  Thus,  by  turning  the  cock  through  a  quarter  of 
a  reivolution  toward  the  termination  of  each  stroke,  the  operation  of  the  machine 
would  be  continued. 

One  of  the  disadvantages  which  is  inseparable  from  the  use  of  a  four-way 
cock  for  this  purpose  is  the  loss  of  the  steam  at  each  stroke,  which  fills  the 
tubes  between  the  cock  and  the  ends  of  the  cylinder.  This,disadvantage  could 
only  be  avoided  by  the  substitution  of  two  two-way  cocks  instead  of  a  four-way 
cock.  A  two-way  cock  at  the  top  of  the  cylinder  would  open  an  alternate  com- 
munication between  the  cylinder  and  steam-pipe,  and  the  cylinder  and  con- 
denser, while  a  similar  office  would  be  performed  by  another  two-way  cock  at 
the  other  end. 

The  friction  on  cocks  of  this  description  is  more  than  on  other  valves  ;  but 
this  is  in  some  degree  compensated  by  the  great  simplicity  of  the  instrument. 
When  the  cock  is  truly  ground  into  its  seat,  being  slightly  conical  in  its  form, 
the  pressure  of  the  steam  has  a  tendency  to  keep  the  surfaces  in  contact ;  but 
this  pressure  also  increases  the  friction,  and  has  a  tendency  to  wear  the  seat 
of  the  cock  into  an  elliptical  shape.  Consequently,  such  cocks  require  to  be 
occasionally  ground  and  refitted. 


THE   STEAM-ENGINE. 


The  four-way  cock,  as  above  described,  admits  the  steam  to  one  end  of  the 
piston  at  the  same  moment  that  it  stops  it  at  the  other  end.  It  would  therefore 
be  inapplicable  where  steam  is  worked  expansively.  A  slight  modification, 
however,  analogous  to  that  already  described  in  the  slides,  will  adapt  it  to 
expansive  action.  This  will  be  accomplished  by  giving  to  one  of  the  pas- 
sages through  the  cock  one  aperture  larger  than  the  other,  and  working  the 
cock  so  that  this  passage  shall  always  be  used  to  conduct  steam  to  the 
cylinder  ;  also  by  enlarging  both  apertures  of  the  other  passage,  and  using 
it  always  to  conduct  steam  from  the  cylinder.  The  effect  of  such  an  arrange- 
ment will  be  readily  understood. 

Let  the  position  of  the  cock  at  the  commencement  of  the  descending  stroke 
be  represented  in  fig.  42.     Steam  flows  from  S  through  T  to  the  top  of  the 


Fig.  43. 


cylinder,  while  it  escapes  from  B  through  C  from  the  bottom  of  the  cylinder. 
When  the  piston  has  arrived  at  that  point  at  which  the  steam  is  to  be  cut  off, 
let  the  cock  be  shifted  to  the  position  represented  in  fig.  43.  The  passage  of 
steam  from  the  boiler  is  now  stopped,  but  the  escape  of  steam  from  the  bottom 
of  the  cylinder  through  C  continues,  and  the  cock  is  maintained  in  this  position 
until  the  piston  approaches  the  bottom  of  the  cylinder,  when  it  is  further  shifted 
to  the  position  represented  in  fig.  44.     Steam  now  flows  from  S  through  B 


Fig.  45. 


to  the  bottom  of  the  cylinder,  while  the  steam  from  the  top  of  the  cylinder  es- 
capes through  C  to  the  condenser.  When  the  piston  has  arrived  at  that  point 
where  the  steam  is  to  be  cut  off,  the  cock  is  shifted  to  the  position  represented 
in  fig.  45.  The  communication  between  the  steam  and  the  bottom  of  the  pis- 
ton is  now  stopped,  while  the  communication  between  the  top  of  the  cylinder 
and  the  condenser  is  still  open.  During  the  next  double  stroke  of  the  piston, 
the  position  of  the  cock  is  similarly  changed,  but  in  the  contrary  direction,  and 
in  the  same  way  the  motion  is  continued.     Under  these   circumstances  the 


484 


THE   STEAM-ENGINE. 


cock,  instead  of  being  moved  constantly  in  the  same  direction,  as  in  the  case 
of  the  common  four-way  cock,  will  require  to  be  moved  alternately  in  opposite 
directions. 


The  office  of  a  piston  being  to  divide  a  cylinder  into  two  compartments  by 
a  moveable  partition  which  shall  obstruct  the  passage  of  any  fluid  from  one 
compartment  to  the  other,  it  is  evident  that  the  two  conditions  which  such  an 
instrument  ought  to  fulfil  are — -first,  that  the  contact  of  its  sides  with  the  sur- 
face of  the  cylinder  shall  be  so  close  and  tight  throughout  its  entire  play  that 
no  steam  or  other  fluid  can  pass  between  them  ;  secondly,  that  it  shall  be  so  free 
from  friction,  notwithstanding  this  necessary  tightness,  that  it  shall  not  absorb 
any  injurious  quantity  of  the  moving  power. 

Since,  however  accurately  the  surfaces  of  the  piston  and  cylinder  may 
be  constructed,  there  will  always  be  in  practice  more  or  less  imperfection  of 
form,  it  is  evident  that  the  contact  of  the  surface  of  the  piston  with  the  cylin- 
der throughout  the  stroke  can  only  be  maintained  by  giving  to  the  circumfer- 
ence of  the  piston  sufficient  elasticity  to  accommodate  itself  to  such  inequali- 
ties of  form.  The  substance,  whatever  it  may  be,  used  for  this  purpose,  and 
by  which  the  piston  is  surrounded,  is  called  packing. 

In  steam-pistons  the  material  used  for  packing  must  be  such  as  is  capable 
of  resisting  the  united  effects  of  heat  and  moisture.  Hence  leather  and  other 
animal  substances  are  inapplicable. 

The  packing  used  for  steam-pistons  is  therefore  of  two  kinds,  vegetable  pack- 
ing, usually  hemp,  or  metallic  packing. 

The  bottom  of  the  common  hemp-packed  piston  is  a  circular  plate  just  so 
much  less  in  diameter  than  the  cylinder  as  is  sufficient  to  allow  its  free  motion 
in  ascending  and  descending.  A  little  above  its  lowest  point  this  plate  begins 
gradually  to  diminish  in  thickness,  until  its  diameter  is  reduced  to  from  one  to 
two  inches  less  than  that  of  the  cylinder,  leaving  therefore  around  it  a  hollow 
space,  as  represented  in  fig.  4-6.     The  cover  of  the  piston  is  a  plate  similarly 

Fig-  46. 


formed,  being  in  like  manner  gradually  reduced  in  thickness  downward,  so  as 
to  correspond  with  the  lower  plate.  In  the  hollow  space  which  thus  surrounds 
the  piston  a  packing  of  unspun  hemp  or  soft  rope,  called  gasket,  is  introduced 
by  winding  it  round  the  piston  so  as  to  render  it  an  even  and  compact  mass. 
When  the  space  is  thus  filled  up,  the  top  of  the  piston  is  attached  to  the  bot- 
tom by  screws.  The  curved  form  of  the  space  within  which  the  hempen 
packing  is  confined  is  such  that,  when  the  screws  are  tightened,  that  part  of 
the  packing  which  is  nearest  to  the  top  and  bottom  of  the  piston  is  forced 
against  the  cylinder  so  as  to  produce  upon  the  two  parallel  rings  as  much 
pressure  as  is  necessary  to  render  it  steam-^ight.  When  by  use  the  packing 
is  worn  down  so  as  to  produce  leakage,  the  cover  of  the  cylinder  must  be  re- 
moved, and  the  screws  connecting  the  top  and  bottom  of  the  piston  tightened; 


THE  STEAM-ENGINE. 


485 


this  will  force  out  the  packing  and  render  the  piston  steam-tight.  This  pack- 
ing is  lubricated  by  melted  tallow  let  down  upon  the  piston  from  the  funnel 
inserted  in  the  top  of  the  cylinder,  furnished  with  a  stop-cock  to  prevent  the 
escape  of  steam.  The  lower  end  of  the  piston-rod  is  formed  slightly  conical, 
the  thickest  part  of  the  cone  being  downward.  It  is  passed  up  through  the 
piston,  and  a  nut  or  wedge  between  the  top  and  bottom  is  inserted  so  as  to 
secure  the  piston  in  its  position  upon  the  rod. 

The  process  of  removing  the  top  of  the  cylinder  for  the  purpose  qf  tighten- 
ing the  screws  in  the  piston  is  one  of  so  laborious  a  nature,  that  the  men  in- 
trusted with  the  superintendence  of  these  machines  are  tempted  to  allow  the 
engine  to  work,  notwithstanding  injurious  leakage  at  the  piston,  rather  than 
incur  the  labor  of  tightening  the  screws  as  often  as  it  is  necessary  to  do  so. 

To  avoid  this  inconvenience,  the  following  method  of  tightening  the  pack- 
ing of  the  piston  without  removing  the  lid  of  the  cylinder,  was  contrived  by 
Woolf.  The  head  of  each  of  the  screws  was  formed  into  a  toothed  pinion,  and 
as  these  screws  were  placed  at  equal  distances  from  the  centre  of  the  piston, 
these  several  pinions  were  driven  by  a  large  toothed  wheel,  revolving  on  the 
piston-rod  as  an  axis.  By  such  an  arrangement  it  is  evident  that  if  any  one 
of  the  screws  be  turned,  a  like  motion  will  be  imparted  to  all  the  others  through 
the  medium  of  the  large  central  wheel.  Woolf  accordingly  formed,  on  the 
head  of  one  of  the  screws,  a  square  end.  When  the  piston  was  brought  to  the 
top  of  the  cylinder,  this  square  end  entered  an  aperture  made  in  the  under  side 
of  the  cover  of  the  cylinder.  This  aperture  was  covered  by  a  small  circular 
piece  screwed  into  the  top  of  the  cylinder,  which  was  capable  of  being  re- 
moved so  as  to  render  the  square  head  of  the  screw  accessible.  When  this 
was  done,  a  proper  key  being  applied  to  the  square  head  of  the  screw,  it  was 
turned  ;  and  by  being  turned,  all  the  other  screws  were  in  like  manner  moved. 
In  this  way,  instead  of  having  to  remove  the  cover  of  the  cylinder,  which  in 
large  cylinders  was  attended  with  great  labor  and  loss  of  time,  the  packing  was 
tightened  by  merely  unscrewing  a  piece  in  the  top  of  the  cylinder  not  much 
greater  in  magnitude  than  the  head  of  one  of  the  screws. 

This  method  was  further  simplified  by  causing  the  great  circular  wheel  al- 
ready described  to  move  upon  the  piston-rod,  not  as  an  axis,  but  as  a  screw, 
the  thread  being  cut  upon  a  part  of  the  piston-rod  which  worked  in  a  corre- 
sponding female  screw  cut  upon  the  central  plate.  By  such  means,  the  screw 
whose  head  was  let  into  the  cover  of  the  cylinder  which  turned,  would  cause 
this  circular  plate  to  be  pressed  downward  by  the  force  of  the  screw  construct- 
ed on  the  piston-rod.  This  circular  plate  thus  pressed  downward,  acted  upon 
pins  or  plugs  which  pressed  together  the  top  and  bottom  of  the  cylinder  in  the 
same  manner  as  they  were  pressed  together  by  the  screws  connecting  them 
as  already  described. 

METALLIC    PISTONS. 


The  notion  of  constructing  a  piston  so  as  to  move  steam-tight  in  the  cylin- 
der without  the  use  of  packing  of  vegetable  matter  was  first  suggested  by  the 
Rev.  Mr.  Cartwright,  a  gentleman  well  known  for  other  mechanical  inventions. 
A  patent  was  granted  in  1797  for  a  new  form  of  steam-engine,  in  which  he 
proposed  to  use  the  vapor  of  alcohol  to  work  the  piston  instead  of  the  steam  of 
water  :  and  since  the  principle  of  the  engine  excluded  the  use  of  lubrication 
by  oil  or  tallow,  he  substituted  a  piston  formed  of  metallic  rings  pressed  against 
the  surface  of  the  cylinder  by  springs,  so  as  to  be  maintained  in  steam-tight 
contact  with  it,  independently  either  of  packing  or  lubrication.  Although  the 
engine  for  which  this  form  of  piston  was  intended  never  came  into  practical 


use,  yet  it  is  so  simple  and  elegant  in  its  structure,  and  forms  a  link  so  inter- 
esting in  the  history  of  the  steam-engine,  that  some  explanation  of  it  ought  not 
to  be  omitted  in  this  work. 

The  steam-pipe  from  the  boiler  is  represented  cut  off  at  B,  fig.  47  ;  T  is  a 


Fig.  47. 


M« 


spindle-valve,  for  admitting  steam  above  the  piston,  and  R  is  a  spindle-valve 
in  the  piston  ;  D  is  a  curved  pipe  forming  a  communication  between  the  cyl- 
inder and  the  condenser,  which  is  of  very  peculiar  construction.  Cartwright 
proposed  effecting  a  condensation  without  a  jet,  by  exposing  the  steam  to  con- 
tact with  a  very  large  quantity  of  cold  surface.  For  this  purpose,  he  formed 
his  condenser  by  placing  two  cylinders  nearly  equal  in  size,  one  within  the 
other,  allowing  the  water  of  the  cold  cistern  in  which  they  were  placed  to  flow 
through  the  inner  cylinder,  and  to  surround  the  outer  one.  Thus,  the  thin 
space  between  the  two  cylinders  formed   the  condenser. 

The  air-pump  is  placed  immediately  under  the  cylinder,  and  the  continua- 
tion of  the  piston-rod  works  its  piston,  which  is  solid  and  without  a  valve.  F 
is  the  pipe  from  the  condenser  to  the  air-pump,  through  which  the  condensed 


THE   STEAM-ENGINE. 


[  Steam  is  drawn  off  through  the  valve  G  on  the  ascent  of  the  piston,  and  on  the 
,  descent  is  forced  through  a  tube  into  a  hot  well  H,  for  the  purpose  of  feedino- 
•  the  boiler  through  the  feed-pipe  I.  In  the  top  of  the  hot  well  H  is  a  valve 
,  which  opens  inward,  and  is  kept  closed  by  a  ball  floating  on  the  surface  of  the 
'  liquid.  The  pressure  of  the  condensed  air  above  the  surface  of  the  liquid  in 
,  H  forces  it  through  I  into  the  boiler.  When  the  air  accumulates  in  too  great 
[  a  degree  in  H,  the  surface  of  the  liquid  is  pressed  so  low  that  the  ball  falls 
I  and  opens  the  valve,  and  allows  it  to  escape.  The  air  in  H  is  that  which  is 
\  pumped  from  the  condenser  with  the  liquid,  and  from  which  it  was  disen- 
I  gaged. 

;  Let  us  suppose  the  piston  at  the  top  of  the  cylinder  :  it  strikes  the  tail  of 
the  valve  T,  and  raises  it,  while  the  stem  of  the  piston-valve  R  strikes  the  top 
of  the  cylinder,  and  is  pressed  into  its  seat.  A  free  communication  is  at  the 
same  time  open  between  the  cylinder,  below  the  piston  and  the  condenser, 
through  the  tube  D.  The  pressure  of  the  steam  thus  admitted  above  the  pis- 
ton acting  against  the  vacuum  below  it,  will  cause  its  descent.  On  arrivino- 
at  the  bottom  of  the  cylinder,  the  tail  of  the  piston-valve  R  will  strike  the  bot- 
tom, and  it  will  be  lifted  from  its  seat,  so  that  a  communication  will  be  opened 
through  it  with  the  condenser.  At  the  same  moment,  a  projecting  spring  K, 
attached  to  the  piston-rod,  strikes  the  stem  of  the  steam-valve  T,  and  presses 
it  into  its  seat.  Thus,  while  the  further  admission  of  steam  is  cut  ofl^,  the 
steam  above  the  piston  flows  into  the  condenser,  and  the  piston  being  relieved 
from  all  pressure,  is  drawn  up  by  the  momentum  of  the  fly-wheel,  which  con- 
tinues the  motion  it  received  from  the  descending  force.  On  the  arrival  of  the 
piston  again  at  the  top  of  the  cylinder,  the  valve  T  is  opened  and  R  closed,  and 
the  piston  descends  as  before,  and  so  the  process  is  continued. 

The  mechanism  by  which  motion  is  communicated  from  the  piston  to  the 
fly-wheel  is  peculiarly  elegant.  On  the  axis  of  the  fly-wheel  is  a  small  wheel 
with  teeth,  which  work  in  the  teeth  of  another  larger  wheel  L.  This  wheel 
is  turned  by  a  crank,  which  is  worked  by  a  cross-piece  attached  to  the 
end  of  the  piston-rod.  Another  equal-toothed  wheel  M  is  turned  by  a 
crank,  which  is  worked  by  the  other  end  of  the  cross-arm  attached  to  the 
piston-rod. 

One  of  the  peculiarities  of  this  engine  is,  that  the  liquid  which  is  used 
for  the  production  of   steam    in  the  boiler  circulates    through    the    machine 
without  either  diminution  or  admixture  with  any  other  fluid,  so  that  the  boiler 
never  wants  more  feeding  than  what  can  be  supplied  from  the  hot  well  H.  ^ 
This  circumstance  forms  an  important  feature  in  the  machine,  as  it  allows  of  ' 
ardent  spirits  being  used  in  the  boiler  instead  of  water,  which,  since  they  i 
boil  at  low  heats,  promised  a  saving  of  fuel.     The   inventor  proposed  that  ' 
the  engine  should  be  used  as  a  still,  as  well  as  a  mechanical  power,  in  which  < 
case  the  whole  of  the  fuel  would  be  saved.  * 

That  part  of  Cartwright's  piston  which  in  the  common  piston  is  occupied  < 
by  the  packing  of  gasket,  already  explained,  was  filled  by  a  number  of  rings, 
one  placed  within  and  above  another,  and  divided  into  three  or  four  seg- 
ments. Two  rings  of  brass  were  made  of  the  full  size  of  the  cylinder,  and 
so  ground  as  to  fit  the  cylinder  nearly  steam-tight.  These  were  cut  into 
several  segments  A  A  A,- fig.  48,  and  were  placed  one  above  the  other,  so 
as  to  fill  the  space  between  the  top  and  bottom  plates  of  the  piston.  The 
divisions  of  the  segments  of  the  one  ring  were  made  to  fit  between  the  di- 
visions of  the  other.  Within  these,  another  series  of  rings,  B  B  B,  were 
placed,  similarly  constructed,  so  as  to  fit  within  the  first  series  in  the  same 
manner  as  the  first  series  were  made  to  fit  within  the  cylinder.  The  joints 
of  the  upper  series  of  each  set  of  rings  are  exhibited  in  the  plan  fig.  48  ; 


488 


THE  STEAM-ENGINE. 


Fig.  48. 


the  places  of  the  joints  of  the  lower  series  are  shown  by  dotted  lines  ;  the 
position  of  the  rings  of  each  series  one  above  the  other  is  shown  in  the  sec- 
tion fig.  49.     The  joints  of  the  inner  series  of  rings  are  so  placed  as  to  lie 


Fisr.  49. 


between  those  of  the  outer  series,  to  prevent  the  escape  of  steam  which 
would  take  place  by  one  continued  joint  from  top  to  bottom  of  the  packing. 
The  segments  into  which  the  rings  are  divided  are  pressed  outward  by  steel 
springs  in  the  form  of  the  letter  V,  the  springs  which  act  upon  the  outer 
series  of  segments  abutting  upon  the  inner  series,  and  those  which  act  on  the 
inner  series  abutting  upon  the  solid  centre  of  the  piston  :  these  springs  are 
represented  in  fig.  48. 

An  improved  form  was  given  to  the  metallic  piston  by  Barton.  Barton's 
piston  consists  of  a  solid  cylinder  of  cast-iron,  represented  at  A  in  section  in 
fig.  50,  and  in  plan  in  fig.  51.     In  the  centre  of  this  is  a  conical  hole,  in- 


Fig.  50. 


THE  STEAM-ENGINE. 


489 


Fig.  51. 


creasing  in  magnitude  downward,  to  receive  the  piston-rod,  in  which  the 
latter  is  secured  by  a  cross-pin  B.  A  deep  groove,  square  in  its  section,  is 
formed  around  the  piston,  so  that  while  the  top  and  bottom  of  the  piston 
form  circles  equal  in  magnitude  to  the  section  of  the  cylinder,  the  interme- 
diate part  of  the  body  of  the  piston  ^forms  a  circle  less  than  the  former  by 
the  depth  of  the  groove.  Let  a  ring  of  brass,  cast-iron,  or  cast-steel,  be 
made  to  correspond  in  magnitude  and  form  with  this  groove,  and  let  it  be 
divided  as  represented  in  fig.  51,  into  four  segments  C  C  C  C,  and  four  cor- 
responding angular  pieces  D  D  D  D.  Let  the  groove  which  surrounds  the 
piston  be  filled  by  the  four  segments  with  the  four  wedge-like  angular  pieces 
within  them,  and  let  the  latter  be  urged  against  the  former  by  eight  spiral 
springs,  as  represented  in  fig.  50  and  fig.  51.  These  springs  will  abut 
against  the  solid  centre  by  the  piston,  and  will  urge  the  segments  C  against 
the  cylinder.  The  spiral  springs  which  urge  the  wedges  are  confined  in 
their  action  by  steel  pins  which  pass  through  their  centre,  and  by  being 
confined  in  cylindrical  cavities  worked  into  the  wedges  and  into  correspond- 
ing parts  of  the  solid  centre  of  the  piston,  as  the  segments  C  wear,  the  springs 
urge  the  wedges  outward,  and  the  points  of  the  latter  protruding,  are  gradu- 
ally worn  down  so  as  to  fill  up  the  spaces  left  between  the  segments,  and 
thus  to  complete  the  outer  surface  of  the  piston. 

Various  other  forms  of  metallic  pistons  have  been  proposed,  but  as  they  do 
not  differ  materially  in  principle  from  those  we  have  just  described,  it  will 
not  be  necessary  here  to  describe  them. 


THE    STEAM-ENGINE 


(FOURTH    LECTURE.) 


Analysis  of  Coal. — Process  of  Combustion. — Heat  evolved  in  it. — Form  and  Structure  of  Boiler. — 
Wagon-Boiler — Furnace. — Method  of  Feeding  it. — Combustion  of  Gas  in  Flues. — Williams's 
Patent  for  Method  of  Consuming  unburned  Gases. — Construction  of  Grate  and  Ash-Pit. — Magni- 
tude of  Heating  Surface  of  Boiler. — Steam-Space  and  Water-Space  in  Boiler. — Position  of  Flues. 
— Method  of  Feeding  Boiler. — Method  of  Indicating  the  Level  of  Water  in  Boiler. — Lever 
Gauges. — Self-Regulating  Feeders. — Steam-Gauge. — Barometer-Gauge. — Watt's  Invention  of 
the  Indicator. — Counter. — Safety- Valve. — Fusible  Plugs. — Self-Regulating  Damper. — Bruntou's 
Self  Regulating  Furnace. — Gross  and  Useful  Effect  of  an  Engine. — Power  and  Duty  of  Engines. 
— Hor.se-Power  of  Steam-Enginea. — Table  exhibiting  the  Mechanical  Povrer  of  Water  converted 
into  Steam  at  various  Pressures. — Evaporation  Proportional  to  Horse-Power. — Sources  of  Loss 
of  Pow^er. — Absence  of  good  Practical  Rules  for  Power. — Common  Rules  followed  by  Engine- 
Makers. — Duty  distinguished  from  Power. — Duty  of  Boilers. — Proportion  of  Stroke  to  Diameter 
of  Cylinder. — Duty  of  Engines. — Cornish  System  of  Inspection. — Table  showing  the  Improve- 
ment of  Cornish  Engines. — Beneficial  Effects  of  Cornish  Inspection. — Successive  Improvements 
on  which  the  increased  Duty  of  Engines  depends,  traced  by  John  Taylor  in  his  "  Records  of 
Mining." 


THE  STEAM-ENGINE. 


493 


THE   STEAM-EIGINE 


(FOURTH    LECTURE.) 


The  machinery  which  has  been  explained,  consisting  of  the  cylinder  with 
its  passages  and  valves,  the  piston-rod,  parallel  motion,  beam,  connecting-rod 
and  crank,  together  with  the  condenser,  air-pump,  and  other  appendages,  having 
no  source  of  moving  power  in  themselves,  must  be  regarded  as  mere  instru- 
ments by  which  the  mechanical  effect  developed  by  the  furnace  and  the  boiler 
is  transmitted  to  the  working  point  and  so  modified  as  to  be  adapted  to  the 
uses  to  which  the  machine  is  applied.  The  boiler  is  at  once  a  magazine  in 
which  the  moving  power  is  stored  in  sufficient  quantity  to  supply  the  demands 
of  the  engine  and  an  apparatus  in  which  that  power  is  fabricated.  The  me- 
chanical effect  evolved  in  the  conversion  of  water  into  steam  by  heat,  is  the 
process  by  which  the  power  of  the  steam-engine  is  produced,  and  space  is 
provided  in  the  boiler,  capacious  enough  to  contain  as  much  steam  as  is  neces- 
sary for  the  engine,  besides  a  sufficient  quantity  of  water  to  continue  that  supply 
undiminished,  notwithstanding  the  constant  drafts  made  upon  it  by  the  cylin- 
der :  even  the  water  itself,  from  the  evaporation  of  which  the  mechanical 
power  is  produced,  ought  to  be  regarded  as  an  instrument  by  which  the  effect 
of  the  heat  of  the  combustible  is  rendered  mechanically  efficient,  inasmuch  as 
the  same  heat,  applied  not  only  to  other  liquids  but  even  to  solids,  would  like- 
wise be  productive  of  mechanical  effects.  The  boiler  and  its  furnace  are 
therefore  parts  of  the  steam-engine,  the  construction  and  operation  of  which 
are  entitled  to  especial  attention. 

Coal,  the  combustible  almost  universally  used  in  steam-engines,  is  a  sub- 
stance, the  principal  constituents  of  which  are  carbon  and  hydrogen,  occasional- 
ly mixed  with  sulphur  in  a  small  proportion,  and  earthy  incombustible  matter. 
In  different  sorts  of  coal  the  proportions  of  these  constituents  vary,  but  in  coal 
of  good  quality  about  three  quarters  of  the  whole  weight  of  the  combustible  is 
carbon. 

When  carbon  is  heated  to  a  temperature  of  about  700°  in  an  atmosphere  oi 


494 


THE  STEAM-ENGINE. 


pure  oxygen,  it  will  combine  chemically  with  that  gas,  and  the  product  will 
be  the  gas  called  carbonic  acid.  The  volume  of  carbonic  acid  produced  by 
this  combination,  will  be  exactly  equal  to  that  of  the  oxygen  combined  with 
the  carbon,  and  therefore  the  weight  of  a  given  volume  of  the  gas  will  be  in- 
creased by  the  weight  of  carbon  which  enters  the  combination.  It  is  found 
that  two  parts  by  weight  of  oxygen  combined  with  three  of  carbon,  form  car- 
bonic acid.  The  weight  of  the  carbonic  acid,  therefore,  produced  in  the  com- 
bustion, will  be  greater  than  the  weight  of  the  oxygen,  bulk  for  bulk,  in  the 
proportion  of  five  to  two,  the  volume  being  the  same  and  the  gases  being  com- 
pared at  the  same  temperatures  and  under  equal  pressures.  In  this  combina- 
tion heat  is  evolved  in  very  large  quantities.  This  effect  arises  from  the  heat 
previously  latent  in  the  carbon  and  oxygen  being  rendered  sensible  in  the 
process  of  combustion.  The  carbonic  acid  proceeding  from  the  combustion  is 
by  such  means  raised  to  a  very  high  temperature,  and  the  carbon  during  the 
process  acquires  a  heat  so  intense  as  to  become  luminous  ;  no  flame,  however, 
is  produced. 

Hydrogen,  heated  to  a  temperature  of  about  1,000°,  in  contact  with  oxygen 
will  combine  with  the  latter,  and  a  great  evolution  of  heat  will  attend  the  pro- 
cess ;  the  gases  will  be  rendered  luminous,  and  flame  will  be  produced.  The 
product  of  this  process  will  be  water,  which  being  exposed  to  the  intense  heat 
of  combustion,  will  be  immediately  converted  into  steam.  Hydrogen  combines 
with  eight  times  its  own  weight  of  oxygen,  producing  nine  times  its  own 
weight  of  water. 

Hydrogen  gas  is,  however,  not  usually  disengaged  from  coal  in  a  simple 
form,  but  combined  chemically  with  a  certain  portion  of  carbon,  the  combina- 
tion being  called  carburetted  hydrogen.  Pure  hydrogen  burns  with  a  very 
faintly  luminous  blue  flame,  but  carburetted  hydrogen  gives  that  bright  flame 
occasionally  having  an  orange  or  reddish  tinge,  which  is  seen  to  issue  from 
burning  coals  :  this  is  the  gas  used  for  illumination,  being  expelled  from  the 
coal  by  the  process  of  coking,  and  conducted  to  the  various  burners  through 
proper  pipes. 

The  sulphur,  which  in  a  very  small  proportion  is  contained  in  coals,  is  also 
combustible,  and  combines  in  the  process  of  combustion  with  oxygen,  forming 
sulphurous  acid :  it  is  also  sometimes  evolved  in  combination  with  hydrogen, 
forming  sulphuretted  hydrogen. 

Atmospheric  air  consists  of  two  gases,  azote  and  oxygen,  mixed  together  in 
the  proportion  of  four  to  one  ;  five  cubic  feet  of  atmospheric  air  consisting  of 
four  cubic  feet  of  azote  and  one  of  oxygen.  Any  combustible  will  combine 
with  the  oxygen  contained  in  atmospheric  air,  if  raised  to  a  temperature  some- 
what higher  than  that  which  is  necessary  to  cause  its  combustion  in  an  at- 
mosphere of  pure  oxygen. 

If  coals,  therefore,  or  other  fuel  exposed  to  atmospheric  air,  be  raised  to  a 
sufficiently  high  temperature,  their  combustible  constituents  will  combine  with 
the  oxygen  of  the  atmospheric  air,  and  all  the  phenomena  of  combustion  will 
ensue.  In  order,  however,  that  the  combustion  should  be  continued,  and 
should  be  carried  on  with  quickness  and  activity,  it  is  necessary  that  the 
carbonic  acid,  and  other  products,  should  be  removed  from  the  combustible  as 
they  are  produced,  and  fresh  portions  of  atmospheric  air  brought  into  contact 
with  it;  otherwise  the  combustible  would  soon  be  surrounded  by  an  atmosphere 
composed  chiefly  of  carbonic  acid  to  the  exclusion  of  atmospheric  air,  and 
therefore  of  uncombined  oxygen,  and  consequently  the  combustion  would  cease, 
and  the  fuel  be  extinguished.  To  maintain  the  combustion,  therefore,  a  cur- 
rent of  atmospheric  air  must  be  constantly  carried  through  the  fuel :  the  quanti- 
ty and  force  of  this  current  must  depend  on  the  quantity  and  quality  of  the  fuel 


THE  STEAM-ENGINE. 


495 


to  be  consumed.  It  must  be  such  that  it  shall  supply  sufficient  oxygen  to  the 
fuel  to  maintain  the  combustion,  and  not  more  than  sufficient,  since  any  excess 
would  be  attended  with  the  effect  of  absorbing  the  heat  of  combustion,  without 
contributing  to  the  maintenance  of  that  effect. 

Heat  is  communicated  from  body  to  body  in  two  ways,  by  radiation  and  by 
contact. 

Rays  of  heat  issue  from  a  heated  body,  and  are  dispersed  through  the  sur- 
rounding space  in  a  manner,  and  according  to  laws,  similar  to  those  which 
govern  the  radiation  of  light.  The  heat  thus  radiated  meeting  other  bodies  is 
imparted  to  them,  and  penetrates  them  with  more  or  less  facility  according  to 
their  physical  qualities. 

A  healed  body  also  brought  into  contact  with  another  body  of  lower  tem- 
perature, communicates  heat  to  that  other  body,  and  will  continue  to  do  so 
until  the  temperature  of  the  two  bodies  in  contact  shall  be  equalized.  Heat 
proceeds  from  fuel  in  a  state  of  combustion  in  both  these  ways  :  the  heated 
fuel  radiates  heat  in  all  directions  around  it,  and  the  heat  thus  radiated  will  be 
imparted  to  all  parts  of  the  furnace  which  are  exposed  to  the  fuel. 

The  gases,  which  are  the  products  of  the  combustion,  escape  from  the  fuel 
at  a  very  high  temperature,  and  consequently,  in  acquiring  that  temperature 
they  absorb  a  considerable  quantity  of  the  heat  of  combustion.  But  besides 
the  gases  actually  formed  in  the  process  of  combustion,  the  azote  forming  four 
fifths  of  the  air  carried  through  the  fuel  to  support  the  combustion,  absorbs  heat 
from  the  combustible,  and  rises  into  the  upper  part  of  the  furnace  at  a  high 
temperature.     These  various  gases,  if  conducted  directly  to  the  chimney,  would 


Fig.  52. 


496 


THE  STEAM-ENGINE. 


carry  off  with  them  a  considerable  quantity  of  the  heat.  Provision  should 
therefore  be  made  to  keep  them  in  contact  with  the  boiler  such  a  length  of 
time  as  will  enable  them  to  impart  such  a  portion  of  the  heat  which  they  have 
absorbed  from  the  fuel,  as  will  still  leave  them  at  a  temperature  sufficient,  and 
not  more  than  sufficient,  to  produce  the  necessary  draught  in  the  chimney. 

The  forms  of  boiler  which  have  been  proposed  as  the  most  convenient  for 
the  attainment  of  all  these  requisite  purposes  have  been  very  various.  If 
strength  alone  were  considered,  the  spherical  form  vi'ould  be  the  best ;  and  the 
early  boilers  were  very  nearly  hemispheres,  placed  on  a  slightly  concave  base. 
The  form  adopted  by  Watt,  called  the  wagon-boiler,  consists  of  a  semi- 
cylindrical  top,  flat  perpendicular  sides,  flat  ends,  and  -  a  slightly  concave 
bottom.  The  steam  intended  to  be  used  in  boilers  of  this  description  did  not 
exceed  the  pressure  of  the  external  atmosphere  by  more  than  from  3  to  5  lbs., 
per  square  inch  ;  and  the  flat  sides  and  ends,  though  unfavorable  to  strength, 
could  be  constructed  sufficiently  strong  for  this  purpose.  In  a  boiler  of  this 
sort,  the  air  and  smoke  passing  through  the  flues  that  are  carried  round  it,  are 
in  contact  at  one  side  only  with  the  boiler.  The  brickwork,  or  other  materials 
forming  the  flue,  must  therefore  be  non-conductors  of  heat,  that  they  may  not 
absorb  any  considerable  portion  of  heat  from  the  air  passing  in  contact  with 
them.     A  boiler  of  this  form  is  represented  in  fig.  52. 

The  grate  and  a  part  of  the  flues  are  rendered  visible  by  the  removal  of  a 
portion  of  the  surrounding  masonry  in  which  the  boiler  is  set.  The  interior 
of  the  boiler  is  also  shown  by  cutting  off  one  half  of  the  semi-cylindrical  roof. 
A  longitudinal  vertical  section  is  shown  in  fig.  53,  and  across  section  in  fig.  54. 


Fig.  53 


A  horizontal  section  taken  above  the  level  of  the  grate,  and  below  the  level  of 
the  water  in  the  boiler,  showing  the  course  of  the  flues,  is  given  in  fig.  55. 
The  corresponding  parts  in  all  the  figures  are  marked  by  the  same  letters. 


THE  STEAM-ENGINE. 


Fig.  54. 


N     ^^ 


The  door  by  which  fuel  is  introduced  upon  the  grate  is  represented  at  A, 
and  the  door  leading  to  the  ash-pit  at  B.  The  fire-bars  at  C  slope  downward 
from  the  front  at  an  angle  of  about  25°,  giving  a  tendency  to  the  fuel  to  move 
from  the  front  toward  the  back  of  the  grate.  The  ash-pit  D  is  constructed  of 
such  a  magnitude,  form,  and  depth,  as  to  admit  a  current  of  atmospheric  air  to 
the  grate-bars,  sufficient  to  sustain  the  combustion.  The  form  of  the  ash-pit  is 
usually  wide  below,  contracting  toward  the  cop. 

The  fuel  when  introduced  at  the  fire-door  A,  should  be  laid  on  that  part  of 
the  grate  nearest  to  the  fire-door,  called  the  dead  plates :  there  it  is  submitted 
to  the  process  of  coking,  by  which  the  gases  and  volatile  matter  which  it  con- 
tains are  expelled,  and  being  carried  by  a  current  of  air,  admitted  through  small 
apertures  in  the  fire-door  over  the  burning  fuel  in  the  hinder  part  of  the  grate, 
they  are  burnt.  When  the  fuel  in  front  of  the  grate  has  been  thus  coked,  it  is 
pushed  back,  and  a  fresh  feed  introduced  in  front.  The  coal  thus  pushed  back 
soon  becomes  vividly  ignitefJ,  and  by  continuing  this  process,  the  fuel  spread 
over  the  grate  is  maintained  in  the  most  active  state  of  combustion  at  the 
hinder  part  of  the  grate.  By  such  an  arrangement,  the  smoke  produced  by 
the  combustion  of  the  fuel  may  be  burnt  before  it  enters  the  flues.  The  flame 
and  heated  air  proceeding  from  the  burning  fuel  arising  from  the  grate,  and 
rushing  toward  the  back  of  the  furnace,  passes  over  the  Jire-bridge  E,  and  is 
carried  through  the  flue  F  which  passes  under  the  boiler.  This  flue  (the 
cross  section  of  which  is  shown  in  fig.  54,  by  the  dark  shade  put  under  the 
boiler)  is  very  nearly  equal  in  width  to  the  bottom  of  the  boiler,  the  space  at 
the  bottom  of  the  boiler,  near  the  corners,  being  only  what  is  sufficient  to  give 
the  weight  of  the  boiler  support  on  the  masonry  forming  the  sides  of  the  flue. 
The  bottom  of  the  boiler  being  concave,  the  flame  and  heated  air  as  they  pass 
along  the  flue  rise  to  the  upper  part  by  the  effects  of  their  high  temperature, 
and  lick  the  bottom  of  the  boiler  from  the  fire-bridge  at  E  to  the  further  end  G. 

At  G  the  flue  arises  to  H,  and  turning  to  the  side  of  the  boiler  at  I  I,  con- 

VOL,.  II — 3!i 


THE    STEAM-ENGINE. 


Fig.  55. 


gjm^^ \\\v^^%^  -^^iw^^-wx  -  ?^  mv-x-  A»>m\---'^'^\\¥^:?^;»^^^>f^ 


ducts  the  flame  in  contact  with  the  side  from  the  back  to  the  front;  it  then  passes 
through  the  flue  K  across  the  front,  and  returns  to  the  back  by  the  other  side- 
flue  L.  The  side-flue  is  represented,  stripped  of  the  masonry,  in  fig.  52,  and 
also  appears  in  the  plan  in  fig.  55,  and  in  the  cross  section  in  fig.  54.  The 
course  of  the  air  is  represented  in  fig.  55,  by  the  arrows.  From  the  flue  L  the 
air  is  conducted  into  the  chimney  at  M. 

By  such  an  arrangement,  the  flame  and  heated  air  proceeding  from  the  grate 
are  made  to  circulate  round  the  boiler,  and  the  length  and  magnitude  of  the 
flues  through  which  it  is  conducted  should  be  such,  that  when  it  shall  arrive 
at  the  chimney  its  temperature  shall  be  reduced,  as  nearly  as  is  consistent 
with  the  maintenance  of  draught  in  the  chimney,  to  the  temperature  of  the 
water  with  which  it  is  in  contact. 

The  method  of  feeding  the  furnace,  which  has  been  described  above,  is  one 
which,  if  conducted  with  skill  and  care,  would  produce  a  much  more  perfect 
combustion  of  the  fuel  than  would  attend  the  common  method  of  filling  the 
grate  from  the  back  to  the  front  with  fresh  fuel,  whenever  the  furnace  is  fed. 
This  method,  however,  is  rarely  observed  in  the  management  of  the  furnace. 
It  requires  the  constant  attention  of  the  stokers  (such  is  the  name  given  to 
those  who  feed  the  furnaces).  The  fuel  must  be  supplied,  not  in  large  quanti- 
ties, and  at  distant  intervals,  but  in  small  quantities  and  more  frequently.  On 
the  other  hand,  the  more  common  practice  is  to  allow  the  fuel  on  the  grate  to 
be  in  a  great  degree  burned  away,  and  then  to  heap  on  a  large  quantity  of  fresh 
fuel,  covering  over  with  it  the  burning  fuel  from  the  back  to  the  front  of  the 
grate.  When  this  is  done,  the  heat  of  the  ignited  coal  acting  upon  the  fresh 
fuel  introduced,  expels  the  gases  combined  with  it  and,  mixed  with  these,  a 
quantity  of  carbon,  in  a  state  of  minute  division,  forming  an  opaque  black 
smoke.  This  is  carried  through  the  flues  and  drawn  up  the  chimney.  The 
consequence  is,  that  not  only  a  quantity  of  solid  fuel  is  sent  out  of  the  chimney 
unconsumed,  but  the  hydrogen  and  other  gases  also  escape  unburnt,  and  a 
proportional  waste  of  the  combustible  is  produced  ;  besides  which,  the  nuisance 
of  an  atmosphere  filled  with  smoke  ensues.  Such  effects  are  visible  to  all 
who  observe  the  chimneys  of  steam-vessels,  while  the  engine  is  in  operation. 
When  the  furnaces  are  thus  filled  with  fresh  fuel,  a  large  volume  of  dense 
black  smoke  is  observed  to  issue  from  the  chimney.  This  gradually  subsides 
as  the  fuel  on  the  grate  is  ignited,  and  does  not  reappear  until  a  fresh  feed  is 
introduced. 

This  method  of  feeding,  by  which  the  furnace  would  be  made  to  consume 
its  own  smoke,  and  the  combustion  of  the  fuel  be  rendered  complete,  is  not 


THE  STEAM-ENGINE. 


499 


however  free  from  counteracting  effects.  In  ordinary  furnaces  the  feed  can 
only  be  introduced  by  opening  the  fire-doors,  and  during  the  time  the  fire-doors 
are  opened  a  volume  of  cold  air  rushes  in,  which  passing  through  the  furnace 
is  carried  througii  the  flues  to  the  chimney.  Such  is  the  effect  of  this  in 
lowering  the  temperature  of  the  flues,  that  in  many  cases  the  loss  of  heat  oc- 
casioned is  greater  than  any  economy  of  fuel  obtained  by  the  complete  con- 
sumption of  smoke.  Various  methods,  however,  may  be  adopted  by  which 
fuel  may  be  supplied  to  the  grate  without  opening  the  fire-doors,  and  without 
disturbing  the  supply  of  air  to  the  fire.  A  hopper  built  into  the  front  of  the 
furnace,  with  a  moveable  bottom  or  valve,  by  which  coals  may  be  allowed  to 
drop  in  from  time  to  lime  upon  the  front  of  the  grate,  would  accomplish  this. 

In  order  to  secure  the  combustion  of  the  gases  evolved  from  the  coals 
placed  in  the  front  of  the  grate,  it  is  necessary  that  a  supply  of  atmospheric  air 
should  be  admitted  with  themover  the  burning  fuel.  This  is  effected  by  small 
apertures  or  regulators,  provided  in  the  fire-doors,  governed  by  sliding-plates, 
by  which  they  may  be  opened  or  closed  to  any  required  extent. 

A  patent  has  recently  been  granted  to  Mr.  Williams,  one  of  the  directors 
of  the  city  of  Dublin  steam  navigation  company,  ibr  a  method  of  consuming 
the  unburnt  gases  which  escape  from  the  grate,  and  are  carried  through  the 
flues.  This  method  consists  in  introducing  into  the  flue  tubes  placed  in  a 
vertical  position,  the  lower  ends  of  which  being  inserted  in  the  bottom  of  the 
flue  are  made  to  communicate  with  the  ash-pit,  and  the  upper  ends  of  which 
are  closed.  The  sides  and  tops  of  these  tubes  are  pierced  with  small  holes, 
through  which  atmospheric  air  drawn  from  the  ash-pit  issues  in  jets.  The 
oxygen  supplied  by  this  air  immediately  combines  with  the  carburetted  hydro- 
gen, which  having  escaped  from  the  furnace  unburnt  is  carried  through  the 
flues  at  a  sufficient  temperature  to  enter  into  combination  with  the  oxygen  ad- 
nulted  through  holes  in  the  tubes.  A  number  of  jets  of  flame  thus  proceed 
from  these  holes,  having  an  appearance  similar  to  the  flame  of  a  gas-lamp. 

It  is  evident  that  such  tubes  must  be  inefficient  unless  they  are  placed  in 
the  flues  so  near  the  furnace,  that  the  temperature  of  the  unburnt  gases  shall 
be  sufiiciently  high  to  produce  their  combustion. 

The  magnitude  of  the  grate  and  ash-pit  must  be  determined  by  the  rate  at 
which  the  evaporation  is  required  to  be  conducted  in  the  boiler  and  the  quality 
of  the  fuel.  It  must  be  a  matter  of  regret,  that  the  proportions  of  the  various 
parts  of  steam-engines,  with  their  boilers  and  furnaces,  have  not  been  deter- 
mined by  any  exact  or  satisfactory  experiments  ;  and  those  who  project  and 
manufacture  the  engines  themselves,  are  not  less  in  ignorance  on  those  points 
than  others.  With  coals  of  the  common  quality  a  certain  average  proportion 
must  exist  between  the  necessary  magnitude  of  the  grate-surface  and  the 
quantity  of  water  to  be  evaporated  in  a  given  time  in  the  boiler.  But  what 
that  proportion  is  for  any  given  quality  of  fuel,  is  at  present  unascertained. 
Each  engine-maker  follows  his  own  rule,  and  the  rule  thus  followed  is  in 
most  cases  a  matter  of  bare  conjecture,  unsupported  by  any  experimental 
evidence.  Some  engine-makers  will  allow  a  square  foot  of  grate-surface  for 
every  cubic  foot  of  water  per  hour,  which  is  expected  to  be  evaporated  in  the 
boiler;  others  allow  only  half  a  square  foot;  and  practice  varies  between  these 
limits.  Bituminous  coals  which  melt  and  cake,  and  which  burn  with  much 
flame  and  smoke,  must  be  spread  more  thinly  on  the  grate  than  other  descrip- 
tions of  fuel,  otherwise  a  considerable  quantity  of  combustible  gases  would  be 
dismissed  into  the  flues  unburnt.  Such  coals  therefore,  other  circumstances 
being  the  same,  require  a  larger  portion  of  grate-surface  ;  and  the  same  may 
be  said  of  coals  which  produce  clinkers  in  their  combustion,  and  form  lumps 
of  vitrified   matter  on  the  grate,  by  which  the  spaces  between  the  grate-bars 


THE  STEAM-ENGINE. 


are  speedily  closed  up.  When  such  fuel  is  used,  the  grate-bars  require  to  be  i 
frequently  raked  out,  otherwise  the  spaces  between  them  being  obstructed,  the  ' 
draught  would  become  insufficient  for  the  due  combustion  of  the  fuel.  , 

To  facilitate  the  raking  out  of  the  grate,  the  bars  are  placed  with  their  ends  ' 
toward  the  fire-door  :  they  are  usually   made   of  cast-iron,  from  two  to  two  < 
inches  and  a  half  wide  on  the  upper  surface,  with  intervals  of  nearly  half  an  ' 
inch  between  them.     The   bars   taper  downward,  their  under  surfaces  being 
much  narrower  than  their  upper,  the  spaces  between  them  thus  widening,  to 
facilitate  the  fall  of  the  ashes  between  them.     The  grate-bars  slope  downward 
from  the  front  to  the  back.     The  height  of  the  centre  of  the   bottom  of  the 
boiler,  above  the  front  of  the  grate,  is  usually  about  two  feet,  and  about  three 
feet  above  the  back  of  it.     The  concave  bottom  of  the  boiler,  however,  brings 
its  surfaces  at  the  slide  closer  to  the  grate. 

Between  the  evaporating  power  of  the  boiler,  and  the  magnitude  of  surface 
it  exposes  to  the  action  of  the  furnace,  there  is  a  relation  which,  like  that  of 
the  grate  surface,  has  never  been  ascertained  by  any  certain  or  satisfactory 
experiihental  investigation  ;  much  less  have  the  different  degrees  of  efficiency 
attending  different  parts  of  the  boiler-surface  been  determined.  That  part  of 
the  surface  of  the  boiler  immediately  over  and  around  the  grate,  is  exposed  to 
the  immediate  radiation  of  the  burning  fuel,  and  is  therefore  probably  the  most 
efficient  in  the  production  of  steam.  The  tendency  of  flame  and  heated  air  to 
rise,  would  naturally  bring  them  in  the  flues  into  closer  contact  with  those  parts 
of  the  boiler-surface  which  are  horizontal  in  their  position,  and  which  form 
the  tops  of  the  flues,  than  with  those  which  are  lateral  or  vertical  in  their 
position,  and  which  form  the  sides  of  the  flues.  Jn  a  boiler  constructed  like 
that  already  described,  the  flue-surface,  therefore,  which  would  be  most  efficient, 
would  be  the  concave  bottom  of  the  boiler  extending  from  the  fire-bridge  to  its 
remote  end.  In  some  boilers,  especially  those  in  which  steam  of  high  pressure 
is  produced,  the  fonn  is  cylindrical,  the  middle  flue  being  formed  into  an 
elliptical  tube  the  greater  axis  of  which  is  horizontal  from  end  to  end  of  the 
boiler.  It  seems  doubtful,  however,  whether  in  such  a  boiler  the  heat  produces 
any  useful  effect  on  the  water  below  the  flue,  the  water  above  being  always  at 
a  higher  temperature,  and  therefore  lighter  than  that  below,  and  consequenily 
no  currents  being  established  between  the  upper  and  lower  strata  of -the  water. 

It  was  considered  by  Mr.  Watt,  but  we  are  not  aware  on  what  experi- 
mental grounds,  that  from  eight  to  ten  square  feet  of  heating  surface  were  suf- 
ficient to  produce  the  evaporation  of  one  cubic  foot  of  water  per  hour.  The 
practice  of  engine-makers  since  that  time  has  been  to  increase  the  allowance  of 
heating  surface  for  the  same  rate  of  evaporation.  Engine-builders  have  va- 
ried very  inuch  in  this  respect,  some  allowing  twelve,  fifteen,  and  even  eighteen 
square  feet  of  surface  for  the  same  rate  of  evaporation.  It  must,  however,  still  be 
borne  in  mind,  that  whether  this  increased  allowance  did  or  did  not  produce 
the  actual  evaporation  imputed  to  it,  has  not  been,  as  far  as  we  are  informed, 
ever  accurately  ascertained.  The  production  of  a  given  rate  of  evaporation 
by  a  moderate  heat  diff'used  over  a  larger  surface,  rather  than  by  a  fiercer  tem- 
perature confined  to  a  smaller  surface,  is  attended  with  many  practical  advan- 
tages. The  plates  of  the  boiler  acted  upon  by  the  fire  are  less  exposed  to 
oxydization,  and  the  boiler  will  be  proportionally  more  durable. 

Besides  presenting  to  the  action  of  the  fire  a  sufficient  surface  to  produce 
steam  at  the  required  rate,  the  capacity  of  the  boiler  must  be  proportioned  to 
the  quantity  of  water  to  be  evaporated.  The  space  within  the  boiler  is  appro- 
priated to  a  twofold  purpose  :  first,  to  contain  the  water  to  be  evaporated  ; 
secondly,  to  contain  a  quantity  of  ready-made  steam  for  the  supply  of  the  cyl- 
inder.    If  the  space  appropriated  to  the  steam  did  not  bear  a  considerable  pro- 


THE   STKAM-ENGINE. 


501 


portion  to  the  magnitude  of  the  cylinder,  the  momentary  expansion  of  the  steam 
passing  to  the  cylinder  from  the  boiler  at  each  stroke  would  reduce  the  pres- 
sure of  the  steam  in  a  great  proportion,  and  unless  the  pressure  in  the  boiler 
were  considerably  greater  than  that  which  the  steam  is  intended  to  have  in 
the  cylinder,  the  pressure  in  the  latter  would  be  reduced  below  the  proper 
amount.  The  proportion  of  the  steam-space  in  the  boiler  to  the  magnitude  of 
the  cylinder  has  been  very  variously  estimated,  nor  can  it  be  said  that  any 
practical  rule  of  a  general  kind  has  been  adopted.  It  is  held  by  some  that  the 
steam-space  will  be  sufficient  if  it  contain  five  times  the  quantity  of  steam  con- 
sumed at  each  stroke,  while  others  maintain  that  it  should  contain  a,t  least  ten 
times  that  quantity,  and  opinions  vary  between  these  limits. 

The  proportion  of  water-space  in  the  boiler  to  its  evaporating  power  should 
also  be  regulated,  so  that  the  introduction  of  the  feed  at  a  comparati\'x3ly  low 
temperature  may  not  unduly  chill  the  water  in  the  boiler.  Supposing  the  feed 
to  be  introduced  in  a  low-pressure  boiler  at  the  temperature  of  100°,  and  that 
the  necessary  temperature  within  the  boiler  be  225°,  the  quantity  of  water  it 
contains  should  be  about  five  times  the  quantity  evaporated,  and  therefore  also 
five  limes  the  quantity  introduced  through  the  feed  per  hour.  For  every  cubic 
foot  of  water  per  hour,  therefore,  intended  to  be  evaporated  by  the  boiler,  water- 
space  for  five  cubic  feet  should  be  provided.  It  is,  however,  right  to  repeat 
that  this  (like  almost  every  other  so-called  rule)  is  the  result,  not  of  any  exact 
general  calculation,  but  one  deduced  from  the  custom  which  has  obtained 
among  the  manufacturers  of  steam-engines. 

The  surface  of  the  water  in  the  boiler  should  always  be  above  the  range  of 
the  flues.  When  the  heated  air  in  the  flues  acts  upon  a  part  of  the  boiler 
within  which  water  is  contained,  the  water  within  receiving  an  increased  tem- 
perature becomes,  bulk  for  bulk,  lighter  than  the  strata  of  water  above  it,  and 
ascends.  It  is  replaced  by  the  descending  strata,  which,  in  their  turn  receiv- 
ing increased  temperature,  rise  to  the  surface  ;  or  if  the  action  of  the  heat  con- 
vert the  water  into  steam,  the  bubbles  of  steam  rise  to  the  surface,  fresh  por- 
tions of  water  continually  coming  into  contact  with  the  boiler-plates  on  which 
the  heated  air  or  flame  acts.  By  this  process  the  boiler-plates  are  continually 
cooled,  either  by  being  successively  washed  by  water  at  a  lower  temperature, 
or  by  the  heat  taken  from  them  becoming  latent  in  the  steam-bubbles  formed  in 
contact  with  them.  But  if  the  heat  act  upon  a  part  of  the  boiler  containing 
steam  within  it,  which  steam  being  a  slow  recipient  of  heat,  and  no  currents 
being  established,  nor  any  phenomenon  produced  in  which  heat  is  rendered 
I  latent,  the  heat  of  the  fire  comnmnicated  to  the  boiler-plates  accumulates  in 
'  them,  and  raises  their  temperature  to  an  injurious  degree.  The  plates  may  by 
this  means  be  softened,  so  as  to  cause  the  boiler  to  burst,  or  the  difference  be- 
tween the  expansion  of  the  highly-heated  plates  thus  exposed  to  fire  in  contact 
with  steam  and  that  of  the  plates  which  are  cooled  by  contact  with  water, 
may  cause  the  joinings  of  the  boiler-plates  to  open,  and  the  boiler  to  leak. 
By  whatever  means,  therefore,  the  boiler  be  fed,  care  should  be  taken  that  the 
evaporation  should  not  be  allowed  to  reduce  the  level  of  the  water  in  it  below 
the  highest  flue. 

As  the  water  by  which  the  boiler  is  fed  must  always  have  a  much  lower 
temperature  than  that  at  which  the  boiler  is  maintained,  the  supply  of  the  feed 
will  have  a  constant  tendency  to  lower  the  temperature  of  the  water,  and  this 
tendency  will  be  determined  by  the  proportion  between  the  magnitude  of  the 
feed  and  the  quantity  of  water  in  the  boiler. 

Since  it  is  requisite  that  the  level  of  the  water  in  the  boiler  shall  not  suff'er 
any  considerable  change,  it  is  evident  that  the  magnitude  of  the  feed  must  be 
equal  to  the  quantity  of  water  evaporated.     If  it  were  less,  the  level  of  the 


I 


502 


THE   STEAM-ENGINE. 


water  would  continually  fall  by  reason  of  the  excess  of  the  evaporation  over 
the  feed  ;  and  if  it  were  greater,  the  level  would  rise  by  the  accumulation  of 
water  in  the  boiler.  If,  therefore,  the  quantity  of  water-space  allowed  in  the 
boiler  be  five  times  the  volume  of  water  evaporated  per  hour,  the  quanti- 
ty introduced  by  the  feed  per  hour,  whether  continuously  or  at  intervals,  must 
be  of  the  same  amount.  Since  the  process  of  evaporation  is  continuous, 
the  variation  of  level  of  water  in  the  boiler  will  be  entirely  dependent  on 
the  intervals  between  the  successive  feeds.  If  the  feed  be  continuous, 
and  always  equal  to  the  evaporation,  then  the  level  of  the  water  in  the 
boiler  will  undergo  no  change ;  but  if,  while  the  evaporation  is  continu- 
ous, the  feed  be  made  at  intervals,  then  the  change  of  level  of  water  in  the 
boiler  as  well  as  its  change  of  temperature,  will  be  subject  to  a  variation  pro- 
portional to  the  intervals  between  the  successive  feeds.  It  is  manifest,  there- 
fore, that  the  feed  should  either  be  uninterrupted  or  be  supplied  at  short  inter- 
vals, so  that  the  change  of  level  and  temperature  of  the  water  in  the  boiler 
should  not  be  considerable. 

Different  methods  have  been,  from  time  to  time,  suggested  for  indicating 
the  level  of  the  water  in  the  boiler.  We  have  already  mentioned  the  two 
gauge-pipes  used  in  the  earlier  steam-engines,  and  which  are  still  generally 
continued.  There  are,  however,  some  other  methods  which  merit  our  atten- 
tion. 

A  weight  F,  fig,  56,  half  immersed  in  the  water  in  the  boiler,  is  supported 

Fig.  56.      . 
W 


0^A 

A 

F 

L 

=^=- 

s 

by  a  wire,  which,  passing  steam-tight  through  a  small  hole  in  the  top,  is  con- 
nected by  a  flexible  string  or  chain,  passing  over  a  wheel  W,  with  a  counter- 
poise A,  which  is  just  sufficient  to  balance  F  when  half  immersed.  If  F  be 
raised  above  the  water,  A  being  lighter  will  no  longer  balance  it,  and  F  will  de- 
scend, pulling  up  A,  and  turning  the  wheel  W.  If,  on  the  other  hand,  F  be 
plunged  deeper  in  the  water,  A  will  more  than  balance  it,  and  will  pull  it  up, 
so  that  the  only  position  in  which  F  and  A  will  balance  each  other  is,  when  F 
is  half  immersed.  The  wheel  W  is  so  adjusted,  that  when  two  pins  placed  on 
its  rim  are  in  the  horizontal  position,  the  water  is  at  its  proper  level.  Conse- 
quently it  follows,  that  if  the  water  rise  above  this  level,  the  weight  F  is  lifted 
and  A  falls,  so  that  the  pins  come  into  another  position.  If,  on  the  other  hand, 
the  level  of  the  waterfall,  F  falls  and  A  rises,  so  that  the  pins  assume  a  differ- 
ent position.  Thus,  in  general,  the  position  of  the  pins  becomes  an  indication 
of  the  quantity  of  water  in  the  boiler. 

Another  method  is  to  place  a  glass  tube,  fig.  57,  with  one  end  T  entering 
the  boiler  above  the  proper  level,  and  the  other  end  T'  entering  it  below  the 
proper  level.     It  must  be  evident  that  the  water  in  the  tube  will  always  stand 


THE   STEAM-ENGINE. 


503 


Fig.  57. 


at  the  same  level  as  the  water  in  the  boiler,  since  the  lower  part  has  a  free 
communication  with  that  water,  while  the  surface  is  submitted  to  the  pressure 
of  the  same  steam  as  the  water  in  the  boiler.  This  and  the  last-mentioned 
gauge  have  the  advantage  of  addressing  the  eye  of  the  engineer  at  once,  with- 
out any  adjustment  ;  whereas,  the  gauge-cocks  must  be  both  opened,  whenever 
the  depth  is  to  be  ascertained. 

These  gauges,  however,  require  the  frequent  attention  of  the  engine-man ; 
and  it  becomes  desirable  either  to  find  some  more  effectual  means  of  awaken- 
ing that  attention,  or  to  render  the  supply  of  the  boiler  independent  of  any  at- 
tention. In  order  to  enforce  the  attention  of  the  engine-man  to  replenish  the 
boiler  when  partially  exhausted  by  evaporation,  a  tube  was  sometimes  inserted 
at  the  lowest  level  to  which  it  was  intended  that  the  water  should  be  permit- 
ted to  fall.  This  tube  was  conducted  from  the  boiler  into  the  engine-house, 
where  it  terminated  in  a  mouth-piece  or  whistle,  so  that  whenever  the  water 
fell  below  the  level  at  which  this  tube  was  inserted  in  the  boiler,  the  steam 
would  rush  through  it,  and  issuing  with  great  velocity  at  the  mouth-piece, 
would  summon  the  engineer  to  his  duty  with  a  call  that  would  rouse  him  even 
from  sleep. 

In  the  most  effectual  of  these  methods,  the  task  of  replenishing  the  boiler 
should  still  be  executed  by  the  engineer  ;  and  the  utmost  that  the  boiler  itself 
was  made  to  do,  was,  to  give  due  notice  of  the  necessity  for  the  supply  of 
water.  The  consequence  was,  among  other  inconveniences,  that  the  level  of 
the  water  was  subject  to  constant  variation. 

To  remedy  this,  a  method  has  been  invented,  by  which  the  engine  is  made 
to  feed  its  own  boiler.     The  pipe  G,  fig.  58,  which  leads  from  the  hot-water 


pump,  terminates  in  a  small  cistern  C  in  which  the  water  is  received.  In  the 
bottom  of  this  cistern,  a  valve  V  is  placed,  which  opens  upward,  and  commu- 
nicates with  a  feed-pipe,  which  descends  into  the  boiler  below  the  level  of  the 


504 


THE   STEAM-ENGINE. 


water  in  it.  The  stem  of  the  valve  V  is  connected  with  a  lever  turning  on 
the  centre  D,  and  loaded  with  a  weight  F  dipped  in  the  water  in  the  boiler  in 
a  manner  similar  to  that  described  in  fig.  56,  and  balanced  by  a  counterpoise 
A  in  exactly  the  same  way.  When  the  level  of  the  water  in  the  boiler  falls, 
the  float  F  falls  with  it,  and  pulling  down  the  arm  of  the  lever  raises  the  valve 
V,  and  lets  the  water  descend  into  the  boiler  from  the  cistern  C.  When  the 
boiler  has  thus  been  replenished,  and  the  level  raised  to  its  former  place,  F 
will  again  be  raised,  and  the  valve  V  closed  by  the  weight  A.  In  practice, 
however,  the  valve  V  adjusts  itself  by  means  of  the  effect  of  the  water  on  the 
weight  F,  so  as  to  permit  the  water  from  the  feeding-cistern  C  to  flow  in  a 
continued  stream,  just  sufficient  in  quantity  to  supply  the  consumption  from 
evaporation,  and  to  maintain  the  level  of  the  water  in  the  boiler  constantly  the 
same. 

By  this  arrangement  the  boiler  is  made  to  replenish  itself,  or,  more  properly 
speaking,  it  is  made  to  receive  such  a  supply,  as  that  it  never  wants  replen- 
ishing— an  effect  which  no  effort  of  attention  on  the  part  of  an  engine-man 
could  produce.  But  this  is  not  the  only  good  effect  produced  by  this  contri- 
vance. A  part  of  the  steam  which  originally  left  the  boiler,  and  having  dis- 
charged its  duty  in  moving  the  piston,  was  condensed  and  reconverted  into 
water,  and  lodged  by  the  air-pump  in  the  hot  well,  lig.  58,  is  here  again  re- 
stored to  the  source  from  which  it  came,  bringing  back  all  the  unconsumed 


THE  STEAM-ENGINE. 


505 


portion  of  its  heat  preparatory  to  being  once  more  put  in  circulation  through 
the  machine. 

The  entire  quantity  of  hot  water  pumped  into  the  cistern  C,  is  not  always 
necessary  for  the  boiler.  A  waste-pipe  may  be  provided  for  carrying  off  the 
surplus,  which  may  be  turned  to  any  purpose  for  which  it  may  be  required  ;  or 
it  may  be  discharged  into  a  cistern  to  cool,  preparatory  to  being  restored  to 
the  cold  cistern,  in  case  water  for  the  supply  of  that  cistern  be  not  sufficiently 
abundant. 

Another  method  of  arranging  a  self-regulating  feeder  is  shown  in  fig.  59. 
A  is  a  hollow  ball  of  metal  attached  to  the  end  of  a  lever,  whose  fulcrum  is  at 
B.  The  other  arm  of  the  lever  C  is  connected  with  the  stem  of  a  spindle- 
valve,  communicating  with  a  tube  which  receives  water  from  the  feeding- 
cistern.  Thus,  when  the  level  of  the  water  in  the  boiler  subsides,  the  ball  A 
preponderating  over  the  weight  of  the  opposite  arm,  the  lever  falls,  the  arm  C 
rises  and  opens  the  valve,  and  admits  the  feeding-water.  This  apparatus  will 
evidently  act  in  the  same  manner  and  on  the  same  principle  as  that  already 
described. 

The  mouth  of  the  tube  by  which  the  feed  is  introduced  should  be  placed  at 
that  part  of  the  boiler  which  is  nearest  the  end  of  the  flues  which  issue  into 
the  chimney.  By  such  means  the  temperature  of  the  water  in  contact  with 
those  flues  will  be  lowest  at  the  place  where  the  temperature  of  the  heated  air 
intended  to  act  upon  it  is  also  lowest.  The  difference  of  the  temperatures 
will  therefore  be  greater  than  it  would  be  if  the  point  of  the  boiler  contain- 
ing water  of  a  higher  temperature  was  left  in  contact  with  this  part  of  the  flue. 

It  is  necessary  to  have  a  ready  method  of  ascertaining  at  all  times  the  pres- 
sure of  the  steam  which  is  used  in  working  the  engine.  For  this  purpose  a 
bent  tube  containing  mercury  is  inserted  into  some  part  of  the  apparatus,  which 
has  free  communication  with  the  steam.     Let  ABC,  fig,  60,  be  such  a  tube. 


Fig.  60. 


The  pressure  of  the  steam  forces  the  mercury  down  in  the  leg  A  B,  and  up  in 
the  leg  B  C.  If  the  mercury  in  both  legs  be  at  exactly  the  same  level,  the 
pressure  of  the  steam  must  be  exactly  equal  to  that  of  the  atmosphere  ,  because 


506 


THE  STEAM-ENGINE. 


the  Steam-pressure  on  the  mercury  in  A  B  balances  the  atmospheric  pressure 
on  the  mercury  in  B  C.  If,  however,  the  level  of  the  mercury  in  B  C  be  above 
the  level  of  the  mercury  in  B  A,  the  pressure  of  the  steam  will  exceed  that  of 
the  atmosphere.  The  excess  of  its  pressure  above  that  of  the  atmosphere  may 
be  found  by  observing  the  difference  of  the  level  of  the  mercury  in  the  tubes 
B  C  and  B  A,  allowing  a  pressure  of  one  pound  on  each  square  inch  for  every 
two  inches  in  the  difference  of  the  levels. 

If,  on  the  contrary,  the  level  of  the  mercury  in  B  C  should  fall  below  its 
level  in  A  B,  the  atmospheric  pressure  will  exceed  that  of  the  steam,  and  the 
quantity  of  the  excess  may  be  ascertained  exactly  in  the  same  way. 

If  the  tube  be  glass,  the  difference  of  levels  of  the  mercury  would  be  visible  ; 
but  it  is  most  commonly  made  of  iron  ;  and  in  order  to  ascertain  the  level,  a 
thin  wooden  rod  with  a  float  is  inserted  in  the  open  end  of  B  C,  so  that  the 
portion  of  the  stick  within  the  tube  indicates  the  distance  of  the  level  of  the 
mercury  from  its  mouth.  A  bulb  or  cistern  of  mercury  might  be  substituted 
for  the  leg  A  B,  as  in  the  common  barometer.  This  instrument  is  called  the 
steam-gauge. 

If  the  steam-gauge  be  used  as  a  measure  of  the  strength  of  the  steam  which 
presses  on  the  piston,  it  ought  to  be  on  the  same  side  of  the  throttle-valve 
(which  is  regulated  by  the  governor)  as  the  cylinder  ;  for  if  it  were  on  the 
same  side  of  the  throttle-valve  with  the  boiler,  it  would  not  be  affected  by  the 
changes  which  the  steam  may  undergo  in  passing  through  the  throttle-valve, 
when  partially  closed  by  the  agency  of  the  governor. 

For  boilers  in  which  steam  of  very  high  pressure  is  used,  as  in  those  of 
locomotive  engines,  a  steam-gauge,  constructed  on  the  above  principle,  would 
have  inconvenient  or  impracticable  length.  In  such  boilers  the  pressure  of  the 
steam  is  equal  to  four  or  five  times  that  of  the  atmosphere,  to  indicate  which 
the  column  of  mercury  in  the  steam-gauge  would  be  four  or  five  feet  in  height. 
In  such  cases  a  therraoraeter-gauge  may  be  used  with  advantage.  The  prin- 
ciple of  this  gauge  is  founded  on  the  fact,  that  between  the  pressure  and  tem- 
perature of  steam  produced  in  contact  with  water  there  is  a  fixed  relation,  the 
same  temperature  always  corresponding  to  the  same  pressure.  If,  therefore, 
a  thermometer  be  immersed  in  the  boiler  which  shall  show  the  temperature  of 
the  steam,  a  scale  may  be  attached  to  it,  on  which  shall  be  engraved  the 
corresponding  pressures.  Such  gauges  are  now  very  generally  used  on  locomo- 
tive engines. 

The  force  with  which  the  piston  is  pressed  depends  on  two  things  :  1st,  the 
actual  strength  of  the  steam  which  presses  on  it ;  and,  2dly,  on  the  actual 
strength  of  the  vapor  which  resists  it.  For  although  the  vacuum  produced  by 
the  method  of  separate  condensation  be  much  more  perfect  than  what  had  been 
produced  in  the  atmospheric  engines,  yet  still  some  vapor  of  a  small  degree 
of  elasticity  is  found  to  be  raised  from  the  hot  water  in  the  bottom  of  the  con- 
denser before  it  can  be  extracted  by  the  air-pump.  One  of  these  pressures  is 
indicated  by  the  steam-gauge  already  described  ;  but  still,  before  we  can  es- 
timate the  force  with  which  the  piston  descends,  it  is  necessary  to  ascertain 
the  force  of  the  vapor  which  remains  uncondensed,  and  resists  the  motion  of 
the  piston.  Another  gauge,  called  the  barometer-gauge,  is  provided  for  this 
purpose.  A  glass  tube  A  B,  fig.  61,  more  than  thirty  inches  long  and  open  at 
both  ends,  is  placed  in  an  upright  or  vertical  position,  having  the  lower  end  B 
immersed  in  a  cistern  of  mercury  C.  To  the  upper  end  is  attached  a  metal 
tube,  which  communicates  with  the  condenser,  in  which  a  constant  vacuum, 
or  rather  high  degree  of  rarefaction,  is  sustained.  The  same  vacuum  must 
therefore  exist  in  the  tube  A  B,  above  the  level  of  the  mercury,  and  the  at- 
mospheric pressure  on  the  surface  of  the  mercury  in  the  cistern  C  will  force 


THE  STEAM-ENGINE. 


507 


Fig.  Gl. 


the  mercury  up  in  the  tube  A  B,  until  the  column  which  is  suspended  in  it  is 
equal  to  the  difference  between  the  atmospheric  pressure  and  the  pressure  of 
the  uncondensed  steam.  The  difference  between  the  column  of  mercury  sus- 
tained in  this  instrument  and  in  the  common  barometer,  will  determine  the  strength 
of  the  uncondensed  steam,  allowing  a  force  proportional  to  one  pound  per 
square  inch  for  every  two  inches  of  mercury  in  the  difference  of  the  two 
columns.  In  a  well-constructed  engine  which  is  in  good  order,  there  is  very 
little  difference  between  the  altitude  in  the  barometer-gauge  and  the  common 
barometer. 

To  compute  the  force  with  which  the  piston  descends,  thus  becomes  a  very 
simple  arithmetical  process.  First,  ascertain  the  difference  of  the  levels  of  the 
mercury  in  the  steam-gauge  ;  this  gives  the  excess  of  the  steam  pressure  above 
the  atmospheric  pressure.  Then  find  the  height  of  the  mercury  in  the  barome- 
ter-gauge ;  this  gives  the  excess  of  the  atmospheric  pressure  above  the  uncon- 
densed steam.  Hence,  if  these  two  heights  be  added  together,  we  shall  obtain 
the  excess  of  the  impelling  force  of  the  steam  from  the  boiler,  on  the  one  side 
of  the  piston,  above  the  resistance  of  the  uncondensed  steam  on  the  other  side  : 
this  will  give  the  effective  impelling  force.  Now,  if  one  pound  be  allowed  for 
every  two  inches  of  mercury  in  the  two  columns  just  mentioned,  we  shall  have 
the  number  of  pounds  of  impelling  pressure  on  every  square  inch  of  the  piston. 
Then,  if  the  number  of  square  inches  in  the  section  of  the  piston  be  found,  and 
multiplied  by  the  number  of  pounds  on  each  square  inch,  the  force  with  which 
it  moves  will  be  obtained. 

From  what  we  have  stated  it  appears  that,  in  order  to  estimate  the  force 
with  which  the  piston  is  urged,  it  is  necessary  to  refer  to  both  the  barometer 
and  the  steam-gauge.  This  double  computation  may  be  obviated  by  making 
one  gauge  serve  both  purposes.  If  the  end  C  of  the  steam-gauge,  fig.  60,  in- 
stead of  communicating  with  the  atmosphere  were  continued  to  the  condenser, 
we  should  have  the  pressure  of  the  steam  acting  upon  the  mercury  in  the 
tube  B  A,  and  the  pressure  of  the  uncondensed  vapor  which  resists  the  piston 
acting  on  the  mercury  in  the  tube  B  C.  Hence  the  difference  of  the  levels 
of  the  mercury  in  the  tubes  would  at  once  indicate  the  difference  between  the 
force  of  the  steam  and  that  of  the  uncondensed  vapor,  which  is  the  effective 
force  with  which  the  piston  is  urged. 

But  these  methods  of  determining  the  effective  force  by  which  the  piston  is 
urged,  can  only  be  regarded  as  approximations,  and  not  very  perfect  ones.  If 
the  condensation  of  steam  on  one  side  of  the  piston  were  instantaneously 
effected,  or  the  uncondensed  vapor  were  of  the  same  tension  during  the  whole 
stroke  ;  and  if,  besides  this,  the  pressure  of  steam  on  the  piston  were  of  uni- 
form intensity  from  the  beginning  to  the  end  of  the  stroke,  then  the  steam  and 
barometer  gauges  taken  together  would  become  an  accurate  index  of  the  effec- 


508 


THE   STEAM-ENGINE. 


tive  force  of  steam  on  the  piston  :  but  such  is  not  the  case.  When  the  steam  ) 
is  first  admitted  through  the  steam-valve  it  acts  on  the  piston  with  a  pressure  j 
which  is  first  slightly  diminished,  and  afterward  a  little  increased,  until  it  ) 
arrives  at  that  part  of  the  stroke  at  which  the  steam  valve  is  closed,  after  which  \ 
the  pressure  is  diminished.  The  pressure,  therefore,  urging  the  piston  is  } 
subject  to  variation  ;  but  the  pressure  of  the  uncondensed  vapor  on  the  other  | 
side  of  the  piston  is  subject  to  still  greater  change.  At  the  moment  the  ex-  < 
hausting-valve  is  opened,  the  piston  is  relieved  from  the  pressure  upon  it  by  | 
the  commencement  of  the  condensation  ;  but  this  process  during  the  descent  < 
of  the  piston  is  gradual,  and  the  vacuum  is  rendered  more  and  more  perfect,  j 
until  the  piston  has  nearly  attained  the  limit  of  its  play.  These  variations,  < 
both  as  well  of  the  force  urging  the  piston  as  of  the  force  resisting  it,  are  such  J 
as  not  to  be  capable  of  being  accurately  measured  by  a  mercurial  column,  since  ( 
they  would  produce,  oscillations  in  such  a  column,  which  would  render  any  ] 
observations  of  its  mean  height  impracticable.  i 

To  measure  the  mean  efficient  force  of  the  piston,  taking  into  account  these  ] 
circumstances,  Mr.  Watt  invented  an  instrument,  which,  like  all  his  mechanical  i 
inventions,  has  answered  its  purpose  perfectly,  and  is  still  in  general  use.    This  [ 
instrument,  called  a\\  indicator,  consists  o(  a  cylinder  of  about  1|  inch  in  diame-  i 
ter,  and  8  inches  in  length.     It  is  bored  with  great  accuracy,  and  fitted  with  a  \ 
solid  piston  moving  steam-tight  in  it  with  very  little  friction.     The  rod  of  this  ' 
piston  is  guided  in  the  direction  of  the  axis  of  the  cylinder  through  a  collar  in  \ 
the  top,  so  as  not  to  be  subject  to  friction  in  any  part  of  its  play.     At  the  bot-  ' 
tom  of  the  cylinder  is  a  pipe  governed   by  a  stop-cock  and  turned  in  a  screw,  ' 
by  which  the  instrument  may  be  screwed  on  the  top  of  the  steam-cylinder  of 
the   engine.     In  this   position,  if  the  stop-cock  of  the  indicator  be  opened,  a 
free  communication  will   be  made  between  the  cylinder  of  the   indicator  and 
that  of  the   engine.     The  piston-rod  of  the  indicator  is  attached  to  a  spiral 
spring,  which  is   capable   of  extension   and  compression,  and   which   by  its 
elasticity  is  capable  of  measuring  the  force  which  extends  or  compresses  it  in 
the  same  manner  as  a  spring  steel-yard  or  balance.     If  a  scale  be  attached  to 
the  instrument  at  any  point  on   the  piston-rod  to  which   an  index  might  be 
attached,  then  the  position  of  that  index  upon  the  scale  would  be  governed  by 
the  position  of  the  indicator-piston  in  its  cylinder.     If  any  force  pressed  the 
indicator-piston  upward,  so  as  to  compress  the  spring,  the   index  would  rise 
upon  the  scale ;  and  if,  on  the  other  hand,  a  force  pressed  the  indicator-piston 
downward,  then  the  spiral  spring  would  be   extended,  and  the   index  on  the 
piston-rod  descend  upon   the   scale.     In  each  case  the   force  of  the  spring, 
whether  compressed  or  extended,  would  be  equal  to  the  force  urging  the  indi- 
cator-piston, and  the  scale  might  be  so  divided  as  to  show  the  amount  of  this 
force. 

Now,  let  the  instrument  be  supposed  to  be  screwed  upon  the  top  of  the  cyl- 
inder of  a  steam-engine,  and  the  stop-cock  opened  so  as  to  leave  a  free  com- 
munication between  the  cylinder  of  the  indicator  below  its  piston  and  the  cyl- 
inder of  the  steam-engine  above  the  steam-piston.  At  the  moment  the  upper 
steam-valve  is  opened,  the  steam  rushing  in  upon  the  steam-piston  will  also 
pass  into  the  indicator,  and  press  the  indicator-piston  upward  :  the  index  upon 
its  piston-rod  will  point  upon  the  scale  to  the  amount  of  pressure  thus  exerted. 
As  the  steam-piston  descends,  the  indicator-piston  will  vary  its  position  with 
the  varying  pressure  of  the  steam  in  the  cylinder,  and  the  index  on  the  piston- 

I  rod  will  play  upon  the  scale,  so  as  to  show  the  pressure  of  the  steam  at  each 

I  point  during  the  descent  of  the  piston. 

'       If  it  were  possible  to  observe  and  record  the  varying  position  of  the  index 

I  on  the  piston-rod  of  the  indicator,  and  to  refer  each  of  these  varying  positions 


THE  STEAM-ENGINE. 


509 


to  the  corresponding  point  of  the  descending  stroke,  we  should  then  be  able  t  ^ 
declare  the  actual  pressure  of  the  steam  at  every  point  of  the  stroke.  But  it  ( 
is  evident  that  such  an  observation  would  not  be  practicable.  A  method,  ^ 
however,  was  contrived  by  Mr.  Southern,  an  assistant  of  Messrs.  Boulton  and  ) 
Watt,  by  which  this  is  perfectly  efi'ected.  A  square  piece  of  paper,  or  card,  < 
is  stretched  upon  a  board,  which  slides  in  grooves  formed  in  a  frame.  This  ) 
frame  is  placed  in  a  vertical  position  near  the  indicator,  so  that  the  paper  may  ( 
be  moved  in  a  horizontal  direction  backward  and  forward,  through  a  space  of  ) 
fourteen  or  fifteen  inches.  Instead  of  an  index  a  pencil  is  attached  to  the  in-  S 
dicator  of  the  piston-rod:  this  pencil  is  lightly  pressed  by  a  spring  against  the  ) 
paper  above  mentioned,  and  as  the  paper  is  moved  in  a  horizontal  direction  S 
under  the  pencil,  would  trace  upon  the  paper  a  line.  If  the  pencil  were  sta-  / 
tionarythis  line  would  be  straight  and  horizontal,  but  if  the  pencil  were  subject  S 
to  a  vertical  motion,  the  line  traced  on  the  paper  moved  under  the  pencil  ? 
horizontally  would  be  a  curve,  the  form  of  which  would  depend  on  the  vertical  ) 
motion  of  the  pencil.  The  board  thus  supporting  the  paper  is  put  into  con-  ( 
nexion  by  a  light  cord  carried  over  pulleys  with  some  part  of  the  parallel  mo-  ) 
tion,  by  which  it  is  alternately  moved  to  the  right  and  to  the  left.  As  the  ? 
piston  ascends  or  descends,  the  whole  play  of  the  board  in  the  horizontal  ) 
direction  will  therefore  represent  ihe  length  of  the  stroke,  and  every  fractional  ( 
part  of  that  play  will  correspond  to  a  proportional  part  of  the  stroke  of  the  ; 
steam-piston.  S 

The  apparatus  being  thus  arranged,  let  us  suppose  the  steam-piston  at  the  ) 
top  of  the  cylinder  commencing  its  descent.  As  it  descends,  the  pencil  attach-  s 
ed  to  the  indicator  piston-rod  varies  its  height  according  to  the  varying  pressure  ) 
of  the  steam  in  the  cylinder.  At  the  same  time  the  paper  is  moved  uniformly  ( 
under  the  pencil,  and  a  curved  line  is  traced  upon  it  from  right  to  left.  When  ) 
the  piston  has  reached  the  bottom  of  the  cylinder,  the  upper  exhausting-valve  s 
is  opened,  and  the  steam  drawn  off  to  the  condenser.  The  indicator-piston  ) 
being  immediately  relieved  from  a  part  of  the  pressure  acting  upon  it  descends,  { 
and  with  it  the  pencil  also  descends  ;  but  at  the  same  time  the  steam-piston  ) 
has  begun  to  ascend,  and  the  paper  to  return  from  left  to  right  under  the  pencil.  \ 
While  the  steam-piston  continues  to  ascend,  the  condensation  becomes  more  / 
and  more  perfect,  and  the  vacuum  in  the  cylinder,  and  therefore  also  in  the  j 
indicator,  being  gradually  increased  in  power,  the  atmospheric  pressure  above  ( 
the  indicator-piston  presses  it  downward  and  stretches  the  spring.  The  pencil  | 
meanwhile,  with  a  paper  moving  under  it  from  right  to  left,  traces  a  second  ( 
curve.  As  the  former  curve  showed  the  actual  pressure  of  the  steam  impelling  \ 
the  piston  in  its  descent,  this  latter  will  show  the  pressure  of  the  uncondensed  < 
steam  raising  the  piston  in  its  ascent,  and  a  comparison  of  the  two  will  ex-  j 
I  hibit  the  effective  force  on  the  piston.     Fig.  62  represents  such  a  diagram  as  < 

I  Eig.  62.  j 


Ikihgfedcba 

would  be  produced  by  this  instrument.     A  B  C  is  the  curve  traced  by  the 
pencil  during  the  descent  of  the  piston,  and  C  D  E  that  during  its  ascent.     A 


510 


THE  STEAM-ENGINE. 


is  the  position  of  the  pencil  at  the  mom,ent  the  piston  commences  its  descent, 
B  is  its  position  at  the  middle  of  the  stroke,  and  C  at  the  termination  of  the 
stroke.  On  closing  the  upper  steam-valve  and  closing  the  exhausting-valve, 
the  indicator-piston  being  gradually  relieved  from  the  pressure  of  the  steam 
the  pencil  descends,  and  at  the  same  time  the  paper  moving  from  left  to  right, 
the  pencil  traces  the  curve  C  D  E,  the  gradual  descent  of  this  curve  showing 
the  progressive  increase  of  the  vacuum.  As  the  atmospheric  pressure  con- 
stantly acts  above  the  piston  of  the  indicator,  its  position  will  be  determined 
by  the  difference  between  the  atmospheric  pressure  and  the  pressure  of  the 
steam  below  it ;  and  therefore  the  difference  between  the  heights  of  the  pencil 
at  corresponding  points  in  the  ascending  and  descending  stroke^  will  express 
the  difference  between  the  pressure  of  the  steam  impelling  the  piston  in  the 
ascent  and  resisting  it  in  the  descent  at  these  points.  Thus  at  the  middle  of 
the  stroke,  the  line  B  D  will  express  the  extent  to  which  the  spring  governing 
the  indicator-piston  would  be  stretched  by  the  difference  between  the  force  of 
stfiam  impelling  the  piston  at  the  middle  of  the  descending  stroke,  and  the 
fo'-ce  of  steam  resisting  it  at  the  middle  of  the  ascending  stroke.  The  force 
therefore  measured  by  the  line  B  D  will  be  the  effective  force  on  the  piston 
at  that  point ;  and  the  same  may  be  said  of  every  part  of  the  diagram  produced 
by  the  indicator. 

The  whole  mechanical  effect  produced  by  the  stroke  of  the  piston  being 
composed  of  the  aggregate  of  all  its  varying  effects  throughout  the  stroke, 
the  determination  of  its  amount  is  a  matter  of  easy  calculation  by  the  measure- 
ment of  the  diagram  supplied  by  the  indicator.  ,  Let  the  horizontal  play  of  the 
pencil  from  A  to  C  be  divided  into  any  proposed  number  of  equal  parts,  say 
ten  :  at  the  middle  of  the  stroke,  B  D  expresses  the  effective  force  on  the 
piston,  and  if  this  be  considered  to  be  uniform  through  the  tenth  part  of  the 
stroke,  as  fromy  to  ^,  then  the  number  of  pounds  expressed  by  B  D  multiplied 
by  the  tenth  part  of  the  stroke  expressed  in  parts  of  a  foot,  will  be  the  mechani- 
cal effect  through  that  part  of  the  stroke  expressed  in  pounds'  weight  raised 
one  foot.  In  like  manner  m  n  will  express  the  effective  force  on  the  piston 
after  three  fourths  of  the  stroke  have  been  performed,  and  if  this  be  multiplied 
by  a  tenth  part  of  the  stroke  as  before,  the  mechanical  effect  similarly  express- 
ed will  be  obtained  ;  and  the  same  process  being  applied  to  any  successive 
tenth  part  of  the  stroke,  and  the  numerical  results  thus  obtained  being  added 
together,  the  whole  effect  of  the  stroke  will  be  obtained,  expressed  in  pounds' 
weight  raised  one  foot. 

By  means  of  the  indicator,  the  actual  mechanical  effect  produced  by  each 
stroke  of  the  engine  can  be  obtained,  and  if  the  actual  number  of  strokes  made 
in  any  given  time  be  known,  the  whole  effect  of  the  moving  power  would  be 
determined.  An  instrument  called  a  counter  was  also  contrived  by  Watt,  to  be 
atiaclied  either  to  the  working  beam  or  to  any  other  reciprocating  part  of  the 
engine.  This  instrument  consisted  of  a  train  of  wheel-work  with  governing 
hands  or  indices  moved  upon  divided  dials,  like  the  hands  of  a  clock.  A  record 
of  the  strokes  was  preserved  by  means  precisely  similar  to  those  by  which 
the  hands  of  a  clock  or  timepiece  indicated  and  recorded  the  number  of  vibra- 
tions of  the  pendulum  or  balance-wheel. 

To  secure  the  boiler  from  accidents  arising  from  the  steam  contained  in  it 
acquiring  an  undue  pressure,  a  safety-valve  is  used,  similar  in  principle  to 
those  adopted  in  the  early  engines.  This  valve  is  represented  in  fig.  52,  at 
N.  It  is  a  conical  valve,  kept  down  by  a  weight  sliding  on  a  rod  upon  it. 
When  the  pressure  of  the  steam  overcomes  the  force  of  this  weight,  it  raises 
the  valve  and  escapes,  being  carried  off  through  the  tube. 

With  a  view  to  the  economy  of  heat,  this  waste-steam  tube  is  sometimes 


THE  STEAM-ENGINE. 


conducted  into  the  feeding  cistern,  where  the  steam  carried  off  by   it  is   con- 
densed, and  heats  the  feeding  water. 

The  magnitude  of  the  safety-valve  should  be  such  that,  when  open,  steam 
should  be  capable  of  passing  through  it  as  rapidly  as  it  is  generated  in  the 
boiler.  The  superficial  magnitude,  therefore,  of  such  valves  must  be  propor- 
tional to  the  evaporating  power  of  the  boiler.  In  low-pressure  boilers  the 
steam  is  generally  limited  to  five  or  six  pounds'  pressure  per  square  inch,  and 
consequently  the  load  over  the  safety-valve  in  pounds  would  be  found  by 
multiplying  the  superficial  magnitude  of  its  smallest  part  by  these  numbers. 
In  boilers  in  which  the  steam  is  maintained  at  a  higher  pressure,  it  would  be 
inconvenient  to  place  upon  the  safety-valve  the  necessary  weight.  In  such 
cases  a  lever  is  used,  the  shorter  arm  of  which  presses  down  the  valve,  and 
the  longer  arm  is  held  down  by  a  weight  capable  of  adjustment,  so  that  the 
pressure  on  the  valve  may  be  regulated  at  discretion.  Two  safety-valves 
should  be  provided  on  all  boilers,  one  of  which  should  be  locked  up,  so  that 
the  persons  in  care  of  the  engine  should  have  no  power  to  increase  the  load 
upon  it.  In  such  case,  however,  it  is  necessary  that  a  handle  connected  with 
the  valve  should  project  outside  the  box  containing  it,  so  that  it  may  always 
be  possible  for  the  engineer  to  ascertain  that  the  valve  is  not  locked  in  its  seat, 
a  circumstance  which  is  liable  to  happen. 

Sometimes  also  two  safety-valves  are  provided,  one  loaded  a  little  heavier 
than  the  other.  The  escape  of  steam  from  the  lighter  valve  in  this  case  gives 
notice  to  the  engine-man  of  the  growing  increase  of  pressure,  and  warns  him 
to  check  the  production  of  steam.  The  lever  by  which  the  safety-valve  is 
held  down  is  sometimes  acted  on  by  a  spiral  spring,  capable  of  being  so  ad- 
justed as  to  produce  any  required  pressure  on  the  valve.  This  arrangement  is 
adopted  in  locomotive  engines,  where  steam  of  very  high  pressure  is  used  ;  and 
in  such  cases  also  there  are  always  provided  two  such  valves,  one  of  which 
cannot  be  increased  in  its  pressure. 

The  pipe  by  which  the  boiler  is  fed  with  water  will  necessarily  act  as  a 
safety-valve,  for  when  the  pressure  of  the  steam  increases  in  an  undue  degree, 
it  will  press  the  water  in  the  boiler  up  through  the  feed-pipe,  so  as  to  dis- 
charge it  into  the  feed-cistern,  a  circumstance  which  would  immediately  give 
notice  of  the  internal  stale  of  the  boiler.  The  steam-gauge  already  described, 
fig.  60,  would  also  act  as  a  safety-valve  ;  for  if  the  pressure  of  steam  in  the 
boiler  should  be  so  augmented  as  to  blow  the  mercury  out  of  the  steam-gauge, 
the  steam  would  then  issue  through  the  gauge,  and  the  pressure  of  the  boiler 
be  reduced,  provided  that  the  magnitude  of  the  tube  forming  the  steam-gauge 
were  sufficient  for  this  purpose. 

In  high-pressure  boilers  which  are  exposed  to  extreme  temperatures  and 
pressures,  and  which  are  therefore  subject  to  danger  of  explosion,  a  plug  of 
metal  is  sometimes  inserted,  which  is  capable  of  being  fused  at  a  temperature 
above  which  the  boiler  should  not  be  permitted  to  be  raised.  If  the  pressure 
of  steam  increase  beyond  the  proper  limit,  the  temperature  of  the  wuter  and 
steam  will  undergo  a  corresponding  increase  ;  and  if  the  metal  of  the  plug  be 
capable  of  being  fused  at  such  a  temperature,  the  plug  will  fall  out  of  the 
boiler,  and  the  steam  and  water  will  issue  from  it.  Various  alloys  of  metal 
are  fusible  at  temperatures  sufficiently  low  for  this  purpose.  An  alloy  com- 
posed of  one  part  of  lead,  three  of  tin,  and  five  of  bismuth,  will  fuse  at  the 
common  temperature  of  boiling  water  ;  and  alloys  of  the  same  metals,  in  various 
proportions,  will  fuse  at  different  temperatures  from  200°  to  400°. 

Although  fusible  plugs  may  be  used,  in  addition  to  other  means  of  insuring 
safety,  they  ought  not  to  be  exclusively  relied  on  at  the  ordinary  working 
pressure  of  the  boiler.     The  fusible  plug  ought  to  be  capable  of  more  than  re- 


512 


THE   STEAM-ENGINE. 


sisting  the  pressure  ;  but  if  it  be  so,  its  point  of  fusion  would  be  one  at  which 
the  steam  would  have  a  pressure  of  at  least  two  atmospheres  above  its  work- 
ing pressure.  The  plug  would  therefore  be  capable  of  being  fused  only  as 
soon  as  the  steam  would  acquire  a  pressure  of  30  lbs.  per  inch  above  its  regu- 
lar working  pressure. 

When  a  boiler  ceases  to  be  worked,  and  the  furnace  has  been  extinguished, 
the  space  within  it  appropriated  to  steam  will  be  left  a  vacuum  by  the  conden- 
sation of  the  steam  with  which  it  was  previously  filled.  The  external  pressure 
of  the  atmosphere  acting  on  the  boiler  would,  under  such  circumstances,  have 
a  tendency  to  crush  it  inward.  To  prevent  this,  a  safety-valve  is  provided, 
opening  inward,  and  balanced  by  a  weight  sufficient  to  keep  it  closed  until  it 
be  relieved  from  the  pressure  of  the  steam  below. 

A  large  aperture  closed  by  a  flange  secured  with  screws,  represented  at  0 
in  fig.  52,  called  the  man-hole,  is  provided  to  admit  persons  into  the  boiler  for 
the  purpose  of  cleaning  or  repairing  its  interior. 

The  manner  in  which  the  governor  regulates  the  supply  of  steam  from  the 
boiler  to  the  cylinder,  proportioning  the  quantity  to  the  work  to  be  done,  and 
thereby  sustaining  a  uniform  motion,  has  been  already  explained.  Since, 
then,  the  consumption  of  steam  in  the  engine  is  subject  to  variation,  owing  to 
the  various  quantities  of  work  it  may  have  to  perform,  it  is  evident  that  the 
production  of  steam  in  the  boiler  should  be  subject  to  a  proportional  variation.  . 
For  otherwise,  one  of  two  effects  would  ensue  :  the  boiler  would  either  fail  I 
to  supply  the  engine  with  steam,  or  steam  would  accumulate  in  the  boiler  from 
being  produced  in  too  great  abundance,  and  would  escape  at  the  safety-valve, 
and  thus  be  wasted. 

In  order  to  vary  the  production  of  steam  in  proportion  to  the  demands  of  the 
engine,  it  is  necessary  to  stimulate  or  mitigate  the  furnace,  as  the  evaporation 
is  to  be  augmented  or  diminished. 

The  activity  of  the  furnace  must  depend  on  the  current  of  air  which  is  drawn 
through  the  grate-bars,  and  this  will  depend  on  the  magnitude  of  the  space  af- 
forded for  the  passage  of  that  current  through  the  flues.  A  plate  called  a  damper 
is  accordingly  placed  with  its  plane  at  right  angles  to  the  flue,  so  that  by  rais- 
ing and  lowering  it  in  the  same  manner  as  the  sash  of  a  window  is  raised  or 
lowered,  the  space  allowed  for  the  passage  of  air  through  the  flue  may  be  reg- 
ulated. This  plate  might  be  regulated  by  the  hand,  so  that  by  raising  or  low- 
ering it  the  draught  might  be  increased  or  diminished,  and  a  corresponding 
eff'ect  produced  on  the  evaporation  in  the  boiler  :  but  the  force  of  the  fire  is 
rendered  uniformly  proportional  to  the  rate  of  evaporation  by  the  following  ar- 
rangement, without  the  intervention  of  the  engineer.  The  column  of  water 
sustained  in  the  feed-pipe  (figs.  52,  53)  represents  by  its  weight  the  difference 
between  the  pressure  of  steam  within  the  boiler  and  that  of  the  atmosphere. 
If  the  engine  consumes  steam  faster  than  the  boiler  produces  it,  the  steam  con- 
tained in  the  boiler  acquires  a  diminished  pressure,  and  consequently  the  col- 
umn of  water  in  the  feed-pipe  will  fall.  If,  on  the  other  hand,  the  boiler  pro- 
duce steam  faster  than  the  engine  consumes  it,  the  accumulation  of  steam  in 
the  boiler  will  cause  an  increased  pressure  on  the  water  it  contains,  and  there- 
by increase  the  height  of  the  column  of  water  sustained  in  the  feed-pipe. 
This  column  therefore  necessarily  rises  and  falls  with  every  variation  in  the 
rate  of  evaporation  in  the  boiler.  A  hollow  float  P  is  placed  upon  the  surface 
of  the  water  of  this  column  ;  a  chain  connected  with  this  float  is  carried  up- 
ward, and  passed  over  two  pulleys,  after  which  it  is  carried  downward  through 
an  aperture  leading  to  the  flue  which  passes  beside  the  boiler:  to  this  chain  is 
attached  the  damper.  By  such  an  arrangement  it  is  evident  that  the  damper 
will  rise  when  the  float  P  falls,  and  will  fall  when  the  float  P  rises,  since  the 


THE   STEAM-ENGINE. 


il3 


weight  of  the  damper  is  so  adjusted,  that  it  will  only  balance  the  float  P  when 
the  latter  rests  on  the  surface  of  the  water. 

Whenever  the  evaporation  of  the  boiler  is  insufficient,  it  is  evident  from  what 
has  been  stated,  that  the  float  P  will  fall  and  the  damper  will  rise,  and  will  af- 
ford a  greater  passage  for  air  through  the  flue.  This  will  stimulate  the  furnace, 
will  augment  its  heating  power,  and  will  therefore  increase  the  rate  of  evapo- 
ration in  the  boiler.  If,  on  the  other  hand,  the  production  of  steam  in  the 
boiler  be  more  than  is  requisite  for  the  supply  of  the  engine,  the  float  will  be 
raised  and  the  damper  let  down,  so  as  to  contract  the  flue,  to  diminish  the 
draught,  to  mitigate  the  fire,  and  therefore  to  check  the  evaporation.  In  this 
way  the  excess,  or  defect,  of  evaporation  in  the  boiler  is  made  to  act  upon  the 
fire,  so  as  to  render  the  heat  proceeding  from  the  combustion  as  nearly  as  pos- 
sible proportional  to  the  wants  of  the  engine. 

The  method  of  feeding  the  furnace  by  hand  through  the  fire-door  being  sub- 
ject to  the  double  objection  of  admitting  more  cold  air  over  the  fuel  than  is 
necessary  for  its  combustion,  and  the  impracticability  of  insuring  that  regular 
attendance  on  the  part  of  the  stokers,  directed  the  attention  of  engineers  to 
the  construction  of  self-regulating  furnaces.  The  most  effectual  of  these,  and 
that  which  has  come  into  most  general  use,  was  invented  by  Mr.  William 
Brunton,  of  Birmingham. 

The  advantages  proposed  to  be  attained  by  him  were  those  expressed  in  his 
patent ; — 

"  1.  I  put  the  coal  upon  the  grate  by  small  quantities,  and  at  very  short  in- 
tervals, say  every  two  or  three  seconds.  2.  I  so  dispose  of  the  coals  upon 
the  grate,  that  the  smoke  evolved  must  pass  over  that  part  of  the  grate  upon 
which  the  coal  is  in  full  combustion,  and  is  thereby  consumed.  ,3.  As  the  in- 
troduction of  coal  is  uniform  in  short  spaces  of  time,  the  jntroduction  of  air  is 
also  uniform,  and  requires  no  attention  from  the  fireman. 

"  As  it  respects  economy  :  1.  The  coal  is  put  upon  the  fire  by  an  appara- 
tus driven  by  the  engine,  and  so  contrived  that  the  quantity  of  coal  is  propor- 
tioned to  the  quantity  of  work  which  the  engine  is  performing  ;  and  the  quan- 
tity of  air  admitted  to  consume  the  smoke  is  regulated  in  the  same  manner. 
2.  The  fire-door  is  never  opened,  excepting  to  clean  the  fire ;  the  boiler,  of 
course,  is  not  exposed  to  that  continual  irregularity  of  temperature  which  is 
unavoidable  in  the  common  furnace,  and  which  is  found  exceedingly  injurious 
to  boilers.  3.  The  only  attention  required  is  to  fill  the  coal-receiver  every 
two  or  three  hours,  and  clean  the  fire  when  necessary.  4.  The  coal  is  more 
completely  consumed  than  by  the  common  furnace,  as  all  the  effect  of  what 
is  termed  '  stirring  up  the  fire'  (by  which  no  inconsiderable  quantity  of  coal 
is  passed  into  the  ash-pit),  is  attained  without  moving  the  coal  upon  the 
grate." 

A  circular  grate  is  placed  on  a  vertical  revolving  shaft ;  on  the  lower  part  af 
this  shaft,  under  the  ash-pit,  is  placed  a  toothed  wheel  driven  by  a  pinion. 
This  pinion  is  placed  on  another  vertical  shaft,  which  ascends  above  the 
boiler ;  and  on  the  other  end  of  this  is  placed  a  bevelled  wheel  driven  by  a 
pinion.  This  pinion  is  attached  to  a  shaft,  which  takes  its  motion  from  the 
axis  of  the  fly-wheel,  or  any  other  revolving  shaft  connected  with  the  engine. 
A.  constant  motion  of  revolution  is  therefore  imparted  to  the  circular  grate,  and 
its  velocity  being  proportional  to  that  of  the  engine,  will  necessarily  be  also 
proportional  to  the  quantity  of  fuel  which  ought  to  be  consumed.  Through 
that  part  of  the  boiler  which  is  over  the  fire-grate  a  vertical  tube  or  opening  is 
made  directly  over  that  part  of  the  furnace  which  is  most  distant  from  the  flues. 
Over  this  opening  a  hopper  is  placed,  which  contains  tlje  fuel  by  which  the 
boiler  is  to  be  fed  ;  and  in  the  bottom  of  this  hopper  is  a  sliding- valve,  capable 

VOL.  H.— 33 


514 


THE  STEAM-ENGINE. 


of  being  opened  or  closed,  so  as  to  regulate  the  quantity  of  fuel  supplied  to  the 
fire-grate.  The  fuel  dropping  in,  in  small  quantities,  through  this  open  valve, 
falls  on  the  grate,  and  is  carried  round  by  it,  so  as  to  leave  a  fresh  portion  of 
the  grate  to  receive  succeeding  feeds.  The  coals  admitted  through  the  hopper 
are  previously  broken  to  a  proper  size  ;  and  in  some  forms  of  this  apparatus 
there  are  two  rollers,  at  a  regulated  distance  asunder,  the  surfaces  of  which  are 
formed  into  blunt  angular  points,  and  which  are  kept  in  slow  revolution  by  the 
engine.  Between  these  rollers  the  coals  must  pass  before  they  reach  the 
valve  through  which  the  furnace  is  fed,  and  they  are  thus  broken  and  reduced 
to  a  regulated  size.  The  valve  which  regulates  the  opening  through  which 
the  feed  is  admitted,  is  connected  by  chains  and  pulleys  with  the  self-regula- 
ting damper  already  described,  so  that  in  proportion  as  the  damper  is  raised, 
the  valve  governing  the  feed  may  be  opened.  Thus,  while  the  quantity  of  air 
admitted  by  the  damper  is  increased  according  to  the  demands  of  the  engine, 
the  quantity  of  fuel  admitted  for  the  feed  is  increased  by  opening  the  valve  in 
the  bottom  of  the  hopper  in  the  same  proportion.  Apertures  are  also  provided 
in  the  front  of  the  grate,  governed  by  regulators,  by  which  the  quantity  of 
air  necessary  and  sufficient  to  produce  the  combustion  of  the  gas  evolved  from 
the  fuel  is  admitted,  these  openings  being  also  connected  with  the  self-regula- 
ting damper. 

A  considerable  portion  of  the  heat  imparted  to  the  water  in  the  boiler  es- 
capes by  radiation  from  the  surface  of  the  boiler,  steam-pipes,  and  other  parts 
of  the  machinery  in  contact  with  the  steam  and  hot  water.  The  effects  of  this 
are  rendered  very  apparent  in  marin^e-engines,  where  a  large  quantity  of  water 
is  found  to  be  condensed  in  the  great  steam  pipes  leading  from  the  boiler  lo 
the  cylinder.  In  stationary  land-boilers  this  loss  of  heat  is  usually  diminished, 
and  in  some  cases  in  a  great  degree  removed,  by  surrounding  the  boiler  with 
iron-conducting  substances.  In  some  cases  the  boiler  is  built  round  in  brick- 
work. In  Cornwall,  where  the  economy  is  regarded  perhaps  to  a  greater  ex- 
tent than  elsewhere,  the  boiler  and  steam-pipes  are  surrounded  with  a  packing 
of  sawdust,  which,  being  almost  a  non-conductor  of  heat,  is  impervious  to  the 
heat  proceeding  from  the  surfaces  with  which  it  is  in  contact,  and  consequent- 
ly confines  all  the  heat  within  the  boiler.  In  marine-boilers  it  has  been  the 
practice  recently  to  clothe  the  boiler  and  steam-pipes  with  a  coating  of  felt, 
which  is  attended  with  a  similar  effect.  When  these  remedies  are  properly 
applied,  the  loss  of  heat  proceeding  from  the  radiation  of  the  boiler  is  reduced 
to  an  extremely  small  amount.  The  engine-houses  of  some  of  the  Cornish 
eno-ines,  where  the  boiler  generates  steam  at  a  very  high  temperature,  are 
nevertheless  frequently  maintained  at  a  lower  temperature  than  the  exter- 
nal air,  and  on  entering  them  they  have  in  a  great  degree  the  effect  of  a 
cave. 

All  mechanical  action  is  measured  by  the  amount  of  force  exercised,  or 
resistance  overcome,  and  the  space  through  which  that  force  has  acted,  or 
through  which  the  resistance  has  been  moved. 

The  gross  amount  of  mechanical  action  developed  by  the  moving  power 
of  an  engine,  is  expended  partly  on  moving  the  engine  itself,  and  partly  on 
overcoming  the  resistance  on  which  the  engine  is  intended  to  act.  That 
part  of  the  mechanical  energy  of  the  moving  power  which  is  expended  on 
the  resistance  or  load  which  the  engine  moves  exclusively,  and  of  the  pow- 
er expended  on  moving  the  engine  itself,  is  called  the  useful  effect  of  the 
machine. 

Th«  gross  effect,  therefore,  exceeds  the  useful  effect  by  the  amount  of  power 
spent  in  moving  tha  engine,  or  which  may  be  wasted  or  destroyed  in  any 
way  by  the  engine. 


THE  STEAM-ENGINE. 


5]5 


'  It  is  usual  to  express  and  estimate  all  mechanical  effect  whatever  by  nature 
,  of  the  resistance  overcome — by  an  equivalent  weight  raised  a  certain  height. 
Thus,  if  an  engine  exerts  a  certain  power  in  driving  a  mill,  in  drawing  a  car- 
riage on  a  road,  or  in  propelling  a  vessel  on  water,  the  resistance  against  which 
it  has  to  act  must  be  equal  to  a  definite  amount  of  weight.  If  a  carriage  be 
drawn,  the  traces  are  stretched  by  the  tractive  power,  by  the  same  tension 
that  would  be  given  to  them  if  a  certain  weight  were  appended  to  them.  If 
the  paddle-wheels  of  a  boat  are  made  to  revolve,  the  water  opposes  to  them  a 
resistance  equal  to  that  which  would  be  produced,  if,  instead  of  moving  the 
water,  the  wheel  had  to  raise  some  certain  weight.  In  any  case,  therefore, 
weight  becomes  the  exponent  of  the  energy  of  the  resistance  against  which 
the  moving  power  acts. 

But  the  amount  of  mechanical  effect  depends  conjointly  on  the  amount  of 
resistance,  and  the  space  through  which  that  resistance  is  moved.  The  quan- 
tity of  this  effect,  therefore,  will  be  increased  in  the  same  proportion,  whether 
the  quantity  of  resistance,  or  the  space  through  which  that  resistance  is  moved, 
be  augmented.  Thus,  a  resistance  of  one  hundred  pounds,  moved  through 
two  feet,  is  mechanically  equivalent  to  a  resistance  of  two  hundred  pounds 
moved  through  one  foot,  or  of  four  hundred  pounds  moved  through  six  inches. 
To  simplify,  therefore,  the  expression  of  mechanical  effect,  it  is  usual  to  re- 
duce it  invariably  to  a  certain  weight  raised  one  foot.  If  the  resistance  un- 
der consideration  be  equivalent  to  a  certain  weight  raised  through  ten  feet,  it 
is  always  expressed  by  ten  times  the  amount  of  that  weight  raised  through  one 
foot. 

It  has  also  been  usual  in  the  expression  of  mechanical  effect,  to  take  the 
pound  weight  as  the  unit  of  weight,  and  the  foot  as  the  unit  of  length,  so  that 
all  mechanical  effect  whatsoever  is  expressed  by  a  certain  number  of  pounds 
raised  one  foot. 

The  gross  effect  of  the  moving  power  in  a  steam-engine,  is  the  whole  me- 
chanical force  developed  by  the  evaporation  of  water  in  the  boiler.     A  part  of 
this  effect  is  lost  by  the  partial  condensation  of  the  steam  before  it  acts  upon 
the  piston,  and  by  the  imperfect  condensation  of  it  subsequently  ;  another  por- 
tion is  expended  on  overcoming  the  friction  of  the  different  moving  parts,  and  \ 
in  acting  against  the  resistance  which  the  air  opposes  to  the  machine.     If  the 
motion  be  subject  to  sudden  shocks,  a  portion  of  the  power  is  then  lost  by  the  [ 
destruction  of  momentum  which  such  shocks  produce.     But  if  those  parts  of  i 
the  machine  which  have  a  reciprocating  motion  be,  as  they  ought  to  be,  brought  \ 
gradually  to  rest  at  each  change  of  direction,  then  no  power  is  absorbed  in  this  < 
way.  ( 

The  useful  effect  of  an  engine  is  variously  denominated  according  to  the  re-  ( 
lation  under  which  it  is  considered.  If  it  be  referred  to  the  time  during  ^ 
which  it  is  produced,  it  is  called  power.  j 

If  it  be  referred  to  the  fuel,  by  the  combustion  of  which  the  evaporation  has 
been  effected,  it  is  called  duty.  i 

When  steam-engines  were  first  brought  into  use,  they  were  commonly  ap-  ) 
pUed  to  work  pumps  for  mills  which  had  been  previously  worked  or  driven  by  ( 
horses.  In  forming  their  contracts,  the  first  steam-engine  builders  found  them-  > 
selves  called  upon  to  supply  engines  capable  of  executing  the  same  work  as  < 
was  previously  executed  by  some  certain  number  of  horses.  It  was  therefore  5 
convenient,  and  indeed  necessary,  to  be  able  to  express  the  performance  of  ( 
these  machines  by  comparison  with  the  animal  power  to  which  manufacturers,  ) 
miners,  and  others,  had  been  so  long  accustomed.  When  an  engine,  there-  s 
fore,  was  capable  of  performing  the  same  work  in  a  given  time  as  any  given  ) 
number  of  horses  of  average  strength  usually  performed,  it  was  said  to  be  an  ( 


THE   STEAM-ENGINE. 


I  engine  of  so  many  horses'  power.  Steam-engines  had  been  in  use  for  a  con- 
'  siderable  time  before  this  term  had  acquired  -any  settled  or  uniform  meaning, 
,  and  the  nominal  power  of  engines  was  accordingly  very  arbitrary.  At  length, 
I  however,  the  use  of  steam-engines  became  more  extended,  and  the  confusion 
and  inconvenience  arising  out  of  all  questions  respecting  the  performance  of 
engines,  rendered  it  necessary  that  some  fixed  and  definite  meaning  should  be 
assigned  to  the  terms  by  which  the  powers  of  this  machine  were  expressed. 
To  have  abandoned  the  term  hnrse-power,  which  had  been  so  long  in  use, 
would  have  been  obviously  inconvenient ;  nor  could  there  be  any  objection  to 
its  continuance,  provided  all  engine-makers,  and  all  those  who  used  engines, 
could  be  brought  to  agree  upon  some  standard  by  which  the  unit  of  horse- 
power might  be  defined.  The  performance  of  a  horse  of  average  strength, 
working  for  eight  hours  a  day,  was  therefore  selected  as  a  standard,  or  unit, 
of  steam-engine  power.  Smeaton  estimated  that  such  an  animal,  so  working, 
was  capable  of  performing  a  quantity  of  work  equal  in  its  mechanical  efl^ect  to 
22,916  lbs.  raised  one  foot  per  minute,  while  Desaguliers  estimated  the  same 
power  at  27,500  lbs.  raised  through  the  same  height  in  the  same  time.  The 
discrepancy  between  these  estimates  probably  arose  from  their  being  made 
from  the  performances  of  different  classes  of  horses.  Messrs.  Boulton  and 
Watt  caused  experiments  to  be  made  with  the  strong  horses  used  in  the  brew- 
eries in  London,  and  from  the  result  of  these  trials  they  assigned  33,000  lbs. 
raised  one  foot  per  minute,  as  the  value  of  a  horse's  power.  This  is  the  unit 
of  engine-power  now  universally  adopted  ;  and  when  an  engine  is  said  to  be 
of  so  many  horses'  power,  what  is  meant  is,  that  that  engine,  in  good  working 
order  and  properly  managed,  is  capable  of  moving  a  resistance  equal  to  33,000 
lbs.  through  one  foot  per  minute.  Thus,  an  engine  of  ten-horse  power  is  one 
that  would  raise  330,000  lbs.  weight  one  foot  per  minute. 

Whether  this  estimate  of  an  average  horse's  power  be  correct  or  not,  in 
refereiice  to  the  actual  work  which  the  animal  is  capable  of  executing,  is  a  i 
matter  of  no  present  importance  in  its  application  to  steam-power.  The  steam-  \ 
engine  is  no  longer  used  to  replace  the  power  of  horses,  and  therefore  no  con-  ' 
tracts  are  based  upon  such  a  comparison.  The  term  horse-power,  therefore,  ] 
as  applied  to  steam-engines,  must  be  understood  to  have  no  reference  what-  ' 
ever  to  the  actual  animal  power,  but  must  be  taken  as  a  term  having  no  other  ^ 
meaning  than  the  expression  of  the  ability  of  the  machine  to  move  the  amount  < 
of  resistance  above  mentioned  through  one  foot  per  minute.  , 

It  has  been  already  explained  that  the  conversion  of  a  given  volume  of  wa-  < 
ter  into  steam  is  productive  of  a  certain  definite  amount  of  mechanical  force,  , 
this  amount  depending  on  the  pressure  under  which  the  water  is  evaporated,  .< 
and  the  extent  to  which  the  expansive  principle  is  used  in  working  the  steam.  ( 
It  is  evident  that  this  amount  of  mechanical  effect  is  a  major  limit,  which  can-  J 
not  be  exceeded  by  the  power  of  the  engine.  < 

If  the  steam  be  not  worked  expansively,  then  the  whole  power  of  the  water,  { 
transmitted  in  the  form  of  steam  from  the  boiler  to  the  working  machinery,  will  be  ) 
a  matter  of  easy  calculation,  when  the  pressure  at  which  the  steam  is  worked  is  J 
known.  The  following  table  exhibits  the  mechanical  power  of  a  cubic  foot  of  wa-  ) 
ter  converted  into  steam  at  various  pressures,  expressed  in  an  equivalent  number  S 
of  pounds'  weight  raised  one  foot  high.  Where  much  accuracy  is  sought  for,  ) 
the  pressure  at  which  the  steam  is  used  must  be  taken  into  account ;  but  by  ( 
reference  to  the  table  it  will  be  seen,  that  when  steam  is  worked  without  ex-  ? 
pansion,  its  mechanical  effect  varies  very  little  with  the  pressure.  It  may  s 
therefore  be  assumed,  as  has  already  been  stated,  that  for  every  cubic  inch  of  / 
water  transmitted  in  the  form  of  steam  to  the  cylinders,  a  force  is  produced,  S 
represented  by  a  ton  weight  raised  a  foot  high.     Now,  as  33,000  lbs.  is  very  ( 


THE  STEAM-ENGINE. 


nearly  15  tons,  it  follows  that  15  cubic  inches  of  water  converted  into  steam 
per  minute,  or  900  cubic  inches  per  hour,  will  produce  a  mechanical  force 
equal  to  one  horse.  If,  therefore,  to  900  cubic  inches  be  added  the  quantity 
of  water  per  hour  necessary  to  move  the  engine  itself,  independently  of  its 
load,  we  shall  obtain  the  quantity  of  water  per  hour  which  must  be  supplied 
by  the  boiler  to  the  engine  for  each  horse-power  ;  and  this  will  be  the  same, 
whatever  may  be  the  magnitude  or  proportions  of  the  cylinder  : — 


Total 

pressure 

m   pounds 

per  square 

inch. 

Corresponding 
tamperatare. 

Volume  of  the 
steam  compared 
to  the  volume  of 

the  water  that  ; 
has  produced  it 

Mechanical  ef- 
fect  of  a  cubic 
inch  of  water 
evaporated  in 
pounds  raised 
one  foot. 

Total  pres- 
sure in 
pounds  per 
square  inch. 

Corresponding 
temperature. 

Volume  of  the 
steam  compared 
to  the  volume  of 

the  water  that 
has  produced  it. 

Mechanical  ef- 
fect of  a  cubic 
inch  of  water 
evaporated  in 
pounds  raised 
one  foot. 

1 

1029 

20868 

1739 

58 

292-9 

484 

2339 

2 

126-1 

10874 

1812 

59 

294-2 

477 

2343 

3 

141-0 

7437 

1859 

60 

295-6 

470 

2347 

4 

152-3 

5685 

1895 

61 

296-9 

463 

2351 

5 

161-4 

4617 

1924 

62 

298-1 

456 

2355 

6 

1692 

3897 

1948 

63 

299-2 

449 

2359 

7 

175-9 

3376 

1969 

64 

300-3 

443 

2362 

8 

1820 

2983 

1989 

65 

301-3 

437 

2365 

9 

187-4 

2674 

2006 

66 

302-4 

431 

2369 

10 

192-4 

2426 

2022 

67 

303-4 

425 

2372 

11 

197-0 

2221 

2036 

68 

304-4 

419 

2375 

12 

201-3 

2050 

2050 

69 

305-4 

414 

2378 

13 

205-3 

1904 

2063 

70 

306-4 

408 

2382 

14 

209-1 

1778 

2074 

71 

307-4 

403 

2385 

15 

212-8 

1669 

2086 

72 

308-4 

398 

2388 

16 

216-3 

1573 

2097 

73 

30d-3 

393 

2391 

17 

219.6 

1488 

2107 

74 

310-3 

388 

2394 

18 

222-7 

1411 

2117 

75 

311-2 

383 

2397 

19 

225-6 

1343 

2126 

76 

3122 

379 

2400 

20 

228-5 

1281 

2135 

77 

3131 

374 

2403 

21 

231-2 

1225 

2144 

78 

314-0 

370 

2405 

22 

233-8 

1174 

2152 

79 

314-9 

366 

2408 

23 

236-3 

1127 

2160 

80 

315-8 

362 

2411 

24 

238-7 

1084 

2168 

81 

316-7 

358 

2414 

25 

241-0 

1044 

2175 

82 

317-6 

354 

2417 

26 

243-3 

1007 

2182 

83 

318-4 

350 

2419 

27 

245-5 

973 

2189 

84 

319-3 

346 

2422 

28 

247-6 

941 

2196 

85 

320-1 

342 

2425 

29 

249-6 

911 

2202 

86 

321-0 

339 

2427 

30 

251-6 

883 

2209 

87 

321-8 

335 

2430 

31 

253-6 

857 

2215 

88 

322-6 

332 

2432 

32 

255-5 

833 

2221 

89 

323-5 

328 

2435 

33 

257-3 

810 

2226 

90 

324-3 

325 

2438 

34 

259-1 

788 

2232 

91 

325-1 

322 

2440 

35 

260-9 

767 

2238 

92 

325-9 

319 

2443 

36 

262-6 

748 

2243 

93 

326-7 

316 

2445 

37 

264-3 

729 

2248 

94 

327-5 

313 

2448 

38 

265-9 

712 

2253 

95 

328-2 

310 

2450 

39 

267-5 

695 

2259 

96 

329-0 

307 

2453 

40 

2691 

679 

2264 

97 

329-8 

304 

2455 

41 

270-6 

664 

2268 

98 

330-5 

301 

2457 

42 

272-1 

649 

2273 

99 

331-3 

298 

2460 

43 

273-6 

635 

2278 

100 

332-0 

295 

2462 

44 

275-0 

t22 

2282 

110 

339-2 

271 

2486 

45 

276-4 

610 

2287 

120 

345-8 

251 

2507 

46 

277-8 

598 

2291 

130 

352-1 

233 

2527 

47 

279-2 

586 

2296 

140 

357-9 

218 

2545 

48 

280-5 

575 

2300 

150 

363-4 

205 

2561 

49 

281-9 

564 

2304 

160 

368-7 

193 

2577 

50 

283-2 

554 

2308 

170 

373-6 

183 

2593 

51 

284-4 

544 

2312 

180 

378-4 

174 

2608 

52 

285-7 

534 

2316 

190 

382-9 

166 

2622 

53 

286-9 

525 

2320 

200 

387-3 

158 

2636 

54 

288-1 

516 

2324 

210 

391-5 

151 

2650 

55 

289-3 

508 

23-27 

220 

395-5 

145 

2663 

56 

290-5 

500 

2331 

230 

399-4 

140 

2675 

57 

291-7     ' 

492 

2335 

240 

403-1 

134 

2687 

518 


THE  STEAM-ENGINE. 


*  The  quantity  of  power  expended  in  working  the  engine  itself,  independently 

I  of  that  required  to   move  its  load,  will  be  less  in  proportion  to  the  degree  of 

*  perfection  which  may  be  attained  in  the  construction  of  the  engine,  and  to  the 
I  order  in  which  it  is  kept  while  working.     Engines  vary  one  from  another  so 

*  much  in  these  respects,  that  it  is  scarcely  possible  to  lay  down  any  general 
I  rules  for  the  quantity  of  power  to  be  allowed  over  and  above  what  is  necessary 
'  to  move  the  load.  The  means  whereby  mechanical  power  is  expended  in 
I  working  the  engine  may  be  enumerated  as  follows  : — 

'       1.   Steam  in  passing  from  the  boiler  to  the   cylinder  is  liable  to  lose  its 

I  temperature  by  the  radiation  of  the  steam-pipes  and  other  passages  through 

'  which  it  is  conducted.     Since  the  steam  produced  in  the  boiler  is  in  contact 

,  with  water,  it  will  be  common  steam,  and  consequently  the  least  loss  of  heat 

will  cause  a  partial  condensation.     To  whatever  extent  this  condensation  may 

be  carried,  a  proportional  loss  of  power,  in  reference  to  the  heat  obtained  from 

the  fuel,  will  be  entailed  upon  the  engine. 

It  has  been  said  that  the  force  necessary  to  move  the  steam  from  the  boiler 
to  the  cylinder  through  passages  more  or  less  contracted,  subject  to  the  fric- 
tion of  the  pipes  and  tubes  through  which  it  moves,  should  be  taken  into  ac- 
count in  estimating  the  power,  and  a  corresponding  deduction  made.  This, 
however,  is  not  the  case  :  the  steam,  having  passed  into  the  cylinder,  remains 
common  steam,  its  pressure  being  diminished  by  reason  of  the  force  expended 
in  thus  moving  it  from  the  boiler  to  the  cylinder.  But  its  mechanical  efficacy 
at  the  reduced  pressure  is  not  sensibly  different  from  the  efficacy  which  it  had 
in  the  boiler.  If,  at  the  reduced  pressure,  its  volume  were  the  same,  then  a 
loss  of  effect  would  be  sustained  equivalent  to  the  difference  of  the  pressures  ; 
but  its  volume  being  augmented  in  very  nearly  the  same  proportion  as  its 
pressure  is  diminished,  the  mechanical  efficacy  of  a  given  weight  of  steam  in 
the  cylinder  will  be  sensibly  the  same  as  in  the  boiler. 

2.  The  radiation  of  heat  from  the  cylinder  and  its  appendages,  will  cause 
a  partial  condensation  of  steam,  and  thereby  produce  a  diminished  mechanical 
effect. 

3.  The  steam,  which  at  each  stroke  of  the  piston  fills  the  passages  be- 
tween the  steam-valves  and  the  piston,  at  the  moment  the  latter  commences 
the  stroke  will  be  inefficient.  If  it  were  possible  for  the  piston  to  come  into 
steam-tight  contact  with  each  end  of  the  cylinder,  and  that  the  steam-valve 
should  be  in  immediate  contact  with  the  side  or  top  of  the  piston,  then  the 
whole  of  the  steam  which  would  pass  through  the  steam-valve  would  be  effi- 
cient ;  but  as  some  space,  however  small,  must  remain  between  the  piston  and 
the  ends  of  the  cylinder,  and  between  the  side  of  the  cylinder  and  the  steam- 
valve,  there  will  always  be  a  volume  of  steam  bearing  a  sensible  proportion  to 
the  magnitude  of  the  cylinder,  which  at  each  stroke  of  the  piston  will  be  inef- 
ficient.    This  volume  of  steam  is  called  the  clearance. 

4.  Since  the  piston  must  move  in  steam-tight  contact  with  the  cylinder, 
it  must  have  a  definite  amount  of  friction  with  the  sides  of  the  cylinder  by 
whatever  means  it  may  be  packed.  This  friction  will  produce  a  correspond- 
ing resistance  to  the  moving  power. 

5.  The  various  joints  of  the  machinery  where  steam  is  contained  are  sub- 
ject to  leakage,  and  whatever  amount  of  steam  shall  thus  escape  must  be  placed 
to  the  account  of  power  lost. 

6.  When  the  eduction-valve  is  opened  to  admit  the  steam  to  the  condenser, 
a  certain  force  is  required  to  expel  the  steam,  from  the  cylinder.  This  force 
reacts  upon  the  piston,  and  counteracts  to  a  proportional  extent  the  moving 
power  of  the  steam  on  the  other  side.  Besides  this  the  water  in  the  conden- 
ser cannot  be  conveniently  reduced  below  the  temperature  of  about  100°,  and 


THE   STEAM-ENGINE. 


519 


at  this  temperature  steam  has  a  pressure  of  about  one  pound  per  square  inch. 
This  vapor  will  continue  to  fill  the  cylinder,  and  will  resist  the  moving  power 
which  impels  the  piston. 

7.  Power  must  be  provided  for  opening  and  closing  the  valves  or  slides, 
for  working  the  air-pump,  hot-water  pump,  and  cold-water  pump,  and  finally 
to  overcome  the  friction  on  the  journals  and  centres  of  the  parts  of  the  par- 
allel motion,  the  main  axle  of  the  beam,  the  connecting-rod,  crank,  and  fly- 
wheel axle. 

It  will  be  apparent  how  very  much  these  sources  of  resistances  must  vary 
in  diflferent  engines,  and  how  rough  an  approximation  any  general  estimate 
must  be  of  their  gross  amount. 

There  are  many  circumstances  which  obstruct  the  practical  application  of 
any  standard  of  engine-power ;  the  magnitude  of  furnace,  and  the  extent  of 
heating  surface  necessary  to  produce  any  required  rate  of  evaporation  in  the 
boiler,  are  unascertained  ;  each  engine-maker  has  his  own  rule  in  these  mat- 
ters, and  all  the  rules  are  equally  unsupported  by  any  experimental  test  enti- 
tled to  respect.  Thus  the  circumstances  that  govern  the  rate  of  evaporation 
in  the  boiler  may  be  regarded  as  almost  wholly  unknown.  But  supposing  the 
rate  of  evaporation  to  be  ascertained,  the  amount  of  power  absorbed  by  the 
condensation  of  steam  on  its  passage  to  the  cylinder,  the  imperfect  conden- 
sation of  the  same  steam  after  it  has  worked  the  piston,  the  friction  of 
the  various  moving  parts  of  the  machinery,  and,  above  all,  the  difference  of 
effect  of  these  losses  of  power  in  engines  constructed  on  diflferent  scales  of 
magnitude,  are  absolutely  unknown.  We  are,  therefore,  not  placed  in  a  con- 
dition to  assign  anything  more  than  a  general  account  of  what  has  been  the 
practice  of  engine-makers  in  constructing  engines  which  are  nominally  of  a 
certain  power. 

In  common  low-pressure  engines  of  the  larger  kind,  to  which  class  alone 
we  at  present  refer,  it  has  been  usual,  with  the   same   fuel  and  under  like 
circumstances,  to  allow  from  10  to  18   square  feet  of  heating  surface  in  the 
boiler  for  every  nominal  horse-power  of  the  engine.     Within  these  wide  limits 
the  practice  of  engine-makers  has  varied.     It  is  not,  however,  to  be  supposed 
that  the  boiler  with  18  square  feet  of  surface  per  horse-power  has  the  same  evap- 
orating power  as  that  which  has  but  10.     This  difference,  therefore,  amounts  to 
nothing  more  than  different  manufacturers  of  steam-engines  putting  into  circu- 
lation boilers  having  powers  T-eaWy  diflferent  while  they  are  nominally  \he  same. 
The  magnitude  of  the  cylinder  is  regulated  by  the  nominal  power  of  the  engine, 
and  it  is  usual  so  to  regulate  the  evaporating  power  of  the  boiler,  that  the  piston 
shall  move  at  the  average  rate  of  200  feet  per  minute.     This  being  assumed,  it 
is  customary  to  allow  about  22  square  inches  of  piston  surface  for  every  nomi- 
nal horse-power  of  the  engine.     If  this  power  were  in  conformity  to  the  stan- 
dard already  defined,  this  amount  of  surface  moved  at  200  ft.  per  minute  would  be  \ 
impelled  by  a  pressure  amounting  to  7^  lbs.  per  square  inch.     The  safety-valve  ' 
of  the  boiler  of  such  engines  is  usually  loaded  at  from  4  to  5  lbs.  per  square  \ 
inch,  and  consequently  the  steam  in  the  boiler  will  have  a  pressure  of  from  19  < 
to  20  lbs.  per  square  inch.     If,  therefore,  the  eff'ective  pressure  on  the  piston  be  < 
really  only  7^  lbs.  per  square  inch,  the  pressure  expended  in  overcoming  the  < 
friction  of  the  engine,  and  the  loss  consequent  on  the  partial  condensation  of  < 
steam  on  one  side  and  its  imperfect  condensation  on  the  other,  would  amount  to  < 
from  12  to  13  lbs.  per  square  inch,  or  nearly  double  the  assumed  useful  effect  of  / 
the  engine.  \ 

Messrs.  Maudslay  and  Field   are  accustomed  to   allow  an  evaporation  of 
ten  gallons,   or  TG  cubic   feet  of  water  per  hour,  for  each  nominal  horse-  j 
power  of  the  engine.     They  also  allow  about  22   square  inches  of  piston-  ) 


520 


THE    STEAM-ENGINE. 


surface  per  nominal  horse-power,  the  piston  being  supposed  to  move  at  the 
rate  of  200  feet  per  second.* 

The  quantity  of  grate-surface  necessary  in  proportion  to  the  power  of  the 
engine,  has  been  equally  unascertained,  and  engine-makers  vary  in  their 
practice  from  half  a  square  foot  to  one  square  foot  per  nominal  horse- 
power. 

The  proportion  which  the  magnitude  of  the  heating-surface  of  the  boiler,  and 
the  fire-surface  of  the  grate  bears  to  the  evaporating  power  of  the  boiler,  has  not 
been  determined  by  experiment,  nor,  so  far  as  we  are  informed,  by  any  well- 
ascertained  practical  results. 

The  estimates  or  rather  conjectures  of  engine-makers,  of  the  evaporation  ne- 
cessary to  produce  one-horse  power,  vary  from  one  to  two  cubic  feet  of  water 
per  hour.  It  has  been  already  shown  that  the  evaporation  of  900  cubic  inches, 
or  little  more  than  half  a  cubic  foot  per  hour,  evolves  a  gross  mechanical  effect 
representing  one  horse-power ;  from  which  it  appears,  that  if  the  evaporation 
of  the  boilers  of  steam-engines  were  what  engineers  suppose  them  to  be,  the 
gross  mechanical  power  produced  in  them  for  every  nominal  horse  power 
of  the  engine  varies  in  actual  amount  from  the  power  of  two  to  that  of  four 
horses. 

The  above  estimates  must  be  understood  as  referring  to  double-acting  steam- 
engines  above  thirty-horse  power.  The  circumstances  attending  the  per- 
formance of  single-acting  engines  applied  to  the  drainage  of  mines,  have  been 
ascertained  with  much  greater  precision.  This  has  been  mainly  owing  to  a 
spirited  system  of  general  inspection  which  has  been  established  in  Cornwall, 
to  which  we  shall  hereafter  more  particularly  advert. 

In  expressing  the  duty  of  engines,  it  would  have  been  desirable  that  the  duty 
of  the  boiler  should  have  been  separated  from  that  of  the  engine. 

The  duty  of  a  boiler  is  estimated  by  the  volume  of  water  evaporated  by  a 
given  quantity  of  fuel,  independently  of  the  time  which  such  evaporation  may 
take.  The  duty,  therefore,  will  be  expressed  by  the  number  of  cubic  feet  of 
water  evaporated,  divided  by  the  number  of  bushels  of  coal  necessary  for  that 
evaporation,  supposing  the  bushel  of  coal  to  be  the  unit  of  fuel.  It  will  be  ob- 
served that  the  duty  of  an  engine  or  boiler  is  entirely  distinct  from,  and  inde- 
pendent of,  its  power.  One  boiler  may  be  greater  than  another  in  power  to  any 
extent,  while  it  may  be  equal  to  or  less  than  it  in  duty.  A  bushel  of  coals  may 
evaporate  the  same  number  of  cubic  feet  of  water  under  two  boilers,  but  may 
take  twice  as  great  a  time  to  produce  such  evaporation  under  one  than  under  the 
other.  In  such  a  case  the  power  of  one  boiler  will  be  double  that  of  the  other, 
while  their  duty  will  be  the  same. 

In  like  manner,  a  bushel  of  coals  consumed  in  working  two  engines  may  pro- 
duce the  same  useful  effect,  but  it  may  produce  that  useful  effect  in  the  one  in 
half  the  time  it  takes  to  produce  it  in  the  other.  In  that  case  the  duty  of  the 
engines  will  be  the  same,  but  the  power  of  the  one  will  be  double  that  of  the 
other. 

In  fine,  power  has  reference  to  time — duty,  to  fuel.  The  more  rapidly  the 
engine  produces  its  mechanical  effect,  the  greater  its  power  will  be,  whatever 
may  be  the  fuel  consumed  in  working  it.     And,  on  the  other  hand,  the  greater 


*  If  22  square  inches  of  piston-surface  be  allowed  to  represent  a  horse-power,  the  pow^er  of 
an  engine  may  always  be  computed  by  dividing  the  square  of  the  diameter  of  the  piston  ex- 
pressed in  inches  by  28.  And,  on  the  other  hand,  to  find  the  diameter  of  piston  which  would 
correspond  to  any  given  power,  multiply  the  number  of  horses'  power  by  28,  and  take  the  square 
root  of  the  product.  These  rules,  however,  cannot  be  applied  if  the  piston  be  supposed  to 
move  with  any  other  velocity :  since,  in  that  case,  the  same  amount  of  piston-surfuce  would  cease 
to  represent  a  horse-power,  unless  the  effective  pressure  on  the  piston  were  at  the  same  time 
changed. 


THE  STEAM-ENGINE. 


521 


the  useful  effect  produced  by  a  given  weight  of  fuel,  the  greater  will  be  the  duty, 
however  long  the  time  may  be  which  the  fuel  may  take  to  produce  the  useful 
effect. 

The  proportion  of  the  stroke  to  the  diameter  of  the  cylinder  must  be  de- 
termined by  the  velocity  intended  to  be  given  to  the  piston.  With  the 
same  capacity  of  cylinder,  and  the  same  evaporation  in  the  boiler,  the 
velocity  of  the  piston  will  augment  as  the  magnitude  of  its  diameter  is  dimin- 
ished. 

The  proportion  of  the  diameter  to  the  stroke  of  the  cylinder  is  very  various. 
In  engines  used  for  steam-vessels  the  length  of  the  cylinder  very  little  exceeds 
its  diameter.  In  land-engines,  however,  the  proportion  of  the  length  to  the  di- 
ameter is  greater.  It  is  maintained  by  some  that  the  proportion  of  the  diame- 
ter and  length  of  the  cylinder  should  be  such  as  to  render  its  surface  exposed 
to  the  cooling  of  the  external  air,  the  smallest  possible.  Tredgold  has  main- 
tained that  since,  during  the  stroke,  the  steam  is  gradually  exposed  to  contact 
with  the  surface  of  the  cylinder  from  the  top  to  the  bottom,  the  mean  surface 
exposed  in  contact  with  steam  being  half  that  of  the  entire  cylinder,  the  pro- 
portion of  the  diameter  to  the  stroke  should  be  such  that  the  surface  of  half  the 
length  of  the  cylinder,  added  to  the  magnitude  of  the  top  and  bottom,  shall  be 
a  minimum.  If  this  principle  be  admitted,  then  the  best  proportion  of  the  di- 
ameter to  the  stroke  would  be  that  of  one  to  two,  the  length  of  the  stroke  being 
twice  the  diameter  of  the  cylinder  ;  but  since  the  whole  surface  of  the  cylinder 
is  constantly  exposed  to  the  cooling  effects  of  the  air,  and  since  in  the  inter- 
vals of  the  stroke  there  is  no  sensible  change  of  the  temperature  of  the  surface, 
the  loss  of  heat  by  cooling  will  in  effect  be  the  same,  especially  in  double- 
acting  engines,  as  if  the  cylinder  were  constantly  filled  with  steam.  If  this  be 
admitted,  then  the  object  should  be  to  give  the  cylinder  such  a  proportion,  that 
its  entire  surface,  including  the  top  and  bottom,  shall  be  a  minimum.  The 
proportion  given  by  this  condition  would  be  very  nearly  that  which  is  observed 
in  the  cylinders  of  marine-engines,  viz.,  that  the  length  of  the  cylinder  should 
be  equal  to  its  diameter. 

If  in  a  low-pressure  engine  the  pressure  of  steam  in  the  cylinder  be  taken  at 

17  lbs.  per  square  inch,  then  the  volume  of  steam  will  be  about  fifteen  hundred 

times  that  of  the  water  which  produces  it.     For  every  cubic  foot  of  water, 

therefore,  in  the  effective  evaporation  of  the  boiler,  1,500  cubic  feet  of  steam 

will  be  passed  through  the  cylinder.     If  it  be  intended  that  the  motion  of  the 

piston  shall  be  at  the  rate  of  25  strokes  per  minute,  or  1,500  strokes  per  hour, 

then  the  capacity  of  that  portion  of  the  cylinder  between  the  steam-valve  and 

the  piston  at  the  end  of  the  stroke,  must  consist  of  half  as  many  cubic  feet  as 

there  are  cubic  feet  per  hour  evaporated  in  the  boiler.     If  the  steam, therefore, 

I  be  cut  off  at  half  stroke,  the  number  of  cubic  feet  of  space  in  the  cylinder  will 

I  be  equal  to  the  number  of  cubic  feet  of  water  effectively  evaporated  by  the 

I  boiler;   and  if  a  cubic   foot  of  water  effectively  evaporated  be  taken  as  the 

J  measure    of    a    horse-power,   then   there    would   be    as  many   cubic  feet  in 

I  the  capacity  of  the  cylinder  as   is   equal  to  the   nominal   power  of  the  en- 

»  The  duty  of  engines  varies  according  to  their  form  and  magnitude,  the  cir- 
f  cumstances  under  which  they  are  worked,  and  the  purposes  to  which  they  are 
>  applied.  In  double-acting  engines  working  without  expansion,  the  coal  con- 
l  sumed  per  nominal  horse-power  per  hour  varies  from  7  to  12  lbs.  An  exami- 
)  nation  of  the  steam-logs  of  several  government  steamers  made  by  me  a  few 
I  years  since,  gave,  as  the  average  of  consumption  of  fuel  at  that  time  of  the  best 
b  class  of  marine-engines,  about  8  lbs.  per  nominal  horse-power  per  hour.  Since, 
(  however,  no  account  could  be  obtained  of  the  actual  evaporation  of  water  in  the 


522 


THE   STEAM-ENGINE. 


boiler,  nor,  with  the  necessary  degree  of  precision,  of  the  quantity  and  pres- 
sure of  the  steam  which  passed  through  the  cylinders,  this  estimate  must  be 
regarded  as  an  approximation  subject  to  several  causes  of  error.  The  ques- 
tion of  the  duty  of  boilers  and  engines  applied  to  the  general  purposes  of  man- 
ufactures and  navigation,  is  one  which  has  not  yet  been  satisfactorily  investi- 
gated ;  and  it  were  much  to  be  desired  that  the  proprietors  of  such  engines 
should  combine  to  establish  a  strict  analysis  of  their  performance  in  reference 
to  their  consumption  of  fuel,  their  evaporation  of  water,  and  their  useful  effects. 
The  results  of  such  an  investigation,  if  properly  conducted,  would  perhaps  tend 
more  to  the  improvement  of  the  steam-engine  than  any  discoveries  in  science, 
or  inventions  in  mechanical  detail,  likely  to  be  made  in  the  present  stage  of 
the  progress  of  that  machine. 

A  strict  investigation  of  this  kind  has  been  for  many  years  carried  on  re- 
specting the  performance  of  the  steam-engines  used  for  the  drainage  of  the 
mines  in  Cornwall  ;  and  it  has  been  attended  with  effects  the  most  beneficial 
to  the  interests  of  those  concerned  in  them.  The  engines  to  which  this  im- 
portant inquiry  has  been  applied  being  used  for  the  purpose  of  pumping,  are 
generally  single-acting  engines,  in  which  steam  is  used  expansively  to  a  great 
extent.  The  steam  is  produced  under  a  very  high  pressure  in  the  boiler,  and 
being  admitted  to  the  cylinder  is  cut  off  after  a  small  portion  of  the  entire 
stroke  has  been  made,  the  remainder  of  the  stroke  being  produced  by  the  ex- 
pansion of  the  steam. 

About  the  year  1811.  a  number  of  the  proprietors  of  the  principal  Cornish 
mines  agreed  to  establish  this  system  of  inspection,  under  the  management 
and  direction  of  Captain  Joel  Lean,  and  to  publish  monthly  reports.  In  these 
reports  were  stated  the  following  particulars  :  1,  the  load  per  square  inch  on 
the  piston  ;  2,  the  consumption  of  coal  in  bushels  ;  3,  the  number  of  strokes 
made  by  the  engine  ;  4,  the  length  of  the  strokes  in  the  pumps  ;  5,  the  load  in 
pounds  ;  6,  the  duty  of  the  engine,  expressed  by  the  number  of  pounds  raised 
one  foot  high  by  the  consumption  of  a  bushel  of  coals  ;  7,  the  number  of 
strokes  per  minute  ;  8,  the  diameter  and  stroke  of  the  cylinder,'and  a  general 
description  of  the  engine.  When  these  reports  were  commenced,  the  number 
of  engines  brought  under  inspection  was  twenty-one.  In  the  year  1813,  it 
increased  to  twenty-nine  ;  in  1814,  to  thirty-two  ;  in  1820,  the  number  report- 
ed upon  increased  to  forty  ;  in  1828,  the  number  was  fifty-seven  ;  and  in 
1836,  it  was  sixty-one.  This  gradual  increase  in  the  number  of  engines 
brought  under  the  system  of  inspection,  was  produced  by  the  good  effects 
which  attended  it.  These  beneficial  consequences  were  manifested,  not 
only  in  the  improved  performance  of  the  same  engines  thus  reported  upon, 
but  in  the  gradually-improved  efficiency  of  those  which  were  afterward  con- 
structed. 

The  following  table,  taken  from  the  statement  of  the  duty  of  Cornish  en- 
gines, will  show  in  a  striking  manner  the  improvement  of  those  engines, 
from  the  commencement  of  this  system  of  inspection  to  the  present  time. 
The  duty  is  expressed  by  the  number  of  pounds  raised  one  foot  high  by  the 
consumption  of  a  bushel  of  coals. 

As  an  example  of  the  beneficial  effects  produced  upon  the  efficiency  of  an 
individual  engine  by  the  first  application  of  this  system  of  inspection,  the 
case  of  the  Stray  Park  engine  may  be  mentioned.  This  engine,  constructed 
by  Boulton  and  Watt,  had  a  sixty-inch  cylinder,  and  when  first  reported  in 
1811,  its  duty  amounted  to  16,000,000  pounds.  After  having  been  reported 
on  for  three  years,  its  duty  was  found  to  have  increased  to  32,000,000  ;  this 
estimate  being  taken  from  the  average  result  of  twelve  months'  performance. 
Its  duty  was  doubled  in  less  than  three  years. 


THE  STEAM-ENGINE. 


523 


Years. 

No.  of  engines. 

Average  duty  of  the  whole. 

Average  duty  of  the  best  en- 
gine. 

1812 

21 

19,300,000 

1813 

29 

19,500,000 

26,400,000 

1814 

32 

20,600,000 

32,000,000 

1815 

35 

20,500,000 

28,700,000 

1816 

35 

23,000,000 

32,400,000 

1817 

35 

26,500,000 

41,600,000 

1818 

36 

25,400,000 

39,300,000 

1819 

40 

26,300,000 

40,000,000 

1820 

46 

28,700,000 

41,300,000 

1821 

45 

28,200,000 

42,800,000 

1822 

52 

28,900,000 

42,500,000 

1823 

52 

28,200,000 

42,100,000 

1824 

49 

28,300,000 

43,500,000 

1825 

56 

32,000,000 

45,400,000  . 

1826 

51 

30,500,000 

45,200,000 

1827 

51 

32,100,000 

59,700,000 

1828 

57 

37,100,000 

76,800,000 

1829 

53 

41,700,000 

77,000,000 

1830 

56 

43,300,000 

78,000,000 

1831 

58 

43,400,000 

71,100,000 

1832 

59 

45,000,000 

85,000,000 

1833 

56 

46,600,000 

84,300,000 

1834 

52 

47,800,000 

90,900,000 

1835 

51 

47,800,000 

91,700,000 

1836 

61 

46,600,000 

85,400,000 

1837 

58 

47,000,000 

87,200,000 

1838 

61 

48,700,000 

84,200,000 

It  will  appear,  by  inspection  of  the  duties  registered  in  the  preceding  ta- 
ble, that  the  augmentation  of  the  efficiency  of  the  engines  has  not  been  the 
effect  of  any  great  or  sudden  improvement,  but  has  rather  resulted  from  the 
combination  of  a  great  number  of  small  improvements  in  the  details  of  the 
operation  of  these  machines.  In  these  improvements  more  is  due  to  the  suc- 
cessful application  of  practical  experience  than  to  any  new  principles  de- 
veloped by  scientific  research.  Mr.  John  Taylor,  in  his  "  Records  of 
Mining,"  has  traced  the  successive  improvements  on  which  the  increased 
duty  of  engines  depends,  and  has  connected  these  improvements  with  their 
causes  in  the  order  of  their  dates.  The  following  results,  abridged  from  his 
estimates,  may  not  be  uninteresting  : — 

In  1769,  soon  after  the  date  of  the  earliest  discoveries  of  Mr.  Watt,  but 
before  they  had  come  into  practical  application,  Smeaton  computed  that  the 
average  duty  of  fifteen  atmospheric  engines,  working  at  Newcastle-on-Tyne, 
was  5,590,000.  The  duty  of  the  best  of  these  engines  was  7,440,000,  and 
that  of  the  worst  3,220,000. 

In  1772,  Smeaton  commenced  his  improvements  on  the  atmospheric  engine, 
and  raised  the  duty  to  9,450,000. 

In  1776,  Watt  obtained  a  duty  of  21,600,000. 

At  this  time  Smeaton  acknowledged  that  Watt's  engines  gave  a  duty  amount- 
ing to  double  that  of  his  own. 

In  1778-'79,  Watt  reported  a  duty  of  23,400,000. 

From  1779  to  1788,  Watt  introduced  the  application  of  expansion,  and  raised 
the  duty  to  26,600,000. 

In  1798,  an  engine  by  Boulton  and  Watt,  erected  at  Herland,  was  reported 
as  giving  a  duty  of  27,000,000. 

This  engine,  which  was    probably  the  best  which  at  that  time  had  ever 


THE    STEAM-EIGIIE. 


(FIFTH    LECTURE.) 


Railways. — Effects  of  Railway  Transport. — History  of  the  Locomotive  Engine. — Construction  of 
Locomotive  Engine  by  Blinkinsop. — Messrs.  Chapman's  Contrivance. — Walking  Engine. — Mr. 
Stephenson's  Engines  at  Killingworth. — Liverpool  and  Manchester  Railway. — Experimental 
Trial  of  the  "  Rocket,"  "  Sanspareil,"  and  "  Novelty." — Method  of  Subdividing  the  Flue  into 
Tubes. — Progi-essive  Improvement  of  Locomotive  Engines. — Adoption  of  Brass  Tubes. — De- 
tailed Description  of  the  most  Improved  Locomotive  Engines. — Power  of  Locomotive  Engines, 
— Position  of  the  Eccentrics. — Pressure  of  Steam  in  the  Boiler. — Dr.  Lardner's  Experiments  in 
1838. — Resistance  to  Railway  Trains. — Dr.  Lardner's  Experiments  on  the  Great  Western  Rail- 
way.— Experiments  on  Resistance. — Restrictions  on  Gradients. — Compensating  Effect  of  Gradi- 
ents.— Experiment  with  the  "  Hecla." — Disposition  of  Gradients  should  be  Uniform. — Methods  of 
surmounting  Steep  Inclinations. 


THE   STEAM-ENGINE. 


(FIFTH    LECTURE.) 


Capital  and  labor  have  for  the  last  twenty  years  been  directed  with  extra- 
ordinary skill  and  energy  to  the  improvement  of  inland  transport,  and  this 
important  element  of  national  prosperity  and  civilization  has  received  a  pro- 
portionate impulse.  Effects  are  now  witnessed,  which,  had  they  been  even 
hinted  at  as  being  within  the  compass  of  possibility  twenty  years  ago,  would 
have  been  scouted  as  the  dreams  of  a  disordered  imagination  ;  such,  indeed, 
as  no  writer  of  fiction  would  have  dared  to  give  place  to.  Even  so  recently  as 
twenty-five  years  since,  who  would  have  credited  the  possibility  of  a  ponder- 
ous machine,  weighing  some  twenty  tons,  carrying  with  it  several  tons  of  coal 
and  water,  flying  over  the  country  at  the  rate  of  fifty,  or  sixty,  or  seventy 
miles  an  hour,  transporting  some  hundreds  of  passengers  with  their  luggage  ! 
Yet  such  a  spectacle  is  now  of  such  ordinary  occurrence  in  England,  as  to 
excite  no  astonishment.  And  the  art  of  constructing  the  machinery  by  which 
these  extraordinary  results  are  obtained  is  so  far  from  having  reached  maturity, 
that  scarcely  two  practical  men  can  be  found  to  agree  upon  the  mechanical 
conditions  which  shall  best  insure  its  efficiency.  At  the  moment  I  address 
you,  commissions  have  been  confided  in  England  and  elsewhere  to  the  most 
eminent  scientific  and  practical  men,  to  ascertain  by  actual  experiment  what 
these  conditions  are  !  So  complete  was  the  ignorance  of  the  powers  of  loco- 
motion by  steam  which  prevailed,  even  among  engineers,  previous  to  the  open- 
ing of  the  Liverpool  railway,  that  the  transport  of  heavy  goods  was  ret^arded 
as  the  chief  object  of  the  undertaking,  and  its  principal  source  of  revenue. 
The  incredible  speed  of  transport,  effected  even  in  the  very  first  experiments 
in  1830,  burst  upon  the  public,  and  on  the  scientific  world,  with  all  the  effect 
of  a  new  and  unlooked-for  phenomenon.  On  the  unfortunate  occasion  which 
deprived  the  British  nation  of  Mr.  Huskisson,  the  wounded  body  of  that  states- 
man was  transported  a  distance  of  about  fifteen  miles  in  twenty-five  minutes, 
being  at  the  rate  of  thirty-six  miles  an  hour.     The  revenue  of  the  road  arising 


> 


528 


THE  STEAM-ENGINE. 


from  passengers  since  its  opening,  has,  contrary  to  all  that  was  foreseen,  been  . 
vastly  greater  than  that  which  has  been  derived  from  merchandise.  So  great 
was  the  want  of  experience  in  the  construction  of  engines,  that  the  company 
was  at  first  ignorant  whether  they  should  adopt  large  steam-engines  fixed  at 
different  stations  on  the  line,  to  pull  the  carriages  from  station  to  station,  or 
travelling  engines  to  drag  the  loads  the  entire  distance.  Having  decided  on 
the  latter,  they  have,  even  to  the  present  moment,  labored  under  the  disadvantage 
of  the  want  of  that  knowledge  which  experience  alone  can  give.  The  engines 
have  been  constantly  varied  in  their  weight  and  proportions,  in  their  magnitude 
and  form,  as  the  experience  of  each  successive  month  has  indicated.  As  de- 
fects became  manifest  they  were  remedied  ;  improvements  suggested  were 
adopted ;  and  each  year  produced  engines  of  such  increased  power  and  effi- 
ciency, that  their  predecessors  were  abandoned,  not  because  they  were  worn 
out,  but  because  they  had  been  outstripped  in  the  rapid  march  of  improvement. 
Add  to  this,  that  only  one  species  of  travelling  engine  has  been  effectively 
tried  ;  the  capabilities  of  others  remain  still  to  be  developed  ;  and  even  that 
form  of  engine  which  has  received  the  advantage  of  a  course  of  experiments 
on  so  grand  a  scale  to  carry  it  toward  perfection,  is  far  short  of  this  point, 
and  still  has  defects,  many  of  which,  it  is  obvious,  time  and  experience  will 
remove. 

If,  then,  the  locomotive  engine,  subject  thus  to  all  the  imperfections  in- 
separable from  a  novel  contrivance — with  the  restrictions  on  the  free  applica- 
tion of  skill  and  capital,  arising  from  the  nature  of  the  monopolies  granted  to 
railway  companies — with  the  disadvantage  of  very  limited  experience,  the 
great  parent  of  practical  improvement,  having  been  submitted  to  experiments 
hitherto  only  on  a  limited  scale,  and  confined  almost  to  one  form  of  machine  ; 
if,  under  such  disadvantages,  such  effects  have  been  produced  as  are  now  daily 
■witnessed  by  the  public,  what  may  not  be  looked  for  from  this  extraordinary 
power  when  the  enterprise  of  the  country  shall  be  more  unfettered — when 
greater  fields  of  experience  are  opened — when  time,  ingenuity,  and  capital, 
have  removed  or  diminished  existing  imperfections,  and  have  brought  to  light 
new  and  more  powerful  principles  ?  This  is  not  mere  speculation  on  abstract 
possibilities,  but  refers  to  what  is  in  actual  progress.  The  points  of  greatest 
wealth  and  population — the  centres  of  largest  capital  and  most  active  industry 
throughout  the  country — will  soon  be  connected  by  lines  of  railway;  and  vari- 
ous experiments  are  proposed,  with  more  or  less  prospect  of  success,  for  the 
application  of  steam-engines  on  stone  roads  where  the  intercourse  is  not  sufli- 
cient  to  render  railways  profitable. 

The  important  commercial  and  political   effects  attending  such  increased 
facility  and  speed  in  the  transport  of  persons   and   goods,  are  too  obvious  to 
require  any  very  extended  notice  here.     A  part  of  the  price  (and  in  many 
cases  a  considerable  part)  of  every  article  of  necessity  or  luxury,  consists  of 
the  cost  of  transporting  it  from  the  producer  to  the  consumer  ;  and  consequent- 
ly every  abatement  or  saving  in  this  cost  must  produce   a   corresponding  re- 
duction in  the  price  of  every  article  transported  ;  that  is  to  say,  of  everything 
which  is  necessary  for  the  subsistence  of  the  poor,  or  for  the  enjoyment  of  the 
'  rich — of  every  comfort,  and  of  every  luxury  of  life.     The  benefit  of  this  will 
I  extend,  not  to  the  consumer  only,  but  to  the  producer :   by  lowering  the  ex- 
'  pense  of  transport  of  the  produce,  whether  of  the  soil  or  of  the  loom,  a  less 
I  quantity  of  that  produce  will  be  spent  in  bringing  the  remainder  to  market, 
'  and  consequently  a  greater  surplus  will  reward  the  labor  of  the  producer.     The 
I  benefit  of  this  will  be  felt  even  more  by  the  agriculturist  than  by  the  manufac- 
'  turer;  because  the  proportional  cost  of  transport  of  the  produce  of  the  soil  is 
\  greater  than  that  of  manufactures.     If  two  hundred  quarters  of  corn  be  neces- 


sary  to  raise  four  hundred,  and  one  hundred  more  be  required  to  bring  the  four 
hundred  to  market,  then  the  net  surplus  will  be  one  hundred.  But-  if  by  the 
use  of  steam-carriages  the  same  quantity  can  be  brought  to  market  with  an 
expenditure  of  fifty  quarters,  then  the  net  surplus  will  be  increased  from  one 
hundred  to  one  hundred  and  fifty  quarters  :  and  either  the  profit  of  the  farmer, 
or  the  rent  of  the  landlord,  must  be  increased  by  the  same  amount. 

But  the  agriculturist  would  not  merely  be  benefited  by  an  increased  return 
from  the  soil  already  under  cultivation.  Any  reduction  in  the  cost  of  tran- 
sporting the  produce  to  market  would  call  into  cultivation  tracts  of  inferior 
fertility,  the  returns  from  which  would  not  at  present  repay  the  cost  of  cultiva- 
tion and  transport.  Thus  land  would  become  productive  which  is  now  waste, 
and  an  eflect  would  be  produced  equivalent  to  adding  so  much  fertile  soil  to 
the  present  extent  of  the  country.  It  is  well  known,  that  land  of  a  given 
degree  of  fertility  will  yield  increased  produce  by  the  increased  application  of 
capital  and  labor.  By  a  reduction  in  the  cost  of  transport,  a  saving  will  be 
made  which  may  enable  the  agriculturist  to  apply  to  tracts  already  under  cul- 
tivation the  capital  thus  saved,  and  thereby  increase  their  actual  production.  Not 
only,  therefore,  would  such  an  efl^ect  be  attended  with  an  increased  extent  of 
cultivated  land,  but  also  with  an  increased  degree  of  cultivation  in  that  which 
is  already  productive. 

It  has  been  said,  that  in  Great  Britain  there  are  above  a  million  of  horses 
engaged  in  various  ways  in  the  transport  of  passengers  and  goods,  and  that 
to  support  each  horse  requires  as  much  land  as  would,  upon  an  average, 
support  eight  men.  If  this  quantity  of  animal  power  were  displaced  by  steam- 
engines,  and  the  means  of  transport  drawn  from  the  bowels  of  the  earth,  in- 
stead of  being  raised  upon  its  surface,  then,  supposing  the  above  calculation 
correct,  as  much  land  would  become  available  for  the  support  of  human  be- 
ings as  would  sufiice  for  an  additional  population  of  eight  millions  ;  or,  what 
amounts  to  the  same,  would  increase  the  means  of  support  of  the  present 
population  by  about  one  third  of  the  present  available  means.  The  land 
which  now  supports  horses  for  transport  would  then  support  men,  or  produce 
corn  for  food. 

The  objection  that  a  quantity  of  land  exists  in  the  country  capable  of  sup- 
porting horses  alone,  and  that  such  land  would  be  thrown  out  of  cultivation, 
scarcely  deserves  notice  here.  The  existence  of  any  considerable  quantity 
of  such  land  is  extremely  doubtful.  What  is  the  soil  which  will  feed  a  horse 
and  not  feed  oxen  or  sheep,  or  produce  food  for  man  ?  But  even  if  it  be  ad- 
mitted that  there  exists  in  the  country  a  small  portion  of  such  land,  that  portion 
cannot  exceed,  nor  indeed  equal,  what  would  be  sufficient  for  the  number  of 
horses  which  must  after  all  continue  to  be  employed  for  the  purposes  of  pleas- 
ure, and  in  a  variety  of  cases  where  steam  must  necessarily  be  inapplicable. 
It  is  to  be  remeinbered,  also,  that  the  displacing  of  horses  in  one  extensive 
occupation,  by  diminishing  their  price  must  necessarily  increase  the  demand 
for  them  in  others. 

The  reduction  in  the  cost  of  transport  of  manufactured  articles,  by  lowering 
their  price  in  the  market,  will  stimulate  their  consumption.  This  observation 
applies  of  course  not  only  to  home  but  to  foreign  markets.,  In  the  latter  we 
already  in  many  branches  of  manufactures  command  a  monopoly.  The  reduced 
price  which  we  shall  attain  by  cheapness  and  facility  to  transport  will  still 
further  extend  and  increase  our  advantages.  The  necessary  consequence  will 
be,  an  increased  demand  for  manufacturing  population ;  and  this  increased 
population  again  reacting  on  the  agricultural  interests,  will  form  an  increased 
rnarket  for  that  species  of  produce.  So  interwoven  and  complicated  are  the 
fibres  which  form  the  texture  of  the  highly-civilized  and  artificial  community 

VOL.  II.— 34 


530 


THE   STEAM-ENGINE. 


in  which  Ave  live,  that  an  effect  produced  on  any  one  point  is  instantly  trans- 
mitted to  the  most  remote  and  apparently  unconnected  parts  of  the  system. 

The  two  advantages  of  increased  cheapness  and  speed,  besides  extending 
the  amount  of  existing  traffic,  call  into  existence  new  objects  of  commercial 
intercourse.  For  the  same  reason  that  the  reduced  cost  of  transport,  as  we 
have  shown,  calls  new  soils  into  cultivation,  it  also  calls  into  existence  new 
markets  for  manufactured  and  agricultural  produce.  The  great  speed  of 
transit  which  has  been  proved  to  be  practicable,  must  open  a  commerce  be- 
tween distant  points  in  various  articles,  the  nature  of  which  does  not  permit 
them  to  be  preserved  so  as  to  be  tit  for  use  beyond  a  certain  time.  Such  are, 
for  example,  many  species  of  vegetable  and  animal  food,  which  at  present  are 
confined  to  markets  at  a  very  limited  distance  from  the  grower  or  feeder.  The 
truth  of  this  observation  is  manifested  by  the  effects  which  have  followed  the 
intercourse  by  steam  on  the  Irish  channel.  The  western  towns  of  England 
have  become  markets  for  a  prodigious  quantity  of  Irish  produce,  which  it  had 
been  previously  impossible  to  export.  If  animal  food  be  transported  alive 
from  the  grower  to  the  consumer,  the  distance  of  the  market  is  limited  by  the 
power  of  the  animal  to  travel,  and  the  cost  of  its  support  on  the  road.  It  is 
only  particular  species  of  cattle  which  bear  to  be  carried  to  market  on  common 
roads  and  by  horse-carriages.  But  of  the  peculiar  nature  of  a  railway,  the 
magnitude  and  weight  of  the  loads  which  may  be  transported  on  it,  and  the 
prodigious  speed  which  may  be  attained,  render  the  transport  of  cattle,  of  every 
species,  to  almost  any  distance,  both  easy  and  cheap.  In  process  of  time, 
when  the  railway  system  becomes  extended,  the  metropolis  and  populous  towns 
will  therefore  become  markets,  not  as  at  present  to  districts  within  limited  dis- 
tances of  them,  but  to  the  whole  country. 

The  moral  and  political  consequences  of  so  great  a  change  in  the  powers 
of  transition  of  persons  and  intelligence  from  place  to  place  are  not  easily  cal- 
culated. The  concentration  of  mind  and  exertion  which  a  great  metropolis 
always  exhibits,  will  be  extended  in  a  considerable  degree  to  the  whole  realm. 
The  same  effect  will  be  produced  as  if  all  distances  were  lessened  in  the  pro- 
portion in  which  the  speed  and  cheapness  of  transit  are  increased.  Towns 
at  present  removed  some  stages  from  the  metropolis,  will  become  its  suburbs  ; 
others,  now  a;t  a  day's  journey,  will  be  removed  to  its  immediate  vicinity;  busi- 
ness will  be  carried  on  with  as  much  ease  between  them  and  the  metropolis, 
as  it  is  now  between  distant  points  of  the  metropolis  itself.  Let  those  who 
discard  speculations  like  these  as  wild  and  improbable,  recur  to  the  state  of 
public  opinion,  at  no  very  remote  period,  on  the  subject  of  steam  navigation. 
Within  the  memory  of  persons  who  have  not  yet  passed  the  meridian  of  life, 
the  possibility  of  traversing  by  the  steam-engine  the  channels  and  seas  that 
surround  and  intersect  these  islands,  was  regarded  as  the  dream  of  enthusiasts. 
Nautical  men  and  men  of  science  rejected  such  speculations  with  equal  in- 
credulity, and  with  little  less  than  scorn  for  the  understanding  of  those  who  could 
for  a  moment  entertain  them.  Yet  we  have  witnessed  steam-engines  traversing, 
not  these  channels  and  seas  alone,  but  sweeping  the  face  of  the  waters  round 
every  coast  in  Europe.  The  seas  which  interpose  between  the  Asiatic  do- 
minions and  Egypt,  and  those  which  separate  the  British  shores  from  America, 
have  offered  an  equally  ineffectual  barrier  to  its  powers.  If  steam  be  not  used 
as  the  only  means  of  connecting  the  most  distant  points  of  our  planet,  it  is  not 
because  it  is  inadequate  to  the  accomplishment  of  that  end,  but  because  the 
supply  of  the  material,  from  which  at  the  present  moment  it  derives  its  powers, 
is  restricted  by  local  and  accidental  circumstances. 

1  propose  at  present  to  lay  before  you  some  account  of  the  means  whereby 
the  effects  above  referred  to  have  been  produced  ;  of  the  manner  and  degree 


THE   STEAM-ENGINE. 


531 


in  which  the  pubHc  have  availed  themselves  of  these  means  ;  and  of  the  im- 
provements of  which  they  seem  to  lis  to  be  susceptible. 

It  is  a  singular  fact,  that  in  the  history  of  this  invention  considerable  time 
and  great  ingenuity  were  vainly  expended  in  attempting  to  overcome  a  diffi- 
culty, which  in  the  end  turned  out  to  be  purely  imaginary.  To  comprehend 
distinctly  the  manner  in  which  a  wheel-carriage  is  propelled  by  steam,  suppose 
that  a  pin  or  handle  is  attached  to  the  spoke  of  the  wheel  at  some  distance  from  its 
centre,  and  that  a  force  is  applied  to  this  pin  in  such  a  manner  as  to  make  the 
wheel  revolve.  If  the  tire  of  the  wheel  and  the  surface  of  the  road  were 
absolutely  smooth  and  free  from  friction,  so  that  the  face  of  the  tire  would  slide 
without  resistance  upon  the  road,  then  the  effect  of  the  force  thus  applied  would 
be  merely  to  cause  the  wheel  to  turn  round,  the  carriage  being  stationary,  the 
surface  of  the  tire  slipping  or  sliding  upon  the  road  as  the  wheel  is  made  to 
revolve.  But  if,  on  the  other  hand,  the  -pressure  of  the  face  of  the  tire  upon 
the  road  is  such  as  to  produce  between  them  such  a  degree  of  adhesion  as  will 
render  it  impossible  for  the  wheel  to  slide  or  slip  upon  the  road  by  the  force 
which  is  applied  to  it,  the  consequence  will  be,  that  the  wheel  can  only  turn 
round  in  obedience  to  the  force  which  moves  it  by  causing  the  carriage  to  ad- 
vance, so  that  the  wheel  will  roll  upon  the  road,  and  the  carriage  will  be  moved 
forward,  through  a  distance  equal  to  the  circumference  of  the  wheel,  each  time 
it  performs  a  complete  revolution. 

It  is  obvious  that  both  of  these  effects  may  be  partially  produced  ;  the  ad- 
hesion of  the  wheel  to  the  road  may  be  insufficient  to  prevent  slipping  alto- 
gether, and  yet  it  may  be  sufficient  to  prevent  the  wheel  from  slipping  as  fast 
as  it  revolves.  Under  such  circumstances  the  carriage  would  advance  and 
the  wheel  would  slip.  The  progressive  motion  of  the  carriage  during  one 
complete  revolution  of  the  wheel  would  be  equal  to  the  difference  between  the 
complete  circumference  of  the  wheel  and  the  portion  through  which  in  one 
revolution  it  has  slipped. 

When  the  construction  of  travelling  steam-engines  first  engaged  the  attention 
of  engineers,  and  for  a  considerable  period  afterward,  a  notion  was  impressed 
upon  their  minds  that  the  adhesion  between  the  face  of  the  wheel  and  the  sur- 
face of  the  road  must  necessarily  be  of  very  small  amount,  and  that  in  every 
practical  case  the  wheels  thus  driven  would  either  slip  altogether,  and  produce 
no  advance  of  the  carriage,  or  that  a  considerable  portion  of  the  impelling 
power  would  be  lost  by  the  partial  slipping  or  sliding  of  the  wheels.  It  is 
singular  that  it  should  never  have  occurred  to  the  many  ingenious  persons  who 
for  several  years  were  engaged  in  such  experiments  and  speculations,  to  as- 
certain by  experiment  the  actual  amount  of  adhesion  in  any  particular  case 
between  the  wheels  and  the  road.  Had  they  done  so,  we  should  probably 
now  have  found  locomotive  engines  in  a  more  advanced  state  than  that  to  which 
they  have  attained. 

To  remedy  this  imaginary  difficulty,  Messrs.  Trevethick  and  Vivian  pro- 
posed to  make  the  external  rims  of  the  wheels  rough  and  uneven,  by  surround- 
ing them  with  projecting  heads  of  nails  or  bolts,  or  by  cutting  transverse 
grooves  on  them.  They  proposed,  in  cases  where  considerable  elevations 
were  to  be  ascended,  to  cause  claws  or  nails  to  project  from  the  surface  during 
the  ascent,  so  as  to  take  hold  of  the  road. 

In  seven  years  after  the  construction  of  the  first  locomotive  engine  by  these 
engineers,  another  locomotive  engine  was  constructed  by  Mr.  Blinkensop,  of 
Middleton  colliery,  near  Leeds.  He  obtained  a  patent,  in  1811,  for  the  appli- 
cation of  a  rack-rail.  The  railroad  thus,  instead  of  being  composed  of  smooth 
bars  of  iron,  presented  a  line  of  projecting  teeth,  like  those  of  a  cog-wheel, 
which  stretched  along  the   entire  distance  to  be  travelled.     The  wheels   on 


533 


THE  STEAM-ENGINE. 


which  the  engine  rolled  were  furnished  with  corresponding  teeth,  which  work- 
ed in  the  teeth  of  the  railroad,  and,  in  this  way,  produced  a  progressive  motion 
in  the  carriage. 

The  next  contrivance  for  overcoming  this  fictitious  difficulty,  was  that  of 
Messrs.  Chapman,  who,  in  the  year  1812,  obtained  a  patent  for  working  a 
locomotive  engine  by  a  chain  extending  along  the  middle  of  the  line  of  rail- 
road, from  the  one  end  to  the  other.  This  chain  was  passed  once  round  a 
grooved  wheel  under  the  centre  of  the  carriage  ;  so  that,  when  this  grooved 
wheel  was  turned  by  the  engine,  the  chain  being  incapable  of  slipping  upon  it, 
the  carriage  was  consequently  advanced  on  the  road.  In  order  to  prevent  the 
strain  from  acting  on  the  whole  length  of  the  chain,  its  links  were  made  to  fall 
upon  upright  forks  placed  at  certain  intervals,  which  between  those  intervals 
sustained  the  tension  of  the  chain  produced  by  the  engine.  Friction-rollers 
were  used  to  press  the  chain  into  the  groove  of  the  wheel,  so  as  to  prevent  it 
from  slipping.  This  contrivance  was  soon  abandoned,  for  the  very  obvious 
reason  that  a  prodigious  loss  of  force  was  incurred  by  the  friction  of  the  chain. 

The  following  year,  1813,  produced  a  contrivance  of  singular  ingenuity,  for 
overcoming  the  supposed  difficulty  arising  from  the  want  of  adhesion  between 
the  wheels  and  the  road.  This  was  no  other  than  a  pair  of  mechanical  legs 
and  feet,  which  were  made  to  walk  and  propel  in  a  manner  somewhat  re- 
sembling the  feet  of  an  animal. 

A  sketch  of  these  propellers  is  given  in  fig.  63.     A  is  the  carriage  moving 


Fig.  63. 


on  the  railroad,  L  and  L'  are  the  legs,  F  and  F'  the  feet.  The  foot"  F  has  a 
joint  at  0,  which  corresponds  to  the  ankle  ;  another  joint  is  placed  at  K, 
which  corresponds  to  the  knee  ;  and  a  third  is  placed  at  L,  which  corresponds 
to  the  hip.  Similar  joints  are  placed  at  the  corresponding  letters  in  the  other 
leg.  The  knee-joint  K  is  attached  to  the  end  of  the  piston  of  the  cylinder. 
When  the  piston,  which  is  horizontal,  is  pressed  outward,  the  leg  L  presses 
the  foot  F  against  the  ground,  and  the  resistance  forces  the  carriage  A  on- 
ward. As  the  carriage  proceeds,  the  angle  K  at  the  knee  becomes  larger,  so  S 
that  the  leg  and  thigh  take  a  straighter  position  ;  and  this  continues  until  the 
piston  has  reached  the  end  of  its  stroke.  At  the  hip  L  there  is  a  short  lever 
L  M,  the  extremity  of  which  is  connected  by  a  cord  or  chain  with  a  point  S, 
placed  near  the  shin  of  the  leg.  When  the  piston  is  pressed  into  the  cylinder, 
the  knee  K  is  drawn  toward  the  engine,  and  the  cord  M  S  is  made  to  lift  the 
foot  F  from  the  ground  ;  to  which  it  does  not  return  until  the  piston  has  ar- 
rived at  the  extremity  of  the  cylinder.     On  the  piston  being  again' driven  out 


THE  STEAM-ENGINE. 


533 


of  the  cylinder,  the  foot  F,  being  placed  on  the  road,  is  pressed  backward  by 
the  force  of  the  piston-rod  at  K  ;  but  the  friction  of  the  ground  preventing  its 
backward  motion,  the  reaction  causes  the  engine  to  advance  :  and  in  the  same 
manner  this  process  is  continued. 

Attached  to  the  thigh  at  N,  above  the  knee,  by  a  joint,  is  a  horizontal  rod 
N  R,  which  works  a  rack  R.  This  rack  has  beneath  it  a  cog-wheel.  This 
cog-wheel  acts  in  another  rack  below  it.  By  these  means,  when  the  knee  K 
is  driven  from  the  engine,  the  rack  R  is  moved  backward ;  but  the  cog-wheel 
acting  on  the  other  rack  beneath  it,  will  move  the  latter  in  the  contrary  direc- 
tion. The  rack  R  being  then  moved  in  the  same  direction'  with  the  knee  K,  it 
follows  that  the  other  rack  will  always  be  moved  in  a  contrary  direction.  The 
lower  rack  is  connected  by  another  horizontal  rod  with  the  thigh  of  the  leo- 
L  F',  immediately  above  the  knee  at  N'.  When  the  piston  is  forced  inward, 
the  kneeK'  will  thus  be  forced  backward;  and  when  the  piston  is  forced  out- 
ward, the  knee  K''  will  be  drawn  forward.  It  therefore  follows,  that  the  two 
knees  K  and  K'  are  pressed  alternately  backward  and  forward.  The  foot  F', 
when  the  knee  K'  is  drawn  forward,  is  lifted  by  the  means  already  described 
for  the  foot  F. 

It  will  be  apparent,  from  this  description,  that  the  piece  of  mechanism  here 
exhibited  is  a  contrivance  derived  from  the  motion  of  the  legs  of  an  animal, 
and  resembling  in  all  respects  the  fore  legs  of  a  horse.  It  is  however  to  be 
regarded  rather  as  a  specimen  of  great  ingenuity  than  as  a  contrivance  of 
practical  utility. 

It  was  about  this  period  that  the  important  fact  was  first  ascertained  that 
the  adhesion  or  friction  of  the  wheels  with  the  rails  on  which  they  moved  was 
amply  sufficient  to  propel  the  engine,  even  when  dragging  after  it  a  load  of 
great  weight ;  and  that  in  such  case,  the  progressive  motion  would  be  effected 
without  any  slipping  of  the  wheels.  The  consequence  of  this  fact  rendered 
totally  useless  all  the  contrivances  for  giving  wheels  a  purchase  on  the  road, 
such  as  racks,  chains,  feet,  &c.  The  experiment  by  which  this  was  deter- 
mined appears  to  have  been  first  tried  on  the  Wylam  railroad ;  where  it  was 
proved,  that  when  the  road  was  level,  and  the  rails  clean,  the  adhesion  of  the 
wheels  was  sufficient,  in  all  kinds  of  weather,  to  propel, considerable  loads. 
By  manual  labor  it  was  first  ascertained  how  much  weight  the  wheels  of  a 
common  carriage  would  overcome  without  slipping  round  on  the  rail,  and 
having  found  the  proportion  which  that  bore  to  the  weight,  they  then  ascer- 
tained that  the  weight  of  the  engine  would  produce  sufficient  adhesion  to  drag 
after  it  on  the  railroad  the  requisite  number  of  wagons. 

In  1814,  an  engine  was  constructed  at  Killingworth,  by  Mr.  Stephenson, 
having  two  cylinders  with  a  cylindrical  boiler,  and  working  two  pair  of  wheels, 
by  cranks  placed  at  right  angles  ;  so  that  when  the  one  was  in  full  operation, 
the  other  was  at  its  dead  points.  By  these  means  the  propelling  power  was 
always  in  action.  The  cranks  were  maintained  in  this  position  by  an  endless 
chain,  which  passed  round  two  cogged  wheels  placed  under  the  engine,  and 
which  were  fixed  on  the  same  axles  on  which  the  wheels  were  placed.  The 
wheels  in  this  case  were  fixed  on  the  axles,  and  turned  with  them. 

In  an  engine  subsequently  constructed  by  Mr.  Stephenson,  for  the  Killing- 
worth  railroad,  the  mode  adopted  of  connecting  the  wheels  by  an  endless  chain 
and  cog-wheels  was  abandoned  ;  and  the  same  effect  was  produced  by  con- 
necting the  two  cranks  by  a  straight  rod.  AH  such  contrivances,  however, 
have  this  great  defect,  that,  if  the  fore  and  hind  wheels  be  not  constructed 
with  dimensions  accurately  equal,  there  must  necessarily  be  a  slipping  or 
dragging  on  the  road.  The  nature  of  the  machinery  requires  that  each  wheel 
should  perform  its  revolution  exactly  in  the  same  time  ;  and  consequently,  in 


doing  so,  must  pass  over  exactly  equal  lengths  of  the  road.  If,  therefore,  the 
circumference  of  the  wheels  be  not  accurately  equal,  that  wheel  which  has 
the  lesser  circumference  must  be  dragged  along  so  much  of  the  road  as  that 
by  which  it  falls  short  of  the  circumference  of  the  greater  wheel ;  or,  on  the 
other  hand,  the  greater  wheel  must  be  dragged  in  the  opposite  direction,  to 
compensate  for  the  same  difference.  As  no  mechanism  can  accomplish  a 
perfect  equality  in  four,  much  less  in  six  wheels,  it  may  be  assumed  that  a 
great  portion  of  that  dragging  eifect  is  a  necessary  consequence  of  the  principle 
of  this  machine  ;  and  even  were  the  wheels,  in  the  first  instance,  accurately 
constructed,  it  is  not  possible  that  their  wear  could  be  so  exactly  uniform  as 
to  continue  equal. 

The  next  stimulus  which  the  progress  of  this  invention  received,  proceeded 
from  the  great  national  work  undertaken  at  Liverpool,  by  which  that  town  and 
the  extensive  commercial  mart  of  Manchester  were  connected  by  a  double  line 
of  railway.  When  this  project  was  undertaken,  it  was  not  decided  what 
moving  power  it  might  be  most  expedient  to  adopt  as  a  means  of  transport  on 
the  proposed  road :  the  choice  lay  between  horse  power,  fixed  steam-engines, 
and  locomotive  engines  ;  but  the  first,  for  many  obvious  reasons,  was  at  once 
rejected  in  favor  of  one  or  other  of  the  last  two. 

The  steam-engine  may  be  applied,  by  two  distinct  methods,  to  move  wagons 
either  on  a  turnpike  road  or  on  a  railway.  By  the  one  method  the  steam- 
engine  is  fixed,  and  draws  the  carriage  or  train  of  carriages  toward  it  by  a  chain 
extending  the  whole  length  of  road  on  which  the  engine  works.  By  this 
method  the  line  of  road  over  which  the  transport  is  conducted  is  divided  into 
a  number  of  short  intervals,  at  the  extremity  of  each  of  which  an  engine  is 
placed.  The  wagons  or  carriages,  when  drawn  by  any  engine  to  its  own 
station,  are  detached,  and  connected  with  the  extremity  of  the  chain  worked 
by  the  next  stationary  engine  ;  and  thus  the  journey  is  performed,  from  station 
to  station,  by  separate  engines.  By  the  other  method  the  same  engine  draws 
the  load  the  whole  journey,  travelling  with  it. 

The  directors  of  the  Liverpool  and  Manchester  railroad,  when  that  work  was 
advanced  toward  its  completion,  employed,  in  the  spring  of  the  year  1829, 
Messrs.  Stephenson  and  Lock,  and  Messrs.  Walker  and  Rastrick,  experienced 
engineers,  to  visit  the  diff'erent  railways,  where  practical  information  respect- 
ing the  comparative  eiTects  of  stationary  and  locomotive  engines  was  likely  to 
be  obtained ;  and  from  these  gentlemen  they  received  reports  on  the  relative 
merits,  according  to  their  judgment  of  the  two  methods.  The  particulars  of 
their  calculations  are  given  at  large  in  the  valuable  work  of  Mr.  Nicholas 
Wood  on  railways  ;  to  which  we  refer  the  reader,  not  only  on  this,  but  on 
many  other  subjects  connected  with  the  locomotive  steam-engine,  into  which  it 
would  be  foreign  to  our  object  to  enter.  The  result  of  the  comparison  of  the 
two  systems  was,  that  the  capital  necessary  to  be  advanced  to  establish  a  line 
of  stationary  engines  was  considerably  greater  than  that  which  was  necessary 
to  establish  an  equivalent  power  in  locomotive  engines ;  that  the  annual  ex- 
pense by  the  stationary  engines  was  likewise  greater  ;  and  that,  consequently, 
the  expense  of  transport  by  the  latter  was  greater,  in  a  like  proportion. 

The  decision  of  the  directors  was,  therefore,  in  favor  of  locomotive  engines  ; 
and  their  next  measure  was  to  devise  some  means  by  which  the  inventive 
genius  of  the  country  might  be  stimulated  to  supply  them  with  the  best  possible 
form  of  engines  for  this  purpose.  With  this  view,  it  was  proposed  and  carried 
into  effect  to  offer  a  prize  for  the  best  locomotive  engine  which  might  be  pro- 
duced under  certain  proposed  conditions,  and  to  appoint  a  time  for  a  public 
trial  of  the  claims  of  the  candidates.  A  premium  of  five  hundred  pounds  v/as 
accordingly  ofll'ered  for  the  best  locomotive  engine  to  run  on  the  Liverpool  and 


THE  STEAM-ENGINE. 


535 


Manchester  Railway;  under  the  condition  thai  it  should  produc-e  no  smoke ; 
that  the  pressure  of  the  steam  should  be  limited  to  fifty  pounds  on  the  inch  ; 
and  that  it  should  draw  at  least  three  times  its  own  weight,  at  the  rate  of  not 
less  than  ten  miles  an  hour  ;  that  the  engine  should  be  supported  on  springs, 
and  should  not  exceed  fifteen  feet  in  height.  Precautions  were  also  proposed 
against  the  consequences  of  the  boiler  bursting  ;  and  other  matters  not  neces- 
sary to  mention  more  particularly  here.  This  proposal  was  announced  in  the 
spring  of  1829,  and  the  time  of  trial  was  appointed  in  the  following  October. 
The  engines  which  underwent  the  trial  were,  the  Rocket,  constructed  by  Mr. 
Stephenson  ;  the  Sanspareil,  by  Hackworth ;  and  the  Novelty,  by  Messrs. 
Braithwaite  and  Ericsson.  Of  these,  the  Rocket  obtained  the  premium.  A 
line  of  railway  was  selected  for  the  trial,  on  a  level  piece  of  road  about  two 
miles  in  length,  near  a  place  called  Rainhill,  between  Liverpool  and  Manches- 
ter ;  the  distance  between  the  two  stations  was  a  mile  and  a  half,  and  the 
engine  had  to  travel  this  distance  backward  and  forward  ten  times,  which 
made  altogether  a  journey  of  thirty  miles.  The  Rocket  performed  this  journey 
twice  :  the  first  time  in  2  hours  14  minutes  and  8  seconds  ;  and  the  second 
time  in  2  hours  6  minutes  and  49  seconds.  Its  speed  at  different  parts  of  the 
journey  varied  :  its  greatest  rate  of  motion  was  rather  above  29  miles  an  hour ; 
and  its  least,  about  lli  miles  an  hour.  The  average  rate  of  the  one  journey 
was  ISy^  miles  an  hour  ;  and  of  the  other,  142^^  miles.  This  was  the  only 
engine  which  performed  the  complete  journey  proposed,  the  others  having 
been  stopped  from  accidents  which  occurred  to  them  in  the  experiment.  The 
Sanspareil  performed  the  distance  between  the  stations  eight  times,  travelling 
22i  miles  in  1  hour  37  minutes  and  16  seconds.  The  greatest  velocity  to 
which  this  engine  attained  was  something  less  than  23  miles  per  hour.  The 
Novelty  had  only  passed  twice  between  the  stations  when  the  joints  of  the 
boiler  gave  way,  and  put  an  end  to  the  experiment. 

The  great  object  to  be  attained  in  the  construction  of  these  engines  was, 
to  combine  with  sufficient  lightness  the  greatest  possible  heating  power.  The 
fire  necessarily  acts  on  the  water  in  two  ways  :  first,  by  its  radiant  heat ;  and 
second,  by  the  current  of  heated  air  which  is  carried  by  the  draught  through 
the  flues,  and  finally  passes  into  the  chimney.  To  accomplish  this  object, 
therefore,  it  is  necessary  to  expose  to  both  these  sources  of  heat  the  greatest 
possible  quantity  of  surface  in  contact  with  the  water. 

The  superiority  of  the  Rocket  may  be  attributed  chiefly  to  the  greater 
quantity  of  surface  of  the  water  which  was  exposed  to  the  action  of  the  fire. 
With  a  less  extent  of  grate-bars  than  the  Sanspareil,  in  the  proportion  of  three 
to  five,  it  exposed  a  greater  surface  of  water  to  radiant  heat,  in  the  proportion 
of  four  to  three  ;  and  a  greater  surface  of  water  to  heated  air,  in  the  proportion 
of  more  than  three  to  two.  It  was  found  that  the  Rocket,  compared  with  the 
Sanspareil,  consumed  fuel,  in  the  evaporation  of  a  given  quantity  of  water, 
in  the  proportion  of  eleven  to  twenty-eight. 

The  object  to  be  eflfected  in  the  boilers  of  these  engines  is,  to  keep  a  small  i 
quantity  of  water  at  an  excessive  temperature,  by  means  of  a  small  quantity  of  I 
fuel  kept  in  the  most  active  state  of  combustion.  To  accomplish  this,  it  is  > 
necessary,  first,  so  to  shape  the  boiler,  furnace,  and  flues,  that  the  water  shall  ( 
be  in  contact  with  as  extensive  a  surface  as  possible,  every  part  of  which  is  S 
acted  on,  either  immediately,  by  the  heat  radiating  from  the  fire,  or  mediately,  ( 
by  the  air  which  has  passed  through  the  fire,  and  which  finally  rushes  into  the  S 
chimney  :  and,  secondly,  that  such  a  forcible  draught  should  be  maintained  in  ( 
the  furnace,  that  a  quantity  of  heat  shall  be  extricated  from  the  fuel,  by  com-  S 
bustion,  sufficient  to  maintain  the  water  at  the  necessary  temperature,  and  to  ( 
produce    the    steam   with   sufficient  rapidity.     To  accomplish  these  objects,  ; 


therefore,  the  chamber  containing  the  grate  should  be  completely  surrounded 
by  water,  and  should  be  below  the  level  oi'  the  water  in  the  boiler.  The 
magnitude  of  the  surface  exposed  to  radiation  should  be  as  great  as  is  consistent 
with  the  whole  magnitude  of  the  machine.  In  the  next  place,  it  is  necessary 
that  the  heat,  which  is  absorbed  by  the  air  passing  through  the  fuel,  and  keep- 
ing it  in  a  state  of  combustion,  should  be  transferred  to  the  water  before  the 
air  escapes  into  the  chimney.  Air  being  a  bad  conductor  of  heat,  to  accom- 
plish this  it  is  necessary  that  the  air  in  the  flues  should  be  exposed  to  as  great 
an  extent  of  surface  in  contact  with  the  water  as  possible.  No  contrivance 
can  be  less  adapted  for  the  attainment  of  this  end  than  one  or  two  large  tubes 
traversing  the  boiler,  as  in  the  earliest  locomotive  engines  :  the  body  of  air 
which  passed  through  the  centre  of  these  tubes  had  no  contact  with  their 
surface,  and,  consequently,  passed  into  the  chimney  at  nearly  the  same  tem- 
perature as  that  which  it  had  when  it  quitted  the  fire.  The  only  portion  of  air 
which  imparted  its  heat  to  the  water  was  that  portion  which  passed  next  to  the 
surface  of  the  tube. 

Several  methods  suggest  themselves  to  increase  the  surface  of  water  in  con- 
tact with  a  given  quantity  of  air  passing  through  it.  This  would  be  accom- 
plished by  causing  the  air  to  pass  between  plates  placed  near  each  other,  so 
as  to  divide  the  current  into  thin  strata,  having  between  them  strata  of  water, 
or  it  might  be  made  to  pass  between  tubes  differing  slightly  in  diameter,  the 
water  passing  through  an  inner  tube,  and  being  also  in  contact  with  the  exter- 
nal surface  of  the  outer  tube.  Such  a  method  would  be  similar  in  principle  to 
the  steam-jacket  used  in  Watt's  steam-engines,  or  to  the  condenser  of  Cart- 
wright's  engine.  But,  considering  the  facility  of  constructing  small  tubes,  and 
of  placing  them  in  the  boiler,  that  method,  perhaps,  is,  on  the  whole,  the  best 
in  practice  ;  although  the  shape  of  a  tube,  geometrically  considered,  is  most 
unfavorable  for  the  exposure  of  a  fluid  contained  in  it  to  its  surface.  The  air 
which  passes  from  the  fire-chamber,  being  subdivided  as  it  passes  through  the 
boiler  by  a  great  number  of  very  small  tubes,  may  be  made  to  impart  all  its 
excess  of  heat  to  the  water  before  it  issues  into  the  chimney.  This  is  all 
which  the  most  refined  contrivance  can  effect.  The  Rocket  engine  was 
traversed  by  twenty-five  tubes,  each  three  inches  in  diameter  ;  and  the  principle 
has  since  been  carried  to  a  much  greater  extent. 

The  abstraction  of  a  great  quantity  of  heat  from  the  air  before  it  reaches  the 
chimney  is  attended  with  one  consequence,  which,  at  first  view,  would  present 
a  difficulty  apparently  insurmountable  ;  the  chimney  would,  in  fact,  lose  its 
power  of  draught.  This  difficulty,  however,  was  removed  by  using  the  waste 
steam,  which  had  passed  from  the  cylinder  after  working  the  engine,  for  the 
purpose  of  producing  a  draught.  This  steam  was  urged  through  a  jet  presented 
upward  in  the  chimney,  and  driven  out  with  such  force  in  that  direction  as  to 
create  a  sufficient  draught  to  work  the  furnace. 

The  importance  of  this  subject  will  be  understood,  when  it  is  considered 
that  the  only  limit  to  the  attainment  of  speed  by  locomotive  engines  is  the 
power  to  produce,  in  a  given  time,  a  certain  quantity  of  steam.  Each  stroke 
of  the  piston  causes  one  revolution  of  the  wheels,  and  consumes  four  cylinders 
full  of  steam  :  consequently,  a  cylinder  of  steam  corresponds  to  a  certain 
number  of  feet  of  road  travelled  over  :  hence  it  is  that  the  production  of  a  rapid 
and  abundant  supply  of  heat,  and  the  imparting  of  that  heat  quickly  and  effectu- 
ally to  the  water,  is  the  key  to  the  solution  of  the  problem  to  construct  an  engine 
capable  of  rapid  motion. 

The  method  of  subdividing  the  flue  into  tubes  was  carried  much  further  by 
Mr.  Stephenson  after  the  construction  of  the  Rocket ;  and,  indeed,  the  princi- 
ple was  so  obvious,  it  is   only  surprising  that,  in  the  first  instance,  tubes  of 


THE  STEAM-ENGINE. 


537 


smaller  diameter  than  three  inches  were  not  used.     In  engines   since   con-  ( 
striicted,  the  number  of  tubes  vary  from  ninety  to  one  hundred  and  twenty,  the  ^ 
diameter  being  reduced  to  two  inches  or  less  ;  and   in  some  instances  tubes  I 
have  been  introduced,  even  to  the  number  of  one  hundred  and  fifty,  of  one  and 
a  half  inch  diameter. 

Since  the  period  at  which  this  railway  was  opened  for  the  actual  purposes 
of  transport,  the  locomotive  engines  have  been  in  a  state  of  progressive  im- 
provement. Scarcely  a  month  has  passed  without  suggesting  some  change 
in  the  details,  by  which  fuel  might  be  economized,  the  production  of  steam 
rendered  more  rapid,  the  wear  of  the  engine  rendered  slower,  the  proportionate 
strength  of  the  diflerent  parts  improved,  or  some  other  desirable  end  obtained. 

Engines  constructed  in  the  form  of  the  Rocket,  were  subject  to  two  principal 
defects.  The  cylinders,  being  placed  outside  the  engine,  were  exposed  to  the 
cold  of  the  atmosphere,  which  produced  a  waste  of  heat  more  or  less  consider- 
able by  condensation.  The  points  at  which  the  power  of  the  steam  to  turn  the 
wheels  was  applied,  being  at  the  extremities  of  the  axle  and  on  the  exterior  of 
the  wheel,  a  considerable  strain  was  produced,  owing  to  the  distance  of  the 
point  of  application  of  the  power  from  the  centre  of  resistance.  If  it  were 
possible  that  the  impelling  power  could  act  in  drawing  the  train  at  all  times 
with  equal  energy  to  both  sides  of  the  engine,  then  no  injurious  strain  would 
be  produced  ;  but  from  the  relative  position  of  the  points  on  the  opposite  wheels 
to  which  it  was  necessary  to  attach  the  connecting  rods,  it  was  inevitable  that, 
at  the  moment  when  one  of  the  pistons  exerts  its  full  power  in  driving  the 
wheel,  the  other  piston  must  be  altogether  inactive.  The  impelling  power, 
therefore,  at  alternate  moments  of  time,  acted  on  opposite  wheels,  and  on  each 
of  them  at  the  greatest  possible  distance  from  the  centre  of  the  axle. 

The  next  step  in  the  improvement  of  the  machine  was  made  with  a  view  to 
remove  these  two  defects.  The  cylinders  were  transferred  from  the  exterior 
of  the   engine  to  the  interior  of  the  casing  called  the  smoke-box,  B,  fig.  64, 


Fig.  64. 


which  supports  the  chimney,  and  which  receives  the  heated  air  issuing  from 
the  tubes  which  traverse  the  boiler.  Thus  placed,  the  cylinders  are  always 
maintained  as  hot  as  the  air  which  issues  from  the  flues,  and  all  condensation 
of  steam  by  their  exposure  is  prevented.     The  piston-rods  are  likewise  brought 


THE  STEAM-ENGINE. 


closer  together,  and  nearer  the  centre  of  the  engine  :  the  connecting  rods,  no  ] 
lonoer   attached  to  the  wheels,  are  made  to  act  upon  two  cranks  constructed  ' 
upon  the  axle  of  the  wheels,  and  placed  at  right  angles  to  each  other.     From  [ 
the  position  of  these  cranks,  one  would  always  be  at  its  dead  point  when  the  ' 
other  is  in  full  action.     The  action  of  the  steam  upon  them  would,  therefore, 
be  generally  unequal ;  but  this  would  not  produce  the  same  strain  as  when  the 
connecting  rods  are  attached  to  points  upon  the  exterior  of  the  wheels,  owing 
to  the   cranks   being   constructed  on   the  axle   at  points  so  much  nearer  its 
centre.     By  this  means  it  was  found  that  the    working  of  the   machine   was 
more  even,  and  productive  of  much  less  strain,  than  in  the  arrangement  adopted 
in  the   Rocket,  and  the  earlier  engines.     On  the  other  hand,  a  serious  disad- 
vantage was  incurred  by  a  double-cranked  axle.     The  weakness  necessarily 
arising  from  such  a  form  of  axle  could  only  be  removed   by  great  thickness 
and  weight  of  metal ;  and  even  this  precaution,  at  first,  did  not  prevent  their 
occasional  fracture.     The  forging  of  them  was,  however,  subsequently  much 
improved :  the  cranks,  instead  of  being  formed  by  bending  the  metal  when 
softened  by  heat,  were  made  by  cutting  the  square  of  the  crank  out  of  the  solid 
metal  ;  and  now  it  rarely  happens  that  one  of  these  axles  fails. 

The  adoption  of  smaller  tubes,  and  a  greater  number  of  them,  with  a  view 
more  perfectly  to  extract  the  heat  from  the  air  in  passing  to  the  chimney, 
rendered  a  more  forcible  draught  necessary.  This  was  accomplished  by  the 
adoption  of  a  more  contracted  blast-pipe  leading  from  the  eduction-pipes  of  the 
cylinders  and  presented  up  the  chimney.  A  representation  of  such  a  blast-pipe, 
with  the  two  tubes  leading  from  the  cylinders  and  uniting  together  near  the 
point,  which  is  presented  up  the  chimney,  is  given  at  p  p  in  fig.  74.  The  en- 
gine thus  improved  is  represented  in  fig.  64. 

A  represents  the  cylindrical  boiler,  the  lower  half  of  which  is  traversed  by 
tubes.  They  are  usually  from  eighty  to  one  hundred  in  number,  and  about  an 
inch  and  a  half  in  diameter ;  the  boiler  is  about  seven  feet  in  length ;  the  fire- 
chamber  is  attached  to  one  end  of  it,  at  F,  the  cylinders  are  inserted  in  a 
chamber  at  the  other  end,  immediately  under  the  chimney.  The  piston-rods 
are  supported  in  the  horizontal  position  by  guides  ;  and  connecting  rods  extend 
from  them,  under  the  engine,  to  the  two  cranks  placed  on  the  axle  of  the  large 
wheels.  The  effects  of  an  inequality  in  the  road  are  counteracted  by  springs, 
on  which  the  engine  rests  ;  the  springs  being  below  the  axle  of  the  great 
wheels,  and  above  that  of  the  less.  The  steam  is  supplied  to  the  cylinders,  and 
withdrawn,  by  means  of  the  common  sliding  valves,  which  are  worked  by  an 
eccentric  wheel  placed  on  the  axle  of  the  large  wheels  of  the  carriage.  The 
motion  is  communicated  from  this  eccentric  wheel  to  the  valve  by  sliding  rods. 
The  stand  is  placed  for  the  attendant  at  the  end  of  the  engine,  next  the  fire- 
place F  ;  and  two  levers  L  project  from  the  end  which  communicate  with 
the  valves  by  means  of  rods,  by  which  the  engine  is  governed  so  as  to  reverse 
the  motion. 

The  wheels  of  these  engines  have  been  commonly  constructed  of  wood  with 
strong'iron  ties,  furnished  with  flanges  adapted  to  the  rails.  But  Mr.  Stephen- 
son afterward  substituted,  in  some  instances,  wheels  of  iron  with  hollow  spokes. 
The  engine  draws  after  it  a  tender  carriage  containing  the  fuel  and  water;  and, 
when  carrying  a  light  load,  is  capable  of  performing  the  whole  journey  from 
Liverpool  to  Manchester  without  a  fresh  supply  of  water.  When  a  heavy 
load  of  merchandise  is  drawn,  it  is  usual  to  take  in  water  at  the  middle  of 
the  trip. 

In  reviewing  all  that  has  been  stated,  it  will  be  perceived  that  the  efficiency 
of  the  locomotive  engines  used  on  this  railway  is  mainly  owing  to  three  cir- 
cumstances :   1st,  the  unlimited  power  of  draught  in  the  furnace,  by  projecting 


THE   STEAM-ENGINE. 


539 


the  waste  steam  into  the  chimney  ;  2d,  the  almost  imlimited  abstraction  of  heat 
Irom  the  air  passing  from  the  furnace,  by  arrangement  of  tubes  traversing  the 
boiler  ;  and,  3d,  keeping  the  cylinders  warm,  by  immersing  them  in  the  cham- 
ber under  the  chimney.  There  are  many  minor  details  which  might  be  noticed 
with  approbation,  but  these  constitute  the  main  features  of  the  improvements. 

The  great  original  cost,  and  the  heavy  expense  of  keeping  the  engines  used 
on  the  railway  in  repair,  have  pressed  severely  on  the  resources  of  the  under- 
taking. One  of  the  best-constructed  of  the  later  engines  costs  originally 
1,500^.,  and  sometimes  more.  The  original  cost,  however,  is  far  from  being 
the  principal  source  of  expense :  the  wear  and  tear  of  these  machines,  and 
the  occasional  fracture  of  those  parts  on  which  the  greatest  strain  has  been 
laid,  have  greatly  exceeded  what  the  directors  had  anticipated.  Although 
this  source  of  expense  must  be  in  part  attributed  to  the  engines  not  having  yet 
attained  that  state  of  perfection,  in  the  proportion  and  adjustment  of  their  parts, 
of  which  they  are  susceptible,  and  to  which  experience  alone  can  lead,  yet 
there  are  some  obvious  defects  which  demand  attention. 

The  heads  of  the  boilers  are  flat,  and  formed  of  iron,  similar  to  the  material 
of  the  boilers  themselves.  The  tubes  which  traverse  the  boiler  were,  until 
recently,  copper,  and  so  inserted  into  the  flat  head  or  end  as  to  be  water-tight. 
When  the  boiler  was  heated,  the  tubes  were  found  to  expand  in  a  greater  de- 
gree than  the  other  parts  of  the  boiler  ;  which  frequently  caused  them  either 
to  be  loosened  at  the  extremities,  so  as  to  cause  leakage,  or  to  bend  from  want 
of  room  for  expansion.  The  necessity  of  removing  and  refastening  the  tubes 
caused,  therefore,  a  constant  expense. 

The  fireplace  being  situated  at  one  end  of  the  boiler,  immediately  below  the 
mouths  of  the  tubes,  a  powerful  draught  of  air,  passing  through  the  fire,  carries 
with  it  ashes  and  cinders,  which  are  driven  violently  through  the  tubes,  and 
especially  the  lower  ones,  situated  near  the  fuel.  These  tubes  are,  by  this 
means,  subject  to  rapid  wear,  the  cinders  continually  acting  upon  their  interior 
surface.  After  a  short  time  it  becomes  necessary  to  replace  single  tubes,  ac- 
cording as  they  are  found  to  be  worn,  by  new  ones  ;  and  it  not  unfrequently 
happens,  when  this  is  neglected,  that  tubes  burst.  After  a  certain  length  of 
time  the  engines  require  new  tubing.  This  wear  of  the  tubes  might  possibly 
be  avoided  by  constructing  the  fireplace  in  a  lower  position,  so  as  to  be  more 
removed  from  their  mouths  ;  or,  still  more  effectually,  by  interposing  a  casing 
of  metal,  which  might  be  filled  with  water,  between  the  fireplace  and  those 
tubes  which  are  the  most  exposed  to  the  cinders  and  ashes.  The  unequal 
expansion  of  the  tubes  and  boilers  appears  to  be  an  incurable  defect,  if  the 
present  form  of  the  engine  be  retained.  If  the  fireplace  and  chimney  could 
be  placed  at  the  same  end  of  the  boiler,  so  that  the  tubes  might  be  recurved, 
the  unequal  expansion  would  then  produce  no  injurious  effect;  but  it  would  be 
difficult  to  clean  the  tubes,  if  they  were  exposed,  as  they  are  at  present,  to  the 
cinders.  The  next  source  of  expense  arises  from  the  wear  of  the  boiler-heads, 
which  are  exposed  to  the  action  of  the  fire. 

A  considerable  improvement  was  subsequently  introduced  into  the  method 
of  tubing,  by  substituting  brass  for  copper  tubes.  I  am  not  aware  that  the 
cause  of  this  improvement  has  been  discovered  ;  but  it  is  certain,  whatever  be 
the  cause,  that  brass  tubes  are  subject  to  considerably  slower  wear  than  cop- 
per ones. 

Since  the  date  to  which  the  preceding  observations  refer,  the  locomotive 
engine  has  undergone  several  improvements  in  detail  of  considerable  import- 
ance ;  among  which,  the  addition  of  a  third  pair  of  wheels  deserves  to  be 
particularly  mentioned.  An  engine  supported  on  three  pairs  of  wheels  has 
great  security  in  the  event  of  the  fracture  of  any  one  of  the  axles — the  remain- 


THE  STEAM-ENGINE. 


ing  axles  and  wheels  being  sufficient  for  the  support  of  the  machine.  Con-  < 
nected  with  this  change  is  another,  recommended  by  Mr.  Robert  Stephenson,  | 
by  which  the  flanges  are  removed  from  the  driving  wheels,  those  upon  the  i 
remaining  pairs  of  wheels  being  sufficient  to  keep  the  engine  in  its  position  upon  ] 
the  rails.  We  shall  now  describe  a  locomotive  engine  similar  in  construction  i 
to  those  almost  universally  used  at  present  on  railroads,  as  well  in  this  as  in  [ 
other  countries. 

In  fig.  67  is  exhibited  a  vertical  section  of  the  engine  made  by  a  plane  car-  [ 
ried  through  its  length ;  and  in  fig.  68,  is  exhibited  a  corresponding  section  of 
its  tender — the  tender  being  supposed  to  be  joined  on  to  the  engine  at  the  part 
where  the  connecting  points  appear  to  be  broken  in  the  drawing.  In  fig.  69, 
is  exhibited  the  plan  of  the  working  machinery,  including  the  cylinders, 
pistons,  eccentrics,  &c.,  which  are  under  the  boiler,  by  the  operation  of  which 
the  engine  is  driven.     Fig.  70,  represents  the  tender,  also  taken  in  plan. 

In  fig.  71,  is  represented  an  elevation  of  the  hinder  end  of  the  engine  next 
the  fire-box  ;  and  in  fig.  72,  is  represented  a  cross  vertical  section  through  the 
fire-box,  and  at  right  angles  to  the  length  of  the  engine,  showing  the  interior 
of  the  boiler  above  and  beside  the  fire-box,  the  rivets  and  bolts  connecting 
the  internal  and  external  fire-boxes,  the  regulator,  steam  funnel,  and  steam 
dome. 

In  fig.  73,  is  represented  an  elevation  of  the  front  of  the  engine  next  the 
smoke-box,  showing  the  cylinder  covers  W,  buffers  T,  &c.  ;  and  in  fig.  74,  is 
represented  a  section  of  the  interior  of  the  smoke-box,  made  by  a  vertical 
plane  at  right  angles  to  the  engine,  showing  the  tube-plate  forming  the  fore- 
most end  of  the  boiler,  the  branches  S  of  the  steam-pipe  leading  to  the  cylin- 
ders, the  blast-pipe  p,  the  cylinders  H,  and  the  chimney  G. 

The  same  letters  of  reference  are  placed  at  corresponding  parts  in  the  dif- 
ferent figures. 

The  boiler,  as  has  been  explained  in   the   engines  already  described,  is  a 

cylinder  placed  upon  its  side,  the  section  of  which  is  exhibited  at  A,  fig.  67. 

The   fire-box  consists  of  two  casings  of  metal,  one  within  the  other.     The 

fire-grate  is  represented  at  D.     The  tubes  by  which  the  products  of  combustion 

are  drawn  from  the  fire-box  to  the  smoke-box  F  are  represented  at  E.     Upon 

the   smoke-box   is   erected  the  chimney  G.     In  the  engine  from  which  this 

drawing  has  been  taken,  and  which  was  used  on  the  London  and  Birmingham 

railway,  the  boiler  is  a  cylinder,  7^  feet  long,  and  3i  feet  in  diameter.     It  is 

formed  of  wrought-iron  plates  y'^g  of  an  inch  in  thiqkness,  overlapping  each 

other,  and  bound  together  by  iron  rivets  |  of  an  inch  in  diameter  and  If  inch 

apart.     One  of  these  rivets,  as  it  joins  two  plates,  is  represented  in  fig.  65. 

The  boiler  is  clothed  with  a  boarding  of  wood  a,  an  inch  in  thickness,  and 

bound  round  by  iron  hoops  screwed  together  at  the  bottom.     Wood  bemg   a 

slow  conductor  of  heat,  this  covering  has  the  effect  of  keeping  the  boiler  warm, 

and  checking  the  condensation  of  steam  which  would  otherwise  be  produced 

1  by  the  rapid  motion  of  the  engine  through  the  cold  air. 

'       The  external  fire-box,  B  B,  is  a  casing  nearly  square  in  its  plan,  being  four 

1  feet  wide  outside,  and  three  feet  seven  and  a  half  inches  long,  measured  in 

*  the  direction  of  the  boiler.     It  is  constructed  of  wrought-iron  plates,  similar 

I  to  those  of  the  boiler.     This  box  descends  about  two  feet  below  the  boiler, 

'  the  top  being  semi-cylindrical,  as  seen  in  fig.  72,  of  a  somewhat  greater  diameter 

)  than  the  boiler,  and  concentrical  with  it.     The  front  of  the  fire-box  next  the 

[  end  of  the  boiler  has  a  circular  opening  equal  in  size  to  the  end  of  the  boiler. 

>  To  the  edge  of  this  opening  the  boiler  is  fastened  by  angle  irons,  and  rivets 

(  in  the  manner  represented   in  fig.  66.     These  rivets  are  seen  in  section  in 

fig.  67. 


THE   STEAM-ENGINE. 


541 


542 


THE  STEAM-ENGINE. 


Fig.  66. 


The  internal  fire-box  C,  fig.  67,  is  similar  in  shape  to  the  external,  only  it 
is  flat  at  the  top,  and  close  everywhere  except  at  the  bottom.  Between  it 
and  the  external  fire-box  an  open  space  of  three  inches  and  a  half  is  left  all 
round,  and  on  the  side  next  the  boiler  this  space  is  increased  to  four  inches. 
This  internal  fire-box  is  made  of  copper  plates,  -^^  of  an  inch  in  thickness, 
everywhere  except  next  the  boiler,  where  the  thickness  is  |-. 

As  the  sides  and  front  of  the  external  fire-box,  and  all  the  surfaces  bound- 
ing the  internal  fire-box,  are  flat,  their  form  is  unfavorable  for  the  resistance 
of  pressure.  Adequate  means  are,  therefore,  provided  for  strengthening  them. 
The  plates  forming  the  internal  fire-box  are  bent  outward  near  the  bottom, 
until  they  are  brought  into  contact  with  those  of  the  externa]  fire-box,  to  which 
they  are  attached  by  copper  rivets,  as  represented  atf,  in  fig.  67.  The  plates 
forming  the  bounding  surfaces  of  the  two  fire-boxes  are  fastened  together  by 
stays  represented  at  k,  in  figs.  67  and  72.  These  stays,  which  are  of  copper, 
have  a  screw  cut  upon  them  through  their  whole  length,  and  holes  are  made 
through  the  plates  of  both  fire-boxes  tapped  with  corresponding  threads. 
The  copper  screws  are  then  passed  through  them,  and  rivets  formed  on  their 
heads  within  and  without,  as  seen  in  fig.  72.  These  screw  rivets  connect  all 
parts  of  the  plating  of  the  two  fire-boxes  which  are  opposed  to  each  other : 
they  are  placed  at  about  four  inches  apart  over  the  sides  and  back  of  the  in- 
ternal fireplace  and  that  part  of  the  front  which  is  below  the  boiler. 

As  the  top  of  the  internal  fire-box  cannot  be  strengthened  by  stays  of  this 
kind,  ribs  of  wrought  iron,  which  are  seen  in  their  length  at  /,  in  fig.  67,  and 
of  which  an  end  view  is  seen  in  fig.  72,  are  attached  by  bolts  to  it.  These 
ribs  are  hollowed  out,  as  seen  in  fig.  67,  between  bolt  and  bolt,  in  order  to 
break  their  contact  with  th«  roof  of  the  fire-box,  and  allow  a  more  free  passage 
to  the  heat  through  it.  If  they  were  in  continuous  contact  with  the  fire-box, 
the  metal  composing  them  would  become  more  highly  heated,  and  would  soon 
wear  out,  besides  intercepting  heat  from  the  water.  This  part  of  the  fire-box 
is  subject  to  rapid  wear,  unless  care  be  taken  that  the  level  of  the  water  be 
preserved  at  its  proper  height  in  the  boiler.  Even  when  the  boiler  is  properly 
filled,  the  depth  of  water  above  the  roof  of  the  fire-box  is  not  considerable,  and 
on  the  least  neglect  the  roof  may  be  exposed  to  the  contact  of  steam,  in  which 
case  it  will  soon  be  destroyed. 

To  prevent  accidents  arising  from  this  cause,  a  leaden  plug,  represented  at 
m,  figs.  67  and  72,  is  inserted  in  the  roof  of  the  internal  fire-box.  If  the 
water  be  allowed  to  subside,  this  plug  will  melt  out  before  the  copper  is  very 
injuriously  heated,  and  the  steam  rushing  out  at  the  aperture  will  cause  the 
fire  to  be  extinguished. 

Copper  fire-boxes  are  almost  universally  used  ;  but  sometimes,  from  the 
consideration  of  cheapness,  the  internal  fire-box  is  constructed  of  iron. 


THE   STEAM-ENGINE. 


5'43 


Fig.  68. 


TT ~n TT 

LONGITUDINAL    VERTICAL    SECTION    OF    THE    TENDER. 


Fig.  70. 


544 


THE   STEAM-ENGINE. 


In  the  plating  which  forms  the  back  of  the  external  fire-box,  an  oval  aperture 
is  formed,  as  represented  in  the  back  view  of  the  engine,  fig.  71,  for  the  fire- 
door  g.  The  plating  of  the  internal  fire-box  around  this  aperture  is  bent  at 
right  angles  to  meet  that  of  the  external  fire-box,  to  which  it  is  fastened  by  a 
row  of  copper  rivets.  The  fire-door  is  formed  of  two  plates  of  wrought  iron, 
riveted  together  with  a  space  of  nine  inches  and  a  half  between  them.  The 
air  between  these  plates  being  an  imperfect  conductor  of  heat,  keeps  the  outer 
plate  of  the  fire-door  at  a  moderate  temperature. 

In  that  part  of  the  surface  of  the  internal  fire-box  which  forms  the  end  of  the 
boiler,  holes  are  made  to  receive  the  extremities  of  the  tubes,  by  which  the  air 
proceeding  from  the  fire  is  drawn  to  the  smoke-box  at  the  remote  end  of  the 
boiler.  These  tubes  are  represented  in  longitudinal  section  at  E,  fig.  67,  and 
their  ends  are  seen  in  the  surface  of  the  internal  fire-box  in  fig.  72,  and  in  the 
remote  end  of  the  boiler  where  they  terminate  in  the  smoke-box  in  fig.  74. 
These  tubes  are  formed  of  the  best  rolled  brass,  and  their  thickness  in  the 
engine,  to  which  we  now  refer,  is  J^-  of  an  inch.  After  the  brass  plating  is 
bent  into  the  form  of  a  tube,  and  being  overlapped,  is  properly  soldered  together, 
and  the  edges  smoothed  off",  the  tubes  are  made  perfectly  cylindrical  by  being 
drawn  through  a  circular  steel  die. 

The  tube-plates  (as  those  parts  of  the  boiler  ends  in  which  the  tubes  are  in- 
serted are  called)  are  bored  with  holes  in  corresponding  positions,  truly  cylin- 
drical, and  corresponding  in  magnitude  to  the  tubes,  so  that  the  tubes,  when 
passed  into  them,  will  be  just  in  contact  with  them.  The  length  of  the  tubes 
is  so  regulated,  that  when  extending  from  end  to  end  of  the  boiler,  and  passing 
through  the  holes,  they  shall  project  at  each  end  a  little  beyond  the  holes. 
The  manner  of  fastening  them  so  as  to  be  water-tight  is  as  follows :  A  st'=iel 
hoop  or  ferrule,  made  slightly  conical,  a  section  of  which  is  exhibited  at  C, 
fig.  75,  the  smaller  end  of  which  is  a  little  less  than  the  internal  diameter  of 

Fig.  75. 


the  tube,  but  which  increases  toward  the  outer  end,  is  driven  in  as  represented 
in  the  figure.  It  acts  as  a  wedge,  and  forces  the  tube  into  close  contact  with 
the  edges  of  the  hole  in  the  tube-plate. 

When  particular  tubes  in  a  boiler  are  worn  out,  and  require  to  be  replaced, 
their  removal  is  easily  effected.  It  is  only  necessary  to  cut  the  steel  ferrule 
on  the  inside,  and  to  bend  it  off  from  contact  with  the  tube,  by  which  means  it 
can  be  loosened  and  withdrawn,  and  the  tube  removed. 

In  the  engine  to  which  this  description  refers  there  were  one  hundred  and 
twenty-four  tubes,  the  external  diameter  of  which  was  If  inch      "" 


The  distance 


J 


THE  STEAM-ENGINE. 


545 


between  tube  and  tube  was  f  of  an  inch.  The  number  of  tubes  vary  in  dif- 
ferent engines,  some  having  so  many  as  one  hundred  and  fifly,  while  the  num- 
ber in  some  is  less  than  ninety.  The  evaporating  power  of  an  engine  greatly 
depends  on  the  proper  number  and  magnitude  of  its  tubes  ;  and  the  experience 
which  engineers  have  had  on  railways  have  led  them  gradually  to  increase  the 
number  of  tubes,  and  diminish  their  magnitude.  In  the  Rocket,  already  men- 
tioned as  having  gained  the  prize  on  the  opening  of  the  Liverpool  and  Man- 
chester railway,  the  number  of  tubes  was  twenty-four,  and  their  diameter  three 
inches  ;  but  in  all  the  engines  subsequently  made  their  number  was  augmented, 
and  their  diameter  diminished.  The  practical  inconvenience  which  limits  the 
sizp  of  the  tubes  is  their  liability  to  become  choked  by  cinders  and  ashes, 
which  get  wedged  in  them  when  they  are  too  small,  and  thereby  obstruct  the 
draught,  and  diminish  the  evaporating  power  of  the  boiler.  The  tubes  now  in 
use,  of  about  an  inch  and  a  half  internal  diameter,  not  only  require  to  be  cleared 
of  the  ashes  and  cinders,  which  get  fastened  in  them  after  each  journey,  but 
it  is  necessary  throughout  a  journey  of  any  length  that  the  lubes  should  be 
picked  and  cleaned  by  opening  the  fire-door  at  convenient  intervals. 

When  tubes  fail,  they  are  usually  destroyed  by  the  pressure  of  the  water 
crushing  them  inward ;  the  water  enters  through  the  rent  made  in  the  tube, 
and  flowing  upon  the  fire  extinguishes  it.  When  a  single  tube  thus  fails  upon 
a  journey,  the  engine,  notwithstanding  the  accident,  may  generally  be  made  to 
work  to  the  end  of  its  journey  by  plugging  the  ends  of  the  broken  tube  with 
hard  wood  ;  the  water  in  contact  with  which  will  prevent  the  fire  from  burning 
it  away. 

The  tubes  act  as  stays,  connecting  the  ends  of  the  boiler  to  strengthen 
them.  Besides  these,  there  are  rods  of  wrought  iron  extended  from  end  to  end 
of  the  boiler  above  the  roof  of  the  internal  fireplace.  These  rods  are  repre- 
sented at  0  in  their  length  in  fig.  67,  and  an  end  view  of  them  is  seen  in  fig. 
72.  The  smoke-box  F,  figs.  67,  74,  containing  the  cylinders,  steam-pipe,  and 
blast-pipe,  is  four  feet  wide,  and  two  feet  long.  It  is  formed  of  wrought  iron 
plates,  half  an  inch  thick  on  the  side  next  the  boiler,  and  a  quarter  of  an  inch 
elsewhere.  The  plates  are  riveted  in  the  same  manner  as  those  of  the  fire-box 
already  described.  From  the  top  of  the  smoke-box,  which,  like  the  fire-box, 
is  semi-cylindrical,  as  seen  in  elevation  in  fig.  73,  and  in  section  in  fig.  74, 
rises  the  chimney  G,  fifteen  inches  diameter,  and  formed  of  |-  inch  iron  plates, 
riveted  and  bound  round  by  hoops.  It  is  flanged  to  the  top  of  the  smoke-box, 
as  represented  in  fig.  74.  Near  the  bottom  of  the  smoke-box  the  working 
cylinders  are  placed,  side  by  side,  in  a  horizontal  position,  with  the  slide  valves 
upward.  In  the  top  of  the  external  fire-box  a  circular  aperture  is  formed 
fifteen  inches  in  diameter,  and  upon  this  aperture  is  placed  the  steam-dome  T 
figs.  67,  71,  72,  two  feet  high,  and  attached  around  the  circular  aperture  by  a 
flange  and  screw  secured  by  nuts.  This  steam  dome  is  made  of  brass -I  inch 
thick.  In  stationary  boilers,  where  magnitude  is  not  limited,  it  has  been 
already  explained,  that  the  space  allowed  for  steam  is  sufiiciently  large  to 
secure  the  complete  separation  of  the  vapor  from  the  spray  which  is  mixed 
with  it  when  it  issues  immediately  from  the  water.  In  locomotive  boilers 
sufiicient  space  cannot  be  allowed  lor  this,  and  the  separation  of  the  water 
from  the  steam  is  efl'ected  by  the  arrangement  here  represented.  A  funnel- 
shaped  tube  d' ,  figs.  67,  72,  with  its  wide  end  upward,  rises  into  the  steam- 
dome,  and  reaches  nearly  to  the  top  of  it.  This  funnel  bends  toward  the  back 
of  the  fire-box,  and  is  attached  by  a  flange  and  screws  to  the  great  steam-pipe 
S,  which  traverses  the  whole  length  of  the  boiler.  The  steam  rising  from  the 
boiler  fills  the  steam-dome  T,  and  descends  in  the  I'unnel-shaped  tube  d'. 
The  space  it  has  thus  to  traverse  enables  the  steam  to  disengage  itself  almost 

VOL.  II. 35 


546 


THE   STEAM-ENGINE. 


completely  from  the  priming.     The  wider  part  of  the  great  steam-pipe  a  is  ] 
flanged  and  screwed  at  the  hinder  end  to  a  corresponding  aperture  in  the  back  i 
plate  of  the  fire-box.     This  opening  is  covered  by  a  circular  plate,  secured  by  ' 
screws,  having  a  stuffing-box  in  its  centre,  of  the  same  kind  as  is  used  for  the  ( 
piston-rods  of  steam-cylinders.     Through  this  stuffing-box  the  spindle  a"  of   * 
the  regulator  passes,   and  to  its   end  is  attached  a  winch  ^',  by  which  the  i 
spindle  a"  is  capable  of  being  turned.     This  winch  is  limited  in  its  play  to  a  ' 
quarter  of  a  revolution.     The  other  end  of  the  spindle  a'"'' is  attached  to  a  plate  i 
e' ,  seen  edgewise  in  fig.  67,  and  the  face  of  which  is  seen  in  fig.  72  ;   this  cir-  ' 
cular  plate  e'  is  perforated  with  two  apertures  somewhat  less  than  quadrants. 
That  part  of  the  plate,  therefore,  which  remains  not  pierced  forms  two  solid 
pieces  somewhat  greater  than  quadrants.     This  plate  is  ground  so  as  to  move 
in  steam-tight  contact  with  a  fixed  plate  under  it,  which  terminates  at  the  wide 
end  of  the  conical  mouth  of  the  steam-pipe  S.     This   fixed   circular  plate   is 
likewise  pierced  with  two   nearly  quadrantal  apertures,  corresponding  with 
those  in  the  moveable  plate  e' .     When  the  moveable  plate  e'  is  turned  round  by 
the  winch  U,  the  apertures  in  it  may  be  made  to  correspond  with  those  of  the 
fixed  circular  plate  on  which  it  moves,  in  which  position  the  steam-pipe  S 
communicates  with  the  funnel  d'   by  the  two  quadrantal  apertures  thus  open. 
If,  on  the  other  hand,  the  winch  hi  be  moved   from  this   position  through  a 
quarter  revolution,  then  the  quadrantal  openings  in  the  moveable  plate  will  be 
brought  over  the  solid   parts  of  the  fixed  plate  on  which  it  moves,  and  these 
solid  parts  being  a  little  more  than  quadrants,  while  the  openings  are  a  little 
less,  all  communication   between  the  steam-pipe  S  and  the  funnel  d'  will  be 
stopped,  for  in  this   case  the  quadrantal  openings  in  the  fixed  and  moveable 
plates  respectively  will  be  stopped  by  the  solid  parts  of  these  plates.     It  will 
be  evident  that  as  the  winch  h!  of  the  regulator  is   moved   from   the  former 
position  to  the  latter,  in  every  intermediate  position  the  aperture  communicating 
between  the  funnel  d'  and  the  steam-pipe   S  will  be   less  in  magnitude  than 
the  complete  quadrant.     It  will  in  fact  be  composed  of  two  openings   having 
the  form  of  sectors  of  a  circle  less  than  a  quadrant,  and  these  sectors  may 
be  made   of  any  magnitude,  however  small,  until  the  opening  is  altogether 
closed. 

By  such  means  the  admission  of  steam  from  the  boiler  to  the  steam-pipe  S 
may  be  regulated  by  the  winch  h' . 

The  steam  being  admitted  to  the  steam-pipe  passes  through  it  to  the  front 
end  of  the  boiler,  and  the  pipe  being  enclosed  within  the  boiler  the  temperature 
of  the  steam  is  maintained.  The  steam-pipe  passing  through  the  tube-plate 
at  the  front  end  of  the  boiler  is  carried  to  a  small  distance  from  the  tube-plate 
in  the  same  direction,  where  it  is  flanged  on  to  a  cross  horizontal  pipe  pro- 
ceeding to  the  right  and  to  the  left  as  represented  in  fig.  74.  This  cross  pipe 
is  itself  flanged  to  two  curved  steam-pipes,  S,  fig.  74,  by  which  the  steam  is 
conducted  to  the  valve-boxes  V  V.  The  lower  ends  of  these  curved  arms  are 
flanged  on  to  the  valve-boxes  of  the  two  cylinders  at  the  ends  nearest  to  the 
boiler.  The  opening  of  one  of  these  is  exhibited  in  the  right-hand  cylinder 
in  fig,  69.  By  these  pipes  the  steam  is  conducted  into  the  valve-boxes  or 
steam-chests,  from  which  it  is  admitted  by  slide-valves  to  the  cylinders  to 
work  the  pistons  in  the  same  manner  as  has  been  already  described  in  the 
large  stationary  engines. 

On  the  upper  sides  of  the  cylinders  are  formed  the  steam-chests  or  valve- 
boxes,  which  are  exhibited  at  U,  figs.  67,  69,  74.  These  are  made  of  cast- 
iron  half  an  inch  thick,  and  are  bolted  to  the  upper  side  of  each  cylinder.  At 
the  front  end  they  are  also  secured  by  bolts  to  the  smoke-box,  and  at  the 
hinder  end  are  attached  to  the  tube-plate.     These   valve-boxes  communicate 


THE   STEAM-ENGINE. 


547 


548 


THE  STEAM-ENGINE. 


with  the  passages  m  and  n,  fig.  69,  leading  to  the  top  and  bottom  of  the  cylin- 
der: these  are  called  the  steam-ports.  They  also  communicate  with  a  passage 
0  leading  to  the  mouth  of  a  curved  horizontal  pipe  p'  connecting  the  front  ends 
of  the  two  cylinders,  as  seen  in  figs.  69,  74.  These  curved  pipes  unite  in  a 
single  vertical  pipe  p,  called  the  blast-pipe,  seen  in  figs.  67,  74,  this  vertical 
pipe  becomes  gradually  small  toward  the  top,  and  terminates  a  little  above  the 
base  of  the  funnel  or  chimney  G.  In  the  valve-box  is  placed  the  slide-valve 
V  to  which  is  attached  the  spindle  /'.  This  spindle  moves  through  a  stuffing- 
box  ¥,  and  is  worked  by  gearing,  which  will  be  described  hereafter.  Accord- 
ing to  the  position  given  to  the  slide,  a  communication  may  be  opened  between 
the  steam-chest,  or  the  waste-port,  and  either  end  of  the  cylinders.  Thus 
when  the  slide  is  in  the  position  represented  in  fig.  67,  the  steam-chest  com- 
municates with  the  front  end  of  the  cylinder,  while  the  waste-port  communi- 
cates with  the  hinder  end.  If,  on  the  other  hand,  the  spindle  V  being  pressed 
forward,  move  the  slide  to  its  extreme  opposite  position,  the  steam-port  n  would 
communicate  with  the  waste-port  o,  while  the  steam-chest  would  communicate 
with  the  steam-port  m,  steam  would,  therefore,  be  admitted  to  the  hinder  end 
of  the  cylinder,  while  the  foremost  end  would  communicate  with  the  waste- 
port.  It  will  be  perceived  that  this  arrangement  is  precisely  similar  to  that 
of  the  slide-valves  already  described.  The  slide-valve  is  represented  on  a 
larger  scale  in  fig.  76,  where  A  is  the  hinder  steam-port,  B  the  foremost  steam- 


port,  and  C  the  waste-port.  The  surfaces  D,  separating  the  steam-ports  from 
the  waste-ports,  are  called  the  bars  :  they  are  planed  perfectly  smooth,  so  that 
the  surfaces  F  and  G  of  the  slide-valve,  also  planed  perfectly  smooth,  may 
move  in  steam-tight  contact  with  them.  These  surfaces  are  kept  in  contact 
by  the  pressure  of  the  steam  in  the  steam-chest,  by  which  the  slide-valve  is 
always  pressed  down.  In  its  middle  position,  as  represented  by  the  dotted 
lines  in  the  figure,  both  the  steam-ports  are  stopped  by  the  slide-valve,  so  that 
at  that  moment  no  steam  is  admitted  to  either  end  of  the  cylinder.  On  either 
side  of  this  intermediate  position  the  slide  has  an  inch  and  a  half  play,  Avhich 
is  suflicient  to  open  successively  the  two  steam-ports. 

The  cylinders  are  inserted  at  one  end  in  the  plate  of  the  smoke-box,  and  at 
the  other  in  the  tube-plate  of  the  boiler.  They  are  closed  at  either  end  by 
cast-iron  covers,  nearly  an  inch  thick,  flanged  on  by  bolts  and  screws.  In  the 
cover  of  the  cylinder  attached  to  the  tube-plate  is  a  stufling-box,  in  which  the 
piston-rod  plays.  The  metallic  pistons  used  in  locomotive  engines  do  not  dif- 
fer materially  from  those  already  described,  and  therefore  need  not  be  here 
particularly  noticed.  From  their  horizontal  position  they  have  a  tendency  to 
wear  unequally  in  the  cylinders,  their  weight  pressing  them  on  one  side  only  ; 
but  from  their  small  magnitude  this  effect  is  found  to  be  imperceptible  in  prac- 
tice. In  the  engine  here  described  the  stroke  of  the  piston  is  eighteen  inches, 
and  this  is  the  most  usual  length  of  stroke  in  locomotive  engines.  The  piston, 
in  its  play,  comes  at  either  end  within  about  half  an  inch  of  the  inner  surface  ( 
of  the  covers  of  the  cylinders,  this  space  being  allowed  to  prevent  collision. 
In  the   foremost   cover  of  the  cylinder  is  inserted  a  cock  q' ,  figs.  67,  69,  by 


THE   STEAM-ENGINE. 


549 


Fig.  71. 


^..^^^-^1 


ELEVATION    OF    THE   HINDER   END   OF   A   LOCOMOTIVE    ENGINE. 


550 


THE  STEAM-ENGINE. 


Fig.  77. 


which  any  water  which  may  collect  in  the  cylinder  by  condensation  or  priming 
may  be  discharged.  A  cock  r' ,  fig.  67,  communicajing  with  a  small  tube  proceed- 
ing from  the  branches  of  the  waste  pipe  /»',  fig.  74,  is  likewise  provided  to 
discharge  from  that  pipe  any  water  which  may  be  collected  in  it.  After  the 
steam  has  been  admitted  to  work  the  piston  through  tha  slide-valve,  and  has 
been  discharged  through  the  waste-port  by  shifting  that  valve,  it  passes  through 
the  pipey  into  the  blast-pipe  p,  from  the  mouth  of  which  it  issues,  with  great 
force,  up  the  funnel  G.  When  the  motion  of  the  engine  is  rapid,  the  steam 
from  the  two  cylinders  proceeds  in  an  almost  uninterrupted  current  from  the 
blast-pipe,  and  causes  a  strong  draught  up  the  chimney.  The  heated  air 
which  passes  from  the  mouths  of  the  tubes  into  the  smoke-box  is  drawn  up  by 
this  current,  and  a  corresponding  draught  is  produced  in  the  fire-box. 

The  piston-rods  Y  terminate  in  a  fork,  by  which  they  are  attached  to  cross 
heads  Z,  the  ends  of  which  are  confined  by  guide-bars  A',  in  which  they  are 
allowed  to  play  backward  and  forward  through  a  space  equal  to  the  stroke  of 
the  piston.  To  these  cross  heads  Z,  between  the  prongs  of  the  fork  in  which 
the  piston  terminates,  are  attached  the  foremest  ends  of  the  connecting  rods 
B''.  These  rods  are,  therefore,  driven  backward  and  forward  by  the  motion 
imparted  to  the  cross  head  Z  by  the  piston-rods  Y.  The  connecting  rods  B' 
are  attached  at  the  hinder  ends  to  two  cranks  formed  upon  the  axles  C  of  the 
driving  wheels  D^.  These  two  cranks  are  formed  upon  the  axles  precisely  at 
right  angles  to  each  other.  The  left-hand  crank  is  represented  in  its  hori- 
zontal position,  in  fig.  69,  and  the  right-hand  crank  is 
seen  in  its  vertical  position.  A  cranked  axle  is  repre- 
sented on  a  larger  scale  in  fig.  77,  and  the  two  cranks 
are  seen  in  a  position  oblique  to  the  plane  of  the  figure. 
As  this  axle  is  the  instrument  by  which  the  impelling 
force  is  conveyed  to  the  load,  and  as  it  has  to  support  a 
great  portion  of  the  weight  of  the  engine,  it  is  constructed 
with  great  strength  and  precision.  It  is  made  all  in  one 
piece,  and  of  the  best  wrought  iron  called  back  barrow, 
or  scrap  iron.  In  the  engine  here  described  its  extreme 
length  is  six  feet  and  a  half,  and  its  diameter  is  five 
inches.  At  the  centre  part  A  it  is  cylindrical,  and  is 
increased  to  five  inches  and  a  quarter  at  C,  where  the 
cranks  are  formed.  The  sides  D  of  the  cranks  are  four 
inches  thick,  and  the  crank  pins  B,  which  are  truly  cyl- 
indrical, are  five  inches  diameter,  and  three  inches  in 
length,  the  brasses  at  the  extremities  of  the  connecting 
rods  which  play  upon  them  having  a  corresponding 
magnitude.  The  distance  from  the  centre  of  the  crank- 
pins  B  to  the  centre  of  the  axle  A  must  be  exactly  equal 
to  half  the  stroke  of  the  piston,  and  is,  therefore,  in  this 
case  precisely  nine  inches.  Upon  the  parts  F,  which 
are  seven  inches  and  a  half  long,  the  great  driving 
wheels  are  firmly  fastened,  so  as  to  be  prevented  from 
turning  or  shaking  upon  the  axle.  The  axle  projects 
beyond  the  wheels  at  G,  where  it  is  reduced  to  three 
inches  and  an  eighth  diameter.  These  projecting  parts 
G  are  five  inches  long,  having  collars  at  the  outer  ends. 
Brasses  are  fixed  at  the  outside  frame  of  the  engine 
which  rest  upon  these  projections  G  of  the  axle,  and 
upon  these  brasses  the  weight  of  the  engine  is  supported. 
The  entire  axle  is  accurately  turned  in  a  lathe,  and  each 


THE   STEAM-ENGINE. 


531 


Fig.  72. 


CEOSS   VERTICAL    SECTION    OF    THE    ENGINE   THROUGH    THE    FIRE-BOX. 


552 


THE  STEAM-ENGINE. 


of  the  crank-pins  B  is  likewise  turned  by  suspending  the  axle  on  centres  cor- 
responding with  the  centres  of  the  crank-pins,  and  made  on  strong  cast  iron 
arms,  which  are  firmly  fixed  on  the  ends  of  the  axle,  and  project  beyond  the 
cranks  so  as  to  balance  the  axle,  and  enable  it  to  turn  round  on  the  centre  of 
the  crank-pin.  The  axle  is  by  such  means  made  perfectly  true,  and  the  cranks 
are  made  of  exactly  the  proper  length,  and  precisely  at  right  angles  to  each 
other.  The  corners  of  the  cranks  are  champered  off,  as  shown  in  the  figure, 
and  the  ends  of  the  cylindrical  parts  well  rounded  out. 

The  strength  and  accuracy  of  construction  indispensable  in  these  cranked 
axles,  in  order  to  make  them  execute  their  work,  render  them  very  expensive. 
When  properly  constructed,  however,  they  are  seldom  broken,  but  are  some- 
times bent  when  the  engine  escapes  from  the  rails. 

The  proper  motion  to  admit  and  withdraw  the  steam  from  either  end  of  the 
cylinder  is  imparted  to  the  slide-valves  by  eccentrics,  in  a  manner  and  on  a 
principle  so  similar  to  that  already  described  in  large  stationery  engines,  that 
it  will  not  be  necessary  here  to  enter  into  any  detailed  explanation  of  the  ap- 
paratus for  communicating  this  motion,  which  is  exhibited  in  plan  and  section 
in  figs.  67,  69.  The  eccentrics  are  attached  to  the  cranked  axles  at  E^  E". 
The  eccentric  E'  imparts  motion  by  a  rod  e"  to  a  lever  li" ,  formed  on  an  axle 
extending  across  the  frame  of  the  engine.  This  conveys  motion  to  another 
lever  I" ,  projecting  from  the  same  axle.  This  lever  /"  is  jointed  to  horizontal 
links  m",  which  at  the  foremost  ends  are  attached  to  the  spindle  V ,  by 
which  the  slide  is  driven.  By  these  means  the  motion  received  by  the 
eccentric  from  the  great  working  axle  conveys  to  the  spindle  V  an  alternate 
movement  backward  and  forward,  and  the  points  at  which  it  is  reversed  will 
be  regulated  by  the  position  given  to  the  eccentric  upon  the  great  axle.  The 
eccentric  is  formed  in  two  separate  semicircles,  and  is  keyed  on  to  the  great 
axle,  and  consequently  any  position  may  be  given  to  it  which  may  be  required. 
The  position  to  be  given  to  the  eccentrics  should  be  such  that  they  shall  be  at 
right  angles  to  their  respective  cranks,  and  they  should  be  fixed  a  quarter  of 
a  revolution  behind  the  cranks  so  as  to  move  the  slides  to  that  extent  in  ad- 
vance of  the  piston,  since  by  the  position  of  the  levers  h"  and  I" ,  the  motion 
of  the  eccentric  becomes  reversed  before  it  reaches  the  valve  spindle. 

The  performance  of  the  engine  is  materially  affected  by  the  position  of  the 
eccentrics  on  the  working  axle.  The  slide  should  begin  to  uncover  the  steam- 
port  a  little  before  the  commencement  of  the  stroke  of  the  piston,  in  order  that 
the  steam  impelling  the  piston  should  be  shut  off,  and  the  steam  about  to  impel 
it  in  the  contrary  direction  admitted  before  the  termination  of  the  stroke. 
Through  this  small  space  the  steam,  therefore,  must  act  in  opposition  to  the 
motion  of  the  piston.  This  is  called  the  lead  of  the  slide,  and  the  extent 
generally  given  to  it  is  aljout  a  quarter  of  an  inch.  This  is  accomplished  by 
fixing  the  eccentrics  not  precisely  at  right  angles  to  the  respective  cranks,  but 
a  little  in  advance  of  that  position.  The  introduction  of  the  steam  to  the 
piston  before  the  termination  of  the  stroke  has  the  effect  of  bringing  it  gradual- 
ly to  rest  at  the  end  of  the  stroke,  and  thereby  diminishing  the  jerk  or  shock 
produced  by  the  rapid  change  of  motion.  In  stationary  engines,  where  the 
reciprocations  of  the  engine  are  slow,  the  necessity  for  this  provision  does  not 
arise  ;  but  in  locomotive  engines  in  which  the  motion  of  the  piston  is  changed 
from  four  to  six  times  in  a  second,  it  becomes  necessary.  The  steam  admitted 
to  the  piston  before  the  termination  of  the  stroke  acts  as  a  spring-cushion  to 
assist  in  changing  its  motion,  and  if  it  were  not  applied,  the  piston  could  not 
be  kept  tight  upon  the  piston-rod.  Another  advantage  which  is  produced  by 
I  allowing  some  lead  to  the  slide  is  that  the  waste  steam  which  has  just  impelled 
•  the  piston  begins  to  make  its  escape  through  the  waste-port  before  the  com- 


THE   STEAM-ENGINE. 


Fie.  71. 


''■~™v'>>7"^-*v^t^♦--^*^--'VYMJ 


ELEVATION    OF    THE    FOREMOST   END    OF    A   LOCOMOTIVE   ENGINE. 


554 


THE  STEAM-ENGINE. 


mencement  of  the  next  stroke,  so  that  when  the  impelling  steam  begins  to  pro-  S 
duce  the  returning  stroke,  there  is  less  waste  steam  on  the  other  side  of  the  pis-  / 
ton  to  resist  it.  ) 

When  the  motion  of  the  engine  is  very  rapid,  the  resistance  of  the  waste  ) 
steam,  as  it  escapes  from  the  blast-pipe  to  the  piston,  has  been  generally  sup-  { 
posed  to  be  very  considerable,  though  we  are  not  aware  of  any  direct  experi-  ) 
ments  by  which  its  amount  has  been  ascertained.  In  the  account  of  the  loco-  s 
motive  engine  which  has  been  here  described,  supplied  by  Mr.  Stephenson  ) 
for  the  last  edition  of  Tredgold  on  the  steam-engine,  he  states,  that  the  average  S 
resisting  pressure  of  the  waste  steam  throughout  the  stroke  is  6  lbs.  per  square  ? 
inch,  when  running  at  the  usual  rate  of  from  25  to  28  miles  an  hour,  and  that  S 
at  greater  velocities  this  negative  pressure  has  been  found  to  increase  to  more  ( 
than  double  that  amount.  No  experiments  are,  however,  cited  from  which  this  ) 
inference  has  been  drawn.  < 

It  has  been  also  thought  that  the  pressure  of  steam  upon  the  piston  in  the  ) 
cylinder,  at  high  velocities,  is  considerably  below  the  pressure  of  steam  in  the  < 
boiler  ;  but  this  has  not  been,  so  far  as  we  are  informed,  ascertained  by  any  ) 
satisfactory  experimental  test.  Mr.  Stephenson  likewise  states,  that  this  loss  } 
of  pressure,  causes  the  negative  pressure  or  resistance  of  the  vjraste  steam  to  ! 
amount  to  from  30  to  40  per  cent,  of  the  positive  pressure  upon  the  piston  when  < 
the  engine  is  running  very  fast,  and  that  therefore  the  power  of  the  engine  is  \ 
diminished  nearly  one  half.  ( 

But  it  vrill  be   perceived  that  besides  the  uncertainty  which  attends  the  ^ 
estimate  of  the   actual  amount  of  pressure  on  the  piston  compared  with  the  ( 
pressure  of  steam  in  the  boiler,  the  inference  here  drawn  does  not  appear  to  be  ] 
compatible  with  what  has  been  already  proved  respecting  the  mechanical  effect  ' 
of  steam.     No  change  of  pressure  which  may  take  place  between  the  boiler  , 
and  the  cylinder  can  affect  the  practical  efficacy  of  the  steam.     As  the  steam  ' 
passes  through  the  engine,  whatever  change  of  pressure  it  may  be  subject  to,  , 
it  still  remains  common  steam  :  and  though  its  pressure  may  be  diminished,  ' 
its  volume  being  increased  in  a  nearly  equal  proportion,  its  mechanical  effect  , 
will  remain  the  same.     The  power  of  the  engine,  therefore,  estimated  as  it  ' 
ought  to  be,  by  the  whole   mechanical  effect  produced,  will  not  be  altered 
otherwise  than  by  the  effect  of  the  increased  resistance  produced  by  the  blast- 
pipe.     What  that  resistance  is,  we  repeat,  has  not,  so  far  as  we  know,  been 
ascertained  by  direct  experiment,  and  there  are  circumstances  attending  it 
I  which  render  it  probable  that,  even  at  high  velocities,  it  is  less  in  amount  than 
'  Mr.  Stephenson's  estimate. 

]  The  position  of  the  eccentrics  which  is  necessary  to  make  the  pistons  drive 
'  the  engine  forward  must  be  directly  the  reverse  of  that  which  would  cause 
I  them  to  drive  the  engine  backward.     To  be  able,  therefore,  to  reverse  the 

*  motion  of  the  engine^  it  would  only  be  necessary  to  be  able  to  reverse  the 
)  position  of  the  eccentrics,  which   may    be    accomplished    by  either  of  two 

*  expedients. 

i  First,  The  eccentrics  may  be  capable  of  revolving  on  the  great  working 
(  axle,  and  also  of  sliding  upon  it  through  a  small  space.  Their  revolution  on 
)  the  axle  may  be  checked  by  letting  a  pin  attached  to  a  collar  fastened  on  the 
(  axle  fall  into  a  hole  on  the  side  of  the  eccentric.  Such  a  pin  will  drive  the 
S  eccentric  round  with  the  axle,  and  the  position  of  this  pin  and  the  hole  will 
(  determine  the  position  of  the  eccentric  with  reference  to  the  crank.  At  a  short 
)  distance  on  the  other  side  of  the  eccentric  may  be  a  corresponding  collar  with 
(  a  pin  in  the  opposite  position.  By  moving  the  eccentric  longitudmally  on  the 
I  axle,  the  former  pin  may  be  withdrawn  from  the  hole,  and  the  latter  allowed 
(  to  fall  into  the  hole  on  the  other  side.     Proper  mechanism  may  be  provided 


by  which  the  position  of  the  eccentric  may  thus  be  reversed  in  reference  to 
the  crank,  and  by  such  means  the  motion  of  the  engine  may  be  reversed. 

Secondly,  Supposing  the  eccentrics  which  drive  the  engine  forward  to  be 
immoveably  fixed  upon  the  axle,  two  other  eccentrics  may  be  provided  attach- 
ed to  other  parts  of  the  same  axle,  and  having  a  position  exactly  the  reverse 
with  reference  to  the  cranks.  Proper  mechanism  may  be  provided,  by  which 
either  or  both  pairs  of  eccentrics  may  be  thrown  in  or  out  of  gear.  Such  are 
the  means  adopted  in  the  engine  which  has  been  already  described.  The 
eccentrics  for  driving  the  engine  backward  are  placed  outside  the  cranks  at 
F'  F".  A  hand  lever  w" ,  fig.  71,  is  provided,  by  which  the  engine-man  may 
throw  either  pair  of  eccentrics  into  or  out  of  gear,  so  as  to  make  the  engine 
work  either  backward  or  forward. 

As  all  the  moving  parts  of  the  engine  require  to  be  constantly  lubricated  with 
oil  to  diminish  the  friction,  and  keep  them  cool,  oil-cups  for  this  purpose  are 
fixed  upon  them.  In  some  engines  these  oil-cups  are  attached  separately  to 
all  the  moving  parts  :  in  others  they  are  placed  near  each  other  in  a  row  on 
the  boiler,  and  communicate  by  small  tubes  with  the  several  parts  required  to 
be  lubricated.  One  of  these  is  requisite  for  each  end  of  the  connecting  rods, 
for  each  of  the  guides  of  the  piston-rods,  for  the  piston-rod  itself,  the  spindle 
of  the  slide-valve,  and  other  parts.  An  elevation  of  one  of  these  oil-cups  is 
shown  in  fig.  78,  a  vertical  section  in  fig.  79,  and  horizontal  plan  in  fig.  80. 


Pig.  79. 


The  cup  A  is  made  of  brass  with  a  cover  B.  This  cover  has  a  piece  projecting 
from  it  turning  upon  a  pin  in  a  socket  C  at  the  side  of  the  cup  A,  and  square 
at  the  end,  resting  upon  a  small  spring  at  the  bottom  of  the  socket  to  hold  it 
either  open  or  shut.  In  the  bottom  of  the  cup  is  inserted  an  iron  tube  D  ex- 
tending nearly  to  the  top.  This  tube  projects  from  the  bottom  of  the  cup, 
where  it  is  tapped  for  the  purpose  of  fixing  the  cup  on  the  part  of  the  engine 
which  it  is  intended  to  lubricate.  The  hole  into  which  the  cup  is  screwed 
communicates  with  the  rubbing  surface,  and  some  cotton  thread  is  passed 
through  the  tube  dipping  into  the  oil  in  the  cup  at  the  one  end  and  touching 
the  moving  part  at  the  other.  This  thread  acts  as  a  siphon,  and  constantly 
drops  oil  on  the  rubbing  surface. 

The  tender  is  a  carriage  attached  behind  the  engine  and  close  to  it,  carrying 
coke  for  the  supply  of  the  furnace,  and  water  for  the  boiler.  The  coke  is  con- 
tained in  the  space  R",  figs.  68,  70,  surrounded  by  a  tank  V  containing  water 
to  feed  the  boiler.  The  feed  for  the  boiler  is  conducted  from  the  tank  through 
a  pipe  descending  downward  and  in  a  curved  direction,  P"  Q",  fig.  68,  and 
connected  with  a  horizontal  pipe  K,  fig.  67.  A  cock  is  provided  at  P",  by 
which  the  supply  of  water  to  this  pipe  may  be  cut  off  at  pleasure.  Another 
cock  is  provided  at  t',  fig.  67,  where  the  curved  pipe  joins  the  horizontal  pipe 
by  which  the  quantity  of  water  supplied  to  K  may  be  regulated  by  opening 


THE   STEAM-ENGINE. 


the  cock  more  or  less  fully.  The  handle  of  this  cock  rises  through  the  floor 
of  the  engine,  so  that  the  engineer  may  regulate  it  at  discretion.  The  pipe  K 
being  conducted  under  the  engine,  as  represented  in  fig.  67,  terminates 
in  a  vertical  pipe,  of  greater  diameter,  containing  two  valves,  both  of  which 
open  upward,  and  between  these  valves  to  this  vertical  pipe  is  attached  a 
force-pump,  by  which  the  water  is  drawn  from  the  horizontal  pipe  K  into  the 
vertical  pipe  K',  and  from  the  latter  is  driven  into  a  delivery-pipe  by  which  it 
is  forced  into  the  boiler.  The  details  of  the  interior  of  this  feed-pump  are 
represented  on  a  larger  scale  in  fig.  81.     The  extremity  of  the  horizontal  pipe 

Fia:.  81. 


K'  is  represented  in  section  at  H,  where  it  is  joined  on  by  a  screw  to  the 
bottom  of  the  vertical  pipe  which  is  represented  in  fig.  67,  at  K,  and  which  is 
here  represented  in  section.  The  vertical  pipe,  represented  in  fig.  67,  con- 
sists of  several  parts  screwed  together  by  nuts  and  bolts  passing  through 
flanges.  The  lovvest  piece  I  is  attached  by  a  flange  to  the  piece  L  :  within 
these  is  contained  the  valve  Q  resting  in  a  seat  made  conical,  so  that  the  ball 
which  forms  the  valve  shall  rest  in  water-tight  contact  with  it.  The  ball  is 
turned  and  ground  to  an  accurate  sphere,  and  whatever  position  it  assumes 
upon  its  seat  its  contact  will  be  perfect.  It  is  guided  in  its  upward  and  down- 
ward motion  by  several  vertical  bars  which  confine  it,  and  which  are  united  at 
the  top,  so  as  to  limit  the  upward  motion  of  the  ball.  A  screw  V^  is  inserted 
in  the  bottom  of  the  piece  I,  by  removing  which  access  can  be  obtained  to  the 
valve.  The  piece  L  is  secured  to  the  short  pipe  G  by  nuts  and  bolts  passed 
through  a  flange.  The  pipe  G  is  cast  upon  the  end  of  the  feed-pump  A.  On 
the  foremost  end  of  this  feed-pump  is  constructed  a  stuffing-box  C  of  the  usual 
form,  having  a  gland  D  forced  against  packing  by  nuts  and  screws  E.  The 
plunger  B  is  turned  so  as  to  be  truly  cylindrical,  and  moves  in  water-tight 
contact  through  the  gland  D.  The  plunger  not  being  in  contact  with  the  inner 
surface  of  the  pump-barrel  A,  the  latter  heed  not  be  ground.  The  horizontal 
rod  by  which  the  plunger  B  is  driven  is  attached  at  its  foremost  extremity  to 
an  arm  which  projects  from  the  rod  of  the  steam-piston,  and  consequently  this 
plunger  is  moved  through  a  space  equal  to  the  stroke  of  the  steam-piston.     In 


558 


THE  STEAM-ENGINE. 


this  case  that  space  is  eighteen  inches.  The  upper  end  of  the  vertical  tube 
G  is  attached  by  screws  and  a  flange  to  a  piece  P  containing  a  valve  R 
similar  in  all  respects  to  the  lower  valve  Q,  and  like  it  opening  upward.  A 
screw  V  is  introduced  at  the  top  by  which  access  may  be  obtained  to  this 
valve.  This  screw  also  presses  on  the  crown  of  the  guides  of  the  valve,  so 
as  to  hold  it  down  by  regulated  pressure.  At  the  side  of  this  upper  piece  P 
is  inserted  a  horizontal  tube  M  connected  with  the  end  of  the  delivery-pipe  N. 
This  latter  is  continued  to  the  boiler  with  which  it  communicates  at  the  fire- 
box. When  the  plunger  B  is  drawn  out  of  the  pump-barrel  A,  the  spherical 
valve  Q  being  relieved  from  its  downward  pressure  is  raised,  and  water  passes 
from  the  pipe  H  through  the  valve  Q  into  the  vertical  pipe  G;  the  lower  valve 
Q  then  closes  and  stops  the  return  of  the  water.  The  plunger  B  returning 
into  the  pump-barrel  A  then  forces  the  water  against  the  upper  valve  R  and 
drives  it  through  the  delivery-tube  N,  from  which  its  return  is  prevented  by 
the  valve  R.  When  the  delivery-tube  N  is  filled  with  water  throughout  its 
whole  length,  every  stroke  of  the  plunger  will  evidently  drive  into  the  boiler  a 
volume  of  water  equal  to  the  magnitude  of  a  part  of  the  plunger  eighteen  inches 
in  length. 

Until  within  the  last  few  years,  locomotive  engines  were  supported  on  only 
four  wheels  ;  they  are,  however,  now  almost  universally  supported  on  six,  the 
driving  wheels  being  in  the  middle.  To  give  greater  security  to  the  position 
of  the  engine  between  the  rails  it  is  usual  to  construct  flanges  on  the  tires  of  all 
the  six  wheels.  Mr.  Stephenson,  however,  has  been  in  the  practice  of  con- 
structing the  driving  wheels  without  flanges,  and  with  tires  truly  cylindrical, 
depending  on  the  flanges  of  the  two  pairs  of  smaller  wheels  to  maintain  the 
engine  between  the  rails.  The  wheels  of  the  engine  here  described  are  con- 
structed in  this  manner.  The  driving  wheels  D'  are  fixed  on  the  cranked 
axle  C',  and  are  five  feet  in  diameter.  The  other  wheels  L^  M',  the  one  being 
placed  immediately  behind  the  smoke-box,  and  the  other  immediately  behind 
the  fire-box,  are  each  three  feet  six  inches  in  diameter,  and  have  a  flange  upon 
their  tiires,  which  running  on  the  inside  of  each  rail  keeps  the  engine  between 
the  rails.  Each  pair  of  these  small  wheels,  like  the  driving-wheels,  is  fixed 
upon  their  axle.  The  axles  are  3|  inches  diameter,  and  project  beyond  the 
wheels,  the  projecting  part  supporting  the  frame  of  the  engine  and  turning  in 
brasses.  Upon  these  brasses  rest  springs,  which  bear  the  whole  weight  of 
the  engine.  These  springs  having  nothing  between  them  and  the  road  but  the 
wheels  and  axles  intercept  and  equalize  the  sudden  shocks  produced  by  the 
rapid  motion  upon  the  road. 

When  an  engine  is  required  for  the  transport  of  very  heavy  loads,  such  as 
those  of  merchandise,  the  adhesion  of  one  pair  of  working  wheels  is  found  to 
be  insufficient,  and,  in  such  cases,  one  of  the  two  pairs  of  wheels  U  M'  is 
made  of  the  same  diameter  as  the  wheels  which  are  placed  upon  the  working 
axle,  and  a  bar  is  attached  to  points  on  the  outside  of  the  wheels  at  equal  dis- 
tances from  iheir  centre,  connecting  them  in  such  a  manner  that  any  force  ap- 
plied to  make  one  pair  of  wheels  revolve  must  necessarily  impart  the  same 
motion  to  the  other  pair.  By  such  means  the  force  of  the  steam  is  made  to 
drive  both  pairs  of  wheels  and  consequently  a  proportionally-increased  adhesion 
is  obtained. 

The  velocity  which  an  engine  is  capable  of  imparting  to  the  load  which  it 
draws  depends  upon  the  rate  at  which  the  pistons  are  capable  of  being  moved 
in  the  cylinders.  By  every  motion  of  each  piston  backward  and  forward  one 
revolution  of  the  driving  wheels  is  produced,  and  by  each  revolution  of  the 
driving  wheels,  supposing  them  not  to  slip  upon  the  rails,  the  load  is  driven 
through  a  distance  upon  the  road  equal  to  their  circumference.     As  the  two 


THE  STEAM-ENGINE. 


559 


cylinders  work  together,  it  follows,  that  a  quantity  of  steam  sufficient  to  fill 
four  cylinders  supplied  by  the  boiler  to  the  engine  will  move  the  train  through 
a  distance  equal  to  the  circumference  of  the  driving  wheels ;  and  in  accom- 
plishing this,  each  piston  must  move  twice  from  end  to  end  of  the  cylinder  ; 
each  cylinder  must  be  twice  filled  with  steam  from  the  boiler  ;  and  that 
steam  must  be  twice  discharged  from  the  cylinder  through  the  blast-pipe  into 
the  chimney. 

If  the  driving-wheels  be  five  feet  in  diameter,  their  circumference  will  be 
fifteen  feet  seven  inches.  To  drive  a  train  with  a  velocity  of  thirty  miles  an 
hour,  it  will  be  necessary  that  the  engine  should  be  propelled  through  a  space 
of  forty-five  feet  per  second.  To  accomplish  this  with  five-feet  wheels,  they 
must  be  therefore  made  to  revolve  at  the  rate  of  very  nearly  three  revolutions 
per  second  ;  and  as  each  revolution  requires  two  motions  of  the  piston  in  the 
cylinder,  it  follows  that  each  piston  must  move  three  times  forward  and  three 
times  backward  in  the  cylinder  in  a  second  ;  that  steam  must  be  admitted  six 
times  per  second  from  the  steam-chest  to  each  cylinder,  and  discharged  six 
times  per  second  from  each  cylinder  into  the  blast-pipe.  The  motion,  there- 
fore, of  each  piston,  supposing  it  to  be  uniform,  must  divide  a  second  into  six 
equal  parts,  and  the  puffs  of  the  blast-pipe  in  the  chimney  must  divide  a  sec- 
ond into  twelve  equal  parts.  The  motion  of  the  slides  and  other  reciprocating 
parts  of  the  machinery  must  consequently  correspond. 

This  motion  of  the  reciprocating  parts  of  the  machinery  being  found  to  be 
injurious  to  it,  and  to  produce  very  rapid  wear,  attempts  have  been  made  to 
remedy  the  defect,  and  to  obtain  greater  speed  with  an  equal  or  diminished 
rate  of  motion  of  the  piston,  by  the  adoption  of  driving-wheels  of  greater  di- 
ameter, and  on  several  of  the  great  lines  of  railway  the  magnitude  of  the  wheels 
for  the  passenger-engines  have  been  increased  to  five  feet  and  a  half  and  six 
feet  in  diameter  ;  but  such  engines  have  not  been  sufficiently  long  in  use  to 
afford  grounds  for  forming  a  practical  estimate  of  their  effects.  Experiments 
of  a  much  bolder  description  have,  however,  been  tried  on  one  of  the  great 
lines  of  railway  by  the  adoption  of  driving-wheels  of  much  greater  diameter. 
In  some  cases  their  magnitude  has  been  increased  even  to  ten  feet ;  but  from 
various  experiments  to  which  these  engines  have  been  submitted  by  myself 
and  others,  as  well  as  from  the  experience  which  appears  to  be  obtained  from 
the  results  of  their  ordinary  work,  it  does  not  appear  that  any  advantages  have 
attended  them,  and  they  have  been  accordingly  for  the  most  part  abandoned. 

The  pressure  of  steam  in  the  boiler  is  limited  by  two  safety-valves,  repre- 
sented in  fig.  67,  at  N  and  0.  The  valve  at  N  is  under  the  control  of  the  en- 
gineer, but  the  valve  at  0  is  inaccessible  to  him.  The  structure  of  the  safety- 
valve  represented  at  N  is  exhibited  on  a  larger  scale  in  fig.  82,  which  repre- 
sents its  section,  and   fig.  83,  which  shows  a  plan  of  the  valve-seat  with  the 


Fig.  82. 


Fig.  83. 


valve  removed.  The  valve  A,  which  is  made  of  brass,  is  mitred  round  the 
edge  at  an  angle  of  forty-five  degrees,  and  has  a  spindle,  or  stalk  B,  cast  upon 
it,  projecting  downward  from  the  middle  of  it.     The  valve-seat  C  is  also  made 


560 


THE  STEAM-ENGINE. 


of  brass,  and  cast  with  a  flange  at  the  bottom  to  attach  it  to  the  boiler.  The 
mitred  surface  of  the  valve  is  ground  into  the  valve-seat,  so  as  to  rest  in  steam- 
tight  contact  with  it.  ,  Across  the  valve-seat,  which  is  two  and  a  half  inches 
in  diameter,  is  cast  a  thin  piece  D,  seen  in  plan  in  tig.  83,  and  in  section  in 
fig.  82.  which  extends  from  the  top  to  the  bottom,  and  has  a  longitudinal  hole 
through  it,  in  which  the  spindle  B  of  the  valve  works  :  by  this  hole  it  is  guided 
when  it  rises  from  its  seat.  A  projection  E  is  cast  upon  the  seat  of  the  valve, 
in  which  a  standard  F  is  inserted.  This  standard  is  forked  at  the  top,  and  re- 
ceives the  end  of  a  lever  G,  which  turns  in  it  upon  a  centre.  A  rod  H  is 
jointed  to  this  lever  by  another  pin  at  three  inches  from  the  former,  and  the 
lower  end  of  this  rod,  ground  to  a  point,  presses  upon  the  centre  of  the  valve 
A.  At  the  other  end  of  the  lever,  which  is  broken  off  in  fig.  82,  at  a  distance 
of  three  feet  from  the  centre  pin,  insetted  in  the  fork  of  the  pillar  F,  the  rod  of 
a  common  spring-balance  w,  fig.  71,  is  attached  by  a  finger-nut  n.  The  bottom 
of  this  spring-balance  is  secured  on  to  the  fire-box.  This  balance  is  screwed 
up  by  the  finger-nut  on  the  valve-lever  until  the  required  pressure  on  the  lever 
is  produced  through  the  medium  of  the  rod  H,  this  pressure  being  generally 
fil\y  pounds  per  square  inch  above  the  atmosphere.  When  the  pressure  of  the 
steam  in  the  boiler  exceeds  this,  the  valve  A  is  raised  from  its  seat,  and  the 
steam  escapes. 

It  is  evident  that  the  sliding-weight  by  which  the  pressure  of  the  safety- 
valve  is  sometimes' regulated  in  stationary  engines  would  not  be  admissible  in 
a  locomotive-engine,  since  the  motion  of  the  engine  would  constantly  jolt  it  up 
and  down,  and  cause  the  steam  to  escape.  One  of  the  disadvantages  attending 
the  use  of  the  spring-valve  is,  that  it  can  not  be  opened  to  let  the  steam  escape 
without  increasing  its  force,  so  that  the  steam,  when  escaping,  must  really 
have  a  greater  pressure  than  that  to  which  the  valve  has  been  previously  ad- 
justed. The  longer  the  lever  is,  the  greater  will  be  this  difference  of  pres- 
sure, inasmuch  as  a  given  elevation  of  the  pin  governing  the  rod  H  would 
cause  a  proportionally  greater  motion  in  that  end  of  the  lever  attached  to  the 
spring. 

The  second  safety-Valve  O  is  enclosed  in  a  case,  so  that  it  is  inaccessible, 
and  its  purpose  is  to  limit  the  power  of  the  engineer  to  increase  the  pressure 
of  steam  in  the  boiler.  This  valve  is  similar  in  construction  to  the  former,  but 
instead  of  being  held  down  by  a  lever,  is  pressed  upon  by  several  small  ellip- 
tical springs  placed  one  above  another  over  the  valve,  and  held  down  by  a 
screw  which  turns  in  a  frame  Y,  fixed  into  the  valve-seat.  By  this  screw'the 
pressure  on  the  valve  can  be  adjusted  to  any  required  degree  ;  and  if  the  open 
safety-valve  be  screwed  down  to  a  greater  pressure,  the  steam  will  begin  to 
escape  from  this  second  valve. 

Also  in  the  case  where  the  boiler  produces  surplus  steam  faster  than  its  es- 
cape can  be  effected  at  the  valve  N,  the  pressure  will  sometimes  be  increased 
until  the  valve  0  is  opened,  and  its  escape  will  take  place  from  both  valves. 

The  whole  weight  of  the  engine  bears  upon  those  parts  of  the  six  axles  R', 
fig.  69,  which  project  beyond  the  wheels.  Boxes  are  formed  in  which  these 
parts  of  the  axles  turn,  and  through  the  medium  of  which  the  weight  of  the  engine 
rests  upon  them.  Over  these  boxes  are  constructed  oil  or  grease  cups,  by  means 
of  which  the  axles  are  constantly  lubricated.  It  is  usual  to  lubricate  the  axles  of 
the  engine  itself  with  oil  :  the  axles  of  the  tender,  and  other  coaches  and  wag- 
ons, are  lubricated  with  a  mixture  of  oil  and  tallow.  In  the  middle  of  the  box 
in  which  the  axle  turns,  and  between  the  two  oil-cups,  is  cast  a  socket,  in 
which  the  end  of  the  spindle  on  which  the  spring  presses  rests.  The  springs  ; 
are  composed  of  a  number  of  steel  plates,  laid,  in  the  usual  manner,  one  above 
the  other,  increasing  in  length  upward.     In  the  engine  here  described,  the  ' 


THE  STEAM-ENGINE. 


561 


plates  forming  the  springs  of  the  driving-wheels  are  thirteen  in  number,  each 
of  which  is  four  inches  in  width,  and  jg-ths  of  an  inch  in  thickness.  The 
springs  upon  the  other  wheels  are  three  inches  in  width.  The  springs  of 
the  driving-wheels  are  below  the  axle,  while  those  of  the  smaller  wheels  are 
above  it. 

Buflers  D"  are  placed  behind  the  tender,  which  act  upon  a  spring  C,  fig.  70, 
to  break  the  collision,  when  the  wagons  or  carriages  strike  upon  the  tender, 
and  similar  buffers  are  attached  to  all  passenger-coaches.  Some  of  these  buf- 
fers are  constructed  with  a  system  of  springs  similar  to  C,  but  more  elastic, 
and  combined  in  greater  number  under  the  framing  of  the  carriage,  so  that  a 
considerable  play  is  allowed  to  them.  In  some  cases  the  rods  of  the  buffers 
are  made  to  act  upon  strong  spiral  springs  inserted  in  the  sides  of  the  framing 
of  the  carriage.  This  arrangement  gives  greater  play  to  the  buffers  ;  and  as 
every  coach  in  a  train  has  several  bufl^ers,  the  combined  effect  of  these  is 
such,  that  a  considerable  shock  given  to  either  end  of  the  train  may  be  ren- 
dered harmless  by  being  spent  upon  the  elasticity  of  these  several  systems  of 
springs. 

In  order  to  give  notice  of  the  approach  of  a  train,  a  steam-whistle  Z',  figs. 
67,  71,  is  placed  immediately  above  the  fire-box  at  the  back  of  the  engine. 
This  is  an  apparatus  composed  of  two  small  hemispheres  of  brass,  separated 
one  from  the  other  by  a  small  space.  Steam  is  made  to  pass  through  a  hol- 
low space  constructed  in  the  lower  hemisphere,  and  escapes  from  a  very  nar- 
row circular  opening  round  the  edge  of  that  hemisphere,  rushing  up  with  a 
force  proportional  to  its  pressure.  The  edge  of  the  upper  hemisphere  pre- 
sented downward  encounters  this  steam,  and  an  eff'ect  is  produced  similar 
to  the  action  of  air  in  organ-pipes.  A  shrill  whistle  is  produced,  which  can 
be  heard  at  a  very  considerable  distance,  and,  differing  from  all  ordinary  sounds, 
it  never  fails  to  give  timely  notice  of  the  approach  of  a  train. 

The  water-tank  1",  figs.  68,  70,  which  is  constructed  on  the  tender,  is 
formed  of  wrought-iron  plates  -|-  of  an  inch  thick,  riveted  at  the  corners  by 
angle-iron  already  described.  This  tank  is  9  feet  long,  6|-  feet  wide,  and  2| 
feet  deep.  The  top  is  covered  with  a  board  K",  and  a  raised  platform  N"  is 
constructed  behind,  divided  into  three  parts,  covered  with  leads,  which  open 
on  hinges.  -  The  middle  lid  covers  an  opening  to  the  tank  by  which  water  is 
let  in  :  the  lids  at  either  side  cover  boxes  in  which  are  contained  the  tools  ne- 
cessary to  be  carried  with  the  engine.  The  curved  pipe  P'',  fig.  68,  leading 
from  the  bottom  of  the  tank  to  the  pipe  Q",  is  of  copper.  The  pipe  Q",  con- 
necting the  latter  with  the  feed-pipe  K',  fig.  69,  is  sometimes  formed  of  leather 
or  India-rubber  cloth,  having  a  spiral  spring  on  the  inside  to  prevent  it  from 
collapsing.  It  is  necessary  that  this  pipe  Q''  should  have  a  power  of  yielding 
to  a  sufficient  degree  to  accommodate  itself  to  the  inequalities  of  motion  between 
the  engine  and  tender.  A  metal  pipe  is  sometimes  used,  supplied  with  a  double 
ball  and  socket,  and  a  telescopic  joint,  having  sufficient  play  to  allow  for  the 
lateral  and  longitudinal  inequalities  of  motion  of  the  engine  and  tender.  The 
weight  of  an  engine,  such  as  that  here  described,  supplied  with  its  proper 
quantity  of  water  and  fuel,  is  about  12  tons  :  the  tender,  when  empty,  weighs 
about  3-^-  tons  ;  and  when  filled  with  water  and  fuel  its  weight  is  7  tons.  The 
tank  contains  700  gallons  of  water,  and  the  tender  is  capable  of  carrying  about 
800  weight  of  coke.  This  supply  is  sufficient  for  a  trip  of  from  thirty  to  forty 
miles  witii  an  ordinary  load. 

It  is  not  usual  to  express  the  power  of  locomotive-engines  in  the  same  man- 
ner as  that  of  other  engines  by  the  term  horse-power.     Indeed,  until  the  actual 
amount  of  resistance  opposed  to  these  machines,  under  the  various  circum- 
stances in  which  they  are  worked,  shall  be  ascertained  with  some  degree  of 
VOL,.  II.— 36 


precision,  it  is  impossible  that  their  power  or  efficiency  can  be  estimated  with 
any  tolerable  degree  of  approximation.  The  quantity  of  water  evaporated,  and 
passed  in  steam  through  the  cylinders,  supplies  a  major  limit  to  the  power  ex- 
erted ;  but  even  this  necessary  element  lor  the  calculation  of  the  efficacy  of 
these  machines  has  not  been  ascertained  by  a  sufficiently  extensive  course  of 
observation  and  experiment.  Mr.  Stephenson  states  that  the  engine  which 
has  been  here  described  is  capable  of  evaporating  77  cubic  feet  of  water  per 
hour,  while  the  early  locomotive  could  only  evaporate  16  cubic  feet  per  hour. 
This  evaporation,  however,  is  inferior  to  that  which  I  have  ascertained  myself 
to  be  produced  by  engines  in  regular  operation  on  some  of  the  northern  rail- 
ways. In  an  experiment  made  in  July,  1839,  with  the  Hecla  engine,  I  found 
that  the  evaporation  in  a  trip  of  ninety-five  miles,  from  Liverpool  to  Birming- 
ham, was  at  the  rate  of  93-2  cubic  feet  per  hour,  and  in  returning  the  same  dis- 
tance it  was  at  the  rate  of  85-7  cubic  feet  per  hour,  giving  a  mean  of  89  cubic 
feet  per  hour  nearly.  The  Hecla  weighed  12  tons ;  and  its  dimensions  and 
proportions  corresponded  very  nearly  with  those  of  the  engine  above  de- 
scribed. 

In  a  course  of  experiments  which  I  made  upon  the  engines  then  in  use  on 
the  Grand  Junction  railway  in  the  autumn  of  1838,  I  found  that  the  ordinary 
evaporating  power  of  these  engines  varied  from  eighty  to  eighty-five  cubic  feet 
per  hour. 

Engines  of  much  greater  dimensions,  and  consequently  of  greater  evapora- 
ting power,  are  used  on  the  Great  Western  railway.  In  the  autumn  of  1838, 
experiments  were  made  upon  these  engines  by  Mr.  Nicholas  Wood  and  my- 
self, when  we  found  that  the  most  powerful  engine  on  that  line,  the  North 
Star,  drawing  a  load  of  11 0^  tons  gross,  engine  and  tender  inclusive,  at  30^ 
m.iles  an  hour,  evaporated  200  cubic  feet  of  water  per  hour.  The  same  engine 
drawing  a  load  of  1941  tons  at  18^  miles  an  hour  evaporated  141  cubic  feet 
per  hour,  and  when  drawing  45  tons  at  38|-  miles  an  hour  evaporated  198  cubic 
feet  of  water  per  hour. 

It  has  been  already  shown  that  a  cubic  foot  of  water  evaporated  per  hour 
produces  a  gross  amount  of  mechanical  force  very  little  less  than  two-horse 
power,  and  consequently  the  gross  amount  of  mechanical  power  evolved  in 
these  cases  by  the  evaporation  of  the  locomotive-boilers  will  be  very  nearly 
twice  as  many  horse-power  as  there  are  cubic  feet  of  water  evaporated  per 
hour.  Thus  the  evaporation  of  the  Hecla,  in  the  experiments  made  in  July, 
1839,  gave  a  gross  power  of  about  one  hundred  and  eighty  horses,  while  the 
evaporation  of  the  North  Star  gave  a  power  of  about  four  hundred  horses.  In 
stationary  engines  about  half  the  gross  power  evolved  in  the  evaporation  is  al- 
lowed for  waste,  friction,  and  other  sources  of  resistance  not  connected  with 
the  load.  What  quantity  should  be  allowed  for  this  in  locomotive-engines  is 
not  yet  ascertained,  and  therefore  it  is  impossible  to  state  what  proportion 
of  the  whole  evaporation  is  to  be  taken  as  representing  the  useful  horse- 
power. 

The  great  uniformity  of  resistance  produced  by  the  traction  of  carriages 
upon  a  railway  is  such  as  to  render  the  application  of  steam-power  to  that 
purpose  extremely  advantageous.  So  far  as  this  resistance  depends  on  me- 
chanical defects,  it  is  probably  rendered  as  uniform  as  is  practicable,  and  in 
proportion  to  the  quantity  of  load  carried  is  reduced  to  as  small  an  amount  as 
it  is  likely  to  attain  under  any  practicable  circumstances.  Until  a  recent  pe- 
riod this  resistance  was  ascribed  altogether,  or  nearly  so,  to  mechanical  causes. 
The  inequalities  of  the  road-surface,  the  friction  of  the  axles  of  the  wheels  in 
their  bearings,  and  the  various  sources  of  resistance  due  to  the  machinery  of 
the  engine,  being  the  principal  of  these  resistances,  were  for  the  most  part  in- 


THE  STEAM-ENGINE. 


56c 


dependent  of  the  speed  with  which  the  train  was  moved  ;  and  it  was  accord- 
ingly assumed  in  all  calculations  respecting  the  power  of  locomotive-enoines 
that  the  resistance  would  be  practically  the  same,  whatever  might  be  the  speed 
of  the  train.  It  had  been  well  understood  that,  so  far  as  the  atmosphere  might 
offer  resistance  to  the  moving  power,  this  would  be  dependent  on  the  speed, 
and  would  increase  in  a  very  high  ratio  with  the  speed  ;  but  it  was  consid- 
ered that  the  part  of  the  resistance  due  to  this  cause  formed  a  fraction  of  the 
whole  amount  so  insignificant  that  it  might  be  fairly  disregarded  in  practice, 
or  considered  as  a  part  of  the  actual  computed  resistance  taken  at  an  average 
speed. 

It  has  been,  until  a  late  period,  accordingly  assumed  that  the  total  amount 
of  resistance  to  railway-trains  which  the  locomotive-engines  have  had  to  over- 
come was  about  the  two  hundred  and  fiftieth  part  of  the  gross  weight  of  the 
load  drawn  :  some  engineers  estimated  it  at  a  two  hundred  and  twentieth  ;  olh'- 
ers  at  a  two  hundred  and  fiftieth  ;  others  at  a  three  hundred  and  thirtieth  part 
of  the  load  ;  and  the  two  hundred  and  fiftieth  part  of  the  gross  load  drawn  may 
perhaps  be  considered  as  a  mean  between  these  much-varying  estimates. 
What  the  experiments  were,  if  any,  on  which  these  rough  estimates  'were 
based,  has  never  appeared.  Each  engineer  formed  his  own  valuation  of  this 
effect,  but  none  produced  the  experimental  grounds  of  their  opinion.  It  has 
been  said  that  the  trains  run  down  the  engine,  or  that  the  drawing-chains  con- 
necting the  engine  slacken  in  descending  an  inclination  of  sixteen  feet  in  a 
mile,  or  ^^.  Numerous  experiments,  however,  made  by  myself,  as  well  as 
the  constant  experience  now  daily  obtained  on  railways,  show  that  this  is  a 
fallacious  opinion,  except  at  velocities  so  low  as  are  never  practised  on  rail- 
ways. 

In  the  autumn  of  1838  a  course  of  experiments  was  commenced  at  the  sug- 
gestion of  some  of  the  proprietors  of  the  Great  Western  railway  company,  with 
a  view  to  determine  various  points  connected  with  the  structure  and  the  work- 
ing of  railways.  A  part  of  these  experiments  were  intended  to  determine  the 
mean  amount  of  the  resisting  force  opposed  to  the  moving  power,  and  this  part 
was  conducted  by  me.  After  having  tried  various  expedients  for  determining 
the  mean  amount  of  resistance  to  the  moving  power,  I  found  that  no  method 
gave  satisfactory  results  except  one  founded  on  observing  the  motion  of  trains 
by  gravity  down  steep  inclined  planes.  When  a  train  of  wagons  or  coaches 
is  placed  upon  an  inclined  plane  so  steep  that  it  shall  descend  by  its  gravity 
without  any  moving  power,  its  motion,  when  it  proceeds  from  a  state  of  rest, 
will  be  gradually  accelerated,  and  if  the  resistance  to  that  motion  was,  as  it 
has  been  commonly  supposed  to  be,  uniform  and  independent  of  the  speed,  the 
descent  would  be  uniformly  accelerated  ;  in  other  words,  the  increase  of  speed 
would  be  proportional  to  the  time  of  the  motion.  Whatever  velocity  the  train 
would  gain  in  the  first  minute,  it  would  acquire  twice  that  velocity  at  the  end 
of  the  second  minute,  three  times  that  velocity  at  the  end  of  the  third  minute, 
and  so  on  ;  and  this  increase  of  velocity  would  continue  to  follow  the  same 
law.  however  extended  the  plane  might  be.  That  such  would  be  the  law 
which  the  descending  motion  of  a  train  would  follow  had  always  been  sup- 
posed, up  to  the  time  of  the  experiments  now  referred  to ;  and  it  was  even 
maintained  by  some  that  such  a  law  was  in  strict  conformity  with  experiments 
made  upon  railways  and  a'uly  reported.  The  first  experiments  instituted  by 
me  at  the  time  just  referred  to  afforded  a  complete  refutation  of  this  doctrine. 
It  was  found  that  the  acceleration  was  not  uniform,  but  that  with  every  in- 
crease of  speed  the  acceleration  was  lessened.  Thus  if  a  certain  speed  were 
gained  by  a  train  in  one  second  when  moving  at  five  miles  an  hour,  a  much 
less  speed  was  gained  in  one  second  when   moving  ten  miles   an  hour,  and  a 


564 


THE    STEAM-ENGINE. 


comparatively  small  speed  was  gained  in  the  same  time  when  moving  at  fif- 
teen miles  an  hour,  and  so  on.  In  fact,  the  augmentation  of  the  rate  of  accel- 
eration appeared  to  diminish  in  a  very  rapid  proportion  as  the  speed  increased  : 
this  suggested  to  me  the  probability  that  a  sufficiently  great  increase  of  speed 
would  destroy  all  acceleration,  and  that  the  train  would  at  length  move  at  a 
uniform  velocity.  In  effect,  since  the  moving  power  which  impels  a  train 
down  an  inclined  plane  of  uniform  inclination  is  that  fraction  of  the  gross 
weight  of  the  train  which  acts  in  the  direction  of  the  plane,  this  moving 
power  must  be  necessarily  invariable  ;  and  as  any  acceleration  which  is  pro- 
duced must  arise  from  the  excess  of  this  moving  power  over  the  resistance 
opposed  to  the  motion  of  the  train,  from  whatever  causes  that  resistance  may 
arise,  whenever  acceleration  ceases,  the  moving  force  must  necessarily  be 
equal  to  the  resistance  ;  and  therefore,  when  a  train  descends  an  inclined 
plane  with  a  uniform  velocity,  the  gross  resistance  to  the  motion  of  the  train 
must  be  equal  to  the  gross  weight  of  the  train  resolved  in  the  direction  of 
the  plane  ;  or,  in  other  words,  it  must  be  equal  to  that  fraction  of  the  whole 
weight  of  the  train  which  is  expressed  by  the  inclination  of  the  plane.  Thus 
if  it  be  supposed  that  the  plane  falls  at  the  rate  of  one  foot  in  one  hundred, 
then  the  force  impelling  the  train  downward  will  be  equal  to  the  hundredth 
part  of  the  weight  of  the  train.  So  long  as  the  resistance  to  the  motion  of 
the  train  continues  to  be  less  than  the  hundredth  part  of  its  weight,  so  long 
will  the  motion  of  the  train  be  accelerated  ;  and  the  more  the  hundredth 
part  of  the  weight  exceeds  the  resistance,  the  more  rapid  will  the  accelera- 
tion be  ;  and  the  less  the  hundredth  part  of  the  weight  exceeds  the  resist- 
ance, the  less  rapid  will  the  acceleration  be.  If  it  be  true  that  the  amount 
•  of  resistance  increases  with  the  increase  of  speed,  then  a  speed  may  at 
length  be  attained  so  great  that  the  amount  of  resistance  to  the  motion  of 
the  train  will  be  equal  to  the  hundredth  part  of  the  weight.  When  that  hap- 
pens, the  moving  power  of  a  hundredth  part  of  the  weight  of  the  train  be- 
ing exactly  equal  to  the  resistance  to  the  motion,  there  is  no  excess  of 
power  to  produce  acceleration,  and  therefore  the  motion  of  the  train  will  be 
uniform. 

Founded  on  these  principles,  a  vast  number  of  experiments  were  made  on 
planes  of  different  inclinations,  and  with  loads  of  various  magnitudes  ;  and  it 
was  found,  in  general,  that  when  a  train  descended  an  inclined  plane,  the 
rate  of  acceleration  gradually  diminished,  and  at  length  became  uniform  ; 
that  the  uniform  speed  thus  attained  depended  on  the  weight,  form,  and 
mao-nitude  of  the  train,  and  the  inclination  of  the  plane  ;  that  the  same  train 
f  on  difierent  inclined  planes  attained  diflerent  uniform  speeds — on  the  steeper 
planes  a  greater  speed  being  attained.  From  such  experiments  it  followed, 
contrary  to  all  that  had  been  previously  supposed,  that  the  amount  of  re- 
sistance to  railway-trains  had  a  dependence  on  the  speed  ;  that  this  de- 
pendence was  of  great  practical  importance,  the  resistance  being  subject  to 
very  considerable  variation  at  different  speeds,  and  that  this  source  of  re- 
sistance arises  from  the  atmosphere  which  the  train  encounters.  This  was 
rendered  obvious  by  the  different  amount  of  resistance  to  the  motion  of  a 
train  of  coaches  and  to  that  of  a  train  of  low  wagons  of  equal  weight. 

The  series  of  experiments  which  have  established  these  general  conclusions 
have  not  yet  been  sufficiently  extended  and  varied  to  supply  a  correct  practi- 
cal estimate  of  the  limit  which  it  would  be  most  advantageous  to  impose  upon  the 
gradients  of  railways  ;  but  it  is  certain  that  railways  may  be  laid  down,  without 
practical  disadvantage,  with  gradients  considerably  steeper  than  those  to  which 
it  has  been  hitherto  the  practice  to  recommend  as  a  limit. 

The  principle  of  compensation  by  varied  speed  being  admitted,  it  will  follow 


THE  STEAM-ENGINE. 


that  the  time- of  transit  between  terminus  and  terminus  of  a  line  of  railway  laid 
down  with  gradients,  varying  from  twenty  to  thirty  feet  a  mile,  will  be  practi- 
cally the  same  as  it  would  be  on  a  line  of  the  same  length  constructed  upoa  a 
dead  level ;  and  not  only  will  the  time  of  transport  be  equal,  but  the  quantity 
of  moving  power  expended  will  not  be  materially  different.  The  difference 
between  the  circumstances  of  the  transport  in  the  two  cases  will  be  merely 
that,  on  the  undulating  line,  a  varying  velocity  will  be  imparted  to  the  tnin  and 
a  varying  resistance  opposed  to  the  moved  power ;  while  on  the  level  line  the 
train  would  be  moved  at  a  uniform  speed,  and  the  engine  worked  aganst  a 
uniform  resistance.  These  conclusions  have  been  abundantly  confirmed  by 
the  experiments  made  in  last  July  with  the  Hecla  engine  above  referred  to. 
The  line  of  railway  between  Liverpool  and  Birmingham  on  which  the  eiperi- 
ment  was  made  extended  over  a  distance  of  ninety-five  miles,  and  the  gra(iients 
on  which  the  effects  were  observed  varied  from  a  level  to  thirty  feet  per  snile, 
a  great  portion  of  the  line  being  a  dead  level.  The  following  table  shows  the 
uniform  speed  with  which  the  train  ascended  and  descended  the  several 
gradients,  and  also  the  mean  of  the  ascent  and  descent  in  each  case,  as  veil 
as  the  speed  upon  the  level  parts  of  the  line  : — 


Speed. 

Ascending. 

Descending. 

Mean. 

One  in 

177 
265 
330 
400 
532 
590 
650 

Miles  per  hour. 

22-25 
24-87 
25-26 
26-87 
27-35 
27-37 
29-03 

Miles  per  hour. 
41-32 
39-13 
37-07 
36-75 
34-30 
33-16 
32-58 

31-78 
32-00 
31-16 
31-81 
30-82 
30-21 
30-80 

Level 

30-93 

From  this  table  it  is  apparent  that  the  gradients  do  possess  the  compensating 
power  with  respect  to  speed  already  mentioned.  The  discrepancies  existing 
among  the  mean  values  of  the  speed  are  only  what  may  be  fairly  ascribed  to 
casual  variations  in  the  moving  power.  The  experiment  was  made  under 
favorable  circumstances  :  little  disturbance  was  produced  from  the  atmosphere; 
the  day  was  quite  calm.  In  the  same  experiment  it  was  found  that  the  water 
evaporated  varied  very  nearly  in  proportion  to  the  varying  resistance,  and  the 
amount  of  that  evaporation  may  be  taken  as  affording  an  approximation  to  the 
mean  amount  of  resistance.  Taking  the  trip  to  and  from  Birmingham  over 
the  distance  of  190  miles,  the  mean  evaporation  per  mile  was  3-36  cubic  feet 
of  water.  The  volume  of  steam  produced  by  this  quantity  of  water  will  be 
determined  approximately  by  calculating  the  number  of  revolutions  of  the 
driving  wheels  necessary  to  move  the  engine  one  mile.  The  driving  wheels 
being  5  feet  in  diameter,  their  circumference  was  15-7  feet,  and  consequently 
in  passing  over  a  mile  they  would  have  revolved  336-3  times.  Since  each 
revolution  consumes  four  cylinders  full  of  steam,  the  quantity  of  steam  supplied 
by  the  boiler  to  the  cylinders  per  mile  will  be  found  by  multiplying  the  con- 
tents of  the  cylinder  by  four  times  336-3,  or  1,345-2. 

The  cylinders  of  the  Hecla  were  121-  inches  diameter,  and  18  inches  in 
length,  and  consequently  their  contents  were  1-28  cubic  feet  for  each  cylin- 
der :  this  being  multiplied  by  1,345-2  gives  1,721-86  or  1,722  cubic  feet  of 
steam  per  mile.  It  appears,  therefore,  that  supposing  the  priming  either 
nothing  or  insignificant,  which  was  considered  to  be  the  case  in  these  experi- 


J66 


THE  STEAM-ENGINE. 


mepts,  3-36  cubic  feet  of  water  produced  1,722  cubic  feet  of  steam,  of  the  $ 
deiisitv  worked  in  the  cylinders.  The  ratio,  therefore,  of  the  volume  of  this 
steam  to  that  of  the  water  producing  it,  was  1,722  to  3-36,  or  512-5  to  1. 
Tlie  pressure  of  steam  of  this  density  would  be  54-5  pounds  per  square  inch. 
Such,  therefore,  was  the  limit  of  the  average  total  pressure  of  the  steam  in  the 
cylinders.  In  this  experiment  the  safety-valve  of  the  boiler  was  screwed  2 
down  to  60  pounds  per  square  inch  above  the  atmospheric  pressure,  which  > 
was  therefore  the  major  limit  of  the  pressure  of  steam  in  the  boiler  ;  but  as 
the  actual  pressure  in  the  boiler  must  have  been  less  than  this  amount,  the 
difference  between  the  pressure  in  the  cylinder  and  boiler  could  not  be  ascer- 
tained. This  difference,  however,  would  produce  no  effect  on  the  moving 
power  of  the  steam,  since  the  pressure  of  steam  in  the  cylinders  obtained  by 
the  shove  calculation  is  quite  independent  of  the  pressure  in  the  boiler,  or  of 
any  source  of  error  except  what  might  arise  from  priming.  The  pressure  of 
54\')  pounds  per  square  inch,  calculated  above,  being  the  total  pressure  of  the 
stetm  on  the  pistons,  let  14'5  pounds  be  deducted  from  it,  to  represent  the 
atmospheric  pressure  against  which  the  piston  must  act,  and  the  remaining  40 
pounds  per  square  inch  will  represent  the  whole  available  force  drawing  the 
train  and  overcoming  all  the  resistances  arising  from  the  machinery  of  the 
ergine,  including  that  of  the  blast-pipe.  The  magnitude  of  a  12^  inch  piston 
being  122-7  square  inches,  the  total  area  of  the  two  pistons  would  be  245-2 
square  inches,  and  the  pressure  upon  each  of  40  pounds  per  inch  would  give 
a  total  force  of  9,816  on  the  two  pistons.  Since  this  force  must  act  through  a 
space  of  three  feet,  while  the  train  is  impelled  through  a  space  of  15-7  feet, 
it  must  be  reduced  in  the  proportion  3  to  15-7,  to  obtain  its  effect  at  the  point 
of  contact  of  the  wheels  upon  the  rails  :  this  will  give  1,875  pounds  as  the 
total  fbrce  exerted  in  the  direction  of  the  motion  of  the  train.  The  gross 
weight  of  the  train  being  80  tons,  including  the  engine  and  tender,  this  would 
give  a  gross  moving  force  along  the  road  of  about  2 34  pounds  per  ton  of  the 
gross  load,  this  force  being  understood  to  include  all  the  resistances  due  to  the 
engine.  This  resistance  corresponds  to  the  gravitation  of  a  plane  rising  at  the 
rate  of  ^-^,  and  therefore  it  appears  that  such  would  be  the  inclination  of  the 
plane  by  the  gravitation  of  which  the  gross  resistance  would  be  doubled,  in- 
stead of  such  inclination  being  about  -^^q,  as  has  been  hitherto  supposed. 

Since  the  remarkable  and  unexpected  results  of  this  series  of  experiments 
became  known  various  circumstances  were  brought  to  light,  which  were  be- 
fore unnoticed,  and  which  abundantly  confirm  them.  Among  these  may  be 
mentioned  the  fact,  that  in  descending  the  Madeley  plane,  on  the  grand  junc- 
tion railway,  which  falls  for  above  three  miles  at  the  rate  of  twenty-nine  feet 
a  mile,  the  steam  can  never  be  entirely  cut  off.  But,  on  the  other  hand,  to 
maintain  the  necessary  speed  in  descending,  the  power  of  the  engine  is  always 
necessary.  As  this  plane  greatly  exceeds  that  which  would  be  sufficient  to 
cause  the  free  motion  of  the  train  down  it,  the  power  of  the  engine  expended 
in  descending  it,  besides  all  that  part  of  the  gravitating  power  of  the  plane  which 
exceeds  the  resistance  due  to  friction  and  other  mechanical  causes  must  be 
worked  against  the  atmosphere. 

This  estimate  of  the  resistance  is  also  in  conformity  with  the  results  of  a 
variety  of  experiments  made  by  me  with  trains  of  different  magnitudes  down 
inclined  planes  of  various  inclinations. 

In  laying  out  a  line  of  railway  the  disposition  of  the  gradients  should  be 
such  as  to  preserve  among  them  as  uniform  a  character  as  is  practicable,  for 
the  weight  and  power  of  the  engine  must  necessarily  be  regulated  by  the 
general  steepness  of  the  gradients.  Thus  if  upon  a  railway  which  is  generally 
level,  like  that  between  Liverpool  and  Manchester,  one  or  two  inclined  planes 


THE  STEAM-ENGINE. 


S  of  a  very  steep  character  occur,  as  happens  upon  that  line,  then  the  engine 
(  which  is  constructed  to  work  upon  the  general  gradients  of  the  road  is  unfit  to 
5  draw  the   same  load   up   those  inclinations  which  form  an  exception  to  the 

<  general  character  of  the  gradients.  In  such  cases  some  extraordinary  means 
;  must  generally  be  provided  for  surmounting  those  exceptionable  inclinations. 

<  Several  expedients  have  been  proposed  for  this  purpose,  among  which  the 
)  following  may  be  mentioned  : — 

<  1.  Upon  arriving  at  the  foot  of  the  plane  the  load  is  divided,  and  the  engine 
5  carries  it  up  in  several  successive  trips,  descending  the  plane  unloaded  after 
i  each  trip.  The  objection  to  this  method  is  the  delay  which  it  occasions — a 
)  circumstance  which  is  incompatible  with  a  large  transport  of  passengers. 
I  From  what  has  been  stated,  it  would  be  necessary,  when  the  engine  is  iully 

loaded  on  a  level,  to  divide  its  load  into  two  or  more  parts,  to  be  successively 
carried  up  when  the  incline  rises  52  feet  per  mile.  This  method  has  been 
practised  in  the  transport  of  merchandise  occasionally,  when  heavy  loads 
were  carried  on  the  Liverpool  and  Manchester  line,  upon  the  Rainhill  incline. 

2.  A  subsidiary  or  assistant'  locomotive  engine  may  be  kept  in  constant 
readiness  at  the  foot  of  each  incline,  for  the  purpose  of  aiding  the  different 
trains,  as  they  arrive,  in  ascending.  The  objection  to  this  method  is  the  cost 
of  keeping  such  an  engine  with  its  boiler  continually  prepared,  and  its  steam 
up.  It  is  necessary  to  keep  its  fire  continually  lighted,  whether  employed  or 
not ;  otherwise,  when  the  train  would  arrive  at  the  foot  of  the  incline,  it  should 
wait  until  the  subsidiary  engine  was  prepared  for  work.  In  cases  where 
trains  would  start  and  arrive  at  stated  times,  this  objection,  however,  would 
have  less  force.  This  method  is  at  present  generally  adopted  on  the  Liverpool 
and  Manchester  line. 

3.  A  fixed  steam-engine  may  be  erected  on  the  crest  of  the  incline,  so  as  to 
communicate  by  ropes  with  the  train  at  the  foot.  Such  an  engine  would  be 
capable  of  drawing  up  one  or  two  trains  together,  with  their  locomotives,  ac- 
cording as  they  would  arrive,  and  no  delay  need  be  occasioned.  This  method 
requires  that  the  fixed  engine  should  be  kept  constantly  prepared  for  work,  and 
the  steam  continually  up  in  the  boiler. 

4.  In  working  on  the  level,  the  communication  between  the  boiler  and  the 
cylinder  in  the  locomotives  may  be  so  restrained  by  partially  closing  the 
throttle-valve,  as  to  cause  the  pressure  upon  the  piston  to  be  less  in  a  consider- 
able degree  than  the  pressure  of  steam  in  the  boiler.  If  under  such  circum- 
stances a  sufficient  pressure  upon  the  piston  can  be  obtained  to  draw  the  load 
on  the  level,  the  throttle-valve  may  be  opened  on  approaching  the  inclined 
plane,  so  as  to  throw  on  the  piston  a  pressure  increased  in  the  same  proportion 
as  the  previous  pressure  in  the  boiler  was  greater  than  that  upon  the  piston. 
If  the  fire  be  sufficiently  active  to  keep  up  the  supply  of  steam  in  this  manner 
during  the  ascent,  and  if  the  rise  be  not  greater  in  proportion  than  the  power 
thus  obtained,  the  locomotive  will  draw  the  load  up  the  incline  without  further 
assistance.  It  is,  however,  to  be  observed,  that  in  this  case  the  load  upon  the 
engine  must  be  less  than  the  amount  which  the  adhesion  of  its  working  wheels 
with  the  railroad  is  capable  of  drawing  ;  for  this  adhesion  must  be  adequate 
to  the  traction  of  the  same  load  up  the  incline,  otherwise,  whatever  increase 
of  power  might  be  obtained  by  opening  the  throttle-valve,  the  drawing  wheels 
would  revolve  without  causing  the  load  to  advance.  This  method  has  been 
generally  practised  upon  the  Liverpool  and  Manchester  line  in  the  transport 
of  passengers  ;  and,  indeed,  it  is  the  only  method  yet  discovered  which  is 
consistent  with  the  expedition  necessary  for  that  species  of  traffic. 

In  the  practice  of  this  method  considerable  aid  may  be  derived  also  by 
suspending  the  supply  of  feeding  water  to  the  boiler  during  the  ascent.     It  will 


568 


THE  STEAM-ENGINE. 


be  recollected  that  a  reservoir  of  cold  water  is  placed  in  the  tender  which 
follows  the  engine,  and  that  the  water  is  driven  from  this  reservoir  into  the 
boiler  by  a  forcing  pump,  which  is  worked  by  the  engine  itself.  This  pump 
is  so  constructed  that  it  will  supply  as  much  cold  water  as  is  equal  to  the 
evaporation,  so  as  to  maintain  constantly  the  same  quantity  of  water  in  the 
boiler.  But  it  is  evident,  on  the  other  hand,  that  the  supply  of  this  water  has 
a  tendency  to  check  the  rate  of  evaporation,  since  in  being  raised  to  the  tem- 
perature of  the  water  with  which  it  mixes  it  must  absorb  a  considerable  portion 
of  the  heat  supplied  by  the  fire.  With  a  view  to  accelerate  the  production  of 
steam,  therefore,  in  ascending  the  inclines,  the  engine  man  may  suspend  the 
action  of  the  forcing  pump,  and  thereby  stop  the  supply  of  cold  water  to  the 
boiler  ;  the  evaporation  will  go  on  with  increased  rapidity,  and  the-exhaustion 
of  water  produced  by  it  will  be  repaid  by  the  forcing  pump  on  the  next  level, 
or  still  more  effectually  on  the  next  descending  incline.  Indeed  the  feeding 
pump  may  be  made  to  act  in  descending  an  incline,  if  necessary,  when  the 
action  of  the  engine  itself  is  suspended,  and  when  the  train  descends  by  its 
own  gravity,  in  which  case  it  will  perform  tRe  part  of  a  brake  upon  the  de- 
scending train. 

5.  The  mechanical  connexion  between  the  piston  of  the  cylinder  and  the 
points  of  contact  of  the  working  wheels  with  the  road  may  be  so  altered,  upon 
arriving  at  the  incline,  as  to  give  the  piston  a  greater  power  over  the  working 
wheels.  This  may  be  done  in  an  infinite  variety  of  ways,  but  hitherto  no 
method  has  been  suggested  sufficiently  simple  to  be  applicable  in  practice  ; 
and  even  were  any  means  suggested  which  would  accomplish  this,  unless  the 
intensity  of  the  impelling  power  were  at  the  same  time  increased,  it  would 
necessarily  follow  that  the  speed  of  the  motion  would  be  diminished  in  exactly 
the  same  proportion  as  the  power  of  the  piston  over  the  working  wheels  would 
be  increased.  Thus,  on  the  inclined  plane,  which  rises  fifty-five  feet  per  mile, 
upon  the  Liverpool  line,  the  speed  would  be  diminished  to  nearly  one  fourth  of 
its  amount  upon  the  level. 


THE  END. 


GREAT  EOOK  FOR  FARMERS! 

LET  EVERY   FARMER   IN   THE   UNITED    STATES   HAVE   A   COPY! 

Let  every  Farmer  ia  the  United  States  subscribe  for  a  Copy  for  his  Son,     It  may  prove  of 

more  vahie  to  him  than  a  Horse,  or  even  a  Farm ! 

THE  FARMERS'  LIBRARY 

AND 

MONTHLY  JOURNAL  OF  AOEICULTURE. 

JOHN  S.  SKINNER,  Editor. 


Each  number  consists  of  two  distinct  parts, 
viz  : 

I.  The  Farmers'  Library,  in  which  are 
published  continuously  the  best  Standard 
IVorka  on  Agriculture,  embracing  those  which, 
by  their  cost  or  the  language  in  which  they  are 
written,  would  otherwise  seem  beyond  the  reach 
of  nearly  all  American  Farmers.  In  this  way 
we  give  for  two  or  three  dollars  the  choicest 
European  treatises  and  researches  in  Agricul- 
ture, costing  ten  times  as  much  in  the  oi'iginal 
editions,  not  easily  obtained  at  any  piice,  and 
virtually  out  of  the  reach  of  men  who  live  by 
foUov/ing  the  plow.  The  -works  published  in 
the  Library  will  fonn  a  complete  series,  explor- 
and  exhibiting  the  whole  field  of  iSTatural 
Science,  and  developing  the  rich  treasures 
which  Chemistry,  Geology,  and  Mechanics 
have  yielded  and  may  yield  to  lighten  the  la- 
bors and  swell  the  harvests  of  the  intelligent 
husbandman. 

IL  The  Monthly  Journal  of  Agricul- 
ture will  likewise  contain  about  50  pages  per 
month,  and  will  comprise,  1.  Foi-eign  :  Selec- 
tions fi-om  the  higher  class  of  British,  French 
and  German  periodicals  devoted  to  Agriculture, 
with  extracts  from  new  books  which  may  not 
be  published  in  the  Librarj',  &c.  &c.  2.  Ameri- 
can :  Editorials,  communicated  and  selected 
accounts  of  experiments,  improved  processes, 
discoveries  in  Agriculture,  new  implements, 
ifcc.  &c.  In  this  department  alone  will  ours  re- 
semble any  American  work  ever  yet  published. 
It  can  hardly  be  necessary  to  add  that  no  Politi- 
Economic,  or  other  controvei'ted  doctrine, 
will  be  inculcated  through  this  magazine. 

Each  number  of  the  Library  is  illustrated  by 
numerous  Engravings,  printed  on  type  obtained 
expressly  for  this  work,  and  on  good  paper — 
the  whole  got  up  as  such  a  work  should  be. 

This  Monthly,  which  is  by  far  the  amplest  and 
most  comprehensive  Agricultural  periodical  ever  es- 
tablished in  America,  was  commenced  in  the  month 
of  July,  1845,  and  before  the  close  of  the  first  year 
among  its  subscribers  were  embraced  many  of  the 
most  intelligent  fanners,  professional  men,  and  re- 
tired gentlemen  in  every  City  and  State  in  the  Union 
The  reprint  of  standard  works  and  the  variety,  ele- 
gance and  costliness  of  the  Engi-avings  will  always, 
render  this  one  of  the  most  useful  and  interesting, 
and.  in  view  of  the  amount  of  reading  matter,  the 
cheapest  Farming  ;]criodical   in   this    or  any  other 


countiy.  The  beautiful  work  of  Petzholdt  on  Ag- 
KicULTURiVL  Chemistky  was  published  complete  in 
the  first  rwo  numbers  of  the  Fakmers'  Library  ;  and 
the  great  work  of  Von  Thaek  on  the  Principles 

OF  AgRICUI  t\  1  E,  TKAJSSLATED  BY  Wm.  ShAW  AND 
CUTHBERT  Jijl^NSON,  WITH  A  MejiOIR  OF  THE  AU- 
THOR, &.C.  was  commenced  in  the  number  of  the  Li- 
brary for  September,  1845,  and  will  be  completed 
entire,  witliout  abridgment,  in  the  June  number  for 
1846.  This  justly  celebrated  work  is  alone  worth  the 
full  subscription  price  of  the  Farmers' Library,  and 
yet  it  is  not  more  than  one-third  of  what  each  sub- 
scriber to  the  Work  receives  for  his  subscription 
money.  This  work  of  Von  Thaer  was  originally 
written  and  published  in  the  Gei-man  language,  ti-ans- 
Isted  and  published  in  the  French  and  afterward  in 
the  English  language.  It  is  pronounced  by  compe- 
tent judges  to  be  the  most  finished  Agiicultuj-al  Buok 
which  has  ever  been  written.  The  London  edition 
is  printed  in  two  octavo  volumes,  and  is  sold  at  about 
S8  per  copy. 

Von  Thaer  was  educated  for  a  Physician,  the  prac- 
tice of  which  he  relinquished  for  the  more  quiet  and 
philosophical  pursuits  of  Agi-iculmre.  Soon  after  he 
commenced  farming  he  inti'oduced  such  decided  ■ 
improvements  upon  his  farm  that  his  fame  was  soon 
known  from  one  end  of  Europe  to  the  other.  The  most 
celebrated  farmers  of  England,  France,  Denmark, 
Germany,  &o.  courted  his  friendship,  and  his  writings 
were  everywhere  sought  and  studied. 

The  following  subjects  are  discussed  in  the  work 
of  Von  Thaer,  and  the  manner  of  treating  each  sub- 
ject is  original,  philosophical  and  practical.  , 

Section  L  The  Fundahientai,  Principles  :— A 
Sketch  of  Systematic  Agriculture;  The  Bases  of  tha 
Science  of  Agriculture  ■"  The  Bases  of  Enterprise ; 
Capital ;  The  FaiTn,  and  the  Manner  of  taking  Pos- 
session of  it ;  Leasehold  Estates;  Hereditary  Leases. 

Sec  II.  The  Economy,  Organization  and  Di- 
rection OF  AN  Agricultural  Enterprise  ; — La- 
borin General;  Dnuuiht  Labor  ;  Manual  Labor  ;  The 
Proper  Methocl  of  kcejjing  the  Journals,  Registers,  and 
Other  Books  connected  wiih  an  Agincultiiral  Under- 
takins; ;  Proportion  of  Manure  to  the  Quantity  of  h  od- 
der and  tlie  nnHihrrof  Cottle;  The  various  Systems 
of  Cultivation.  Class  1-The  Cultivation  of  Corn- 
Alternate  rultivation— Altrniate  Rotations  with  Pas- 
rurago— On  the  Succession  of  Crops — Alternate  Cul- 
tivation, accompanied  by  a  suitable  Succession  of 
Crops  and  Pasturage— Alternate  Cultivation,  with 
Stall-Feeding  of  the  Cattle— Four  Crop  Divisions- 
Five  Crop  Divisions— Six  Crop  Divisions— Seven 
Crop  Divisions— Eiirht  Crop  Divisions— Nine  Crop 
Divisions— Ten  Crop  Divisions — Eleven  Crop  Divis- 
ions—Twelve Crop  Divi-ions- Twelve  Crop  Divis- 
ions—The Transition  from  one  Rotation  to  anther. 

Sec.  Iir.  Agronomy;  or  a  Treatise  on  the 
Constituent  Parts  and  Physical  Properties  of 
the  Soil,  and  the  Best  Method  of  Acquiring 
a  Knowledge  of  the  Different  Earths,  and 
AscERT.iiNiNG  THEIR  VALUE  :— Silica  ;  Alumina; 
Clay  ;  Lime  ;  Gypsum,  or  Sulphate  of  Lime  ;  Marl ; 


Magnesia ;  Iron  ;  Humus  ;  Peat ;  The  Dift>rpnt  Spe- 
cies of  Earths,  their  Value,  Employment,  and  Proper- 
ties, in  their  Relations  to  the  Constituent  Parts  of  the 
Soil. 

Sec.  IV.  Agkicultuke:— Part  I — On  Manxiring 
and  Ameliorating  the  Soil :  Vegetable  Manures — 
Mineral  Manures.  Part  2— On  the  Tillage  of  the 
Soil,  or  its  Mechanical  Amelioration ;  Agricultural 
Implements;  On  Plowing;  On  Clearing  l.antl ; 
Hedges,  Fences  and  Enclosures;  On  the  Draining  of 
Land;  On  the  Draining  of  various  kinds  of  Marshes  ; 
Imgation  ;  On  Earthing  and  Warping;  On  the  Man- 
agement of  Meadow  Land  ;  The  Hay  Hai-vest ;  On 
the  various  kinds  of  Pastures. 

Sec.  V.  On  the  Reproduction  of  Animal  and 
Vegetable  Substances  : — Vegetable  Reproduc- 
tion ;  Wheat :  Spring  Wheat — Spelt — One-grained 
Wheat  (Einhorn  of  the  Gennans.) — Smut,  or  Caries 
in  Wheat  (Brand)  ;—'Ryp.\  Barley:  Common,  or 
Small  Quadrangular  Barley Two-rowed,  Long- 
eared,  or  Large  Fiat  Barley— Siberian,  or  Quadran- 
gular Naked  Barley — Naked  Flat  Barley — Six-rowed, 
or  Winter  Barley; — Oats  (Aveiia  Sativa)  ;  Millet 
(Panicum)  ;  On  the  Cultivation  of  Grain  in  Rows,  or 
with  the  Hoi-se-hoe  ;  Leguminous  Crops  ;  The  Pea ; 
The  Lentil ;  Kidney-Beans,  (Haricots) ;  Beans  (  Vicia 
Fabia)  ;  Vetches:  Common  Vetch  (Vetch,  Sativa); 
Buckwheat  CPolygonwrn  Fagopyriim,)  ;  Meslin — Mix- 


ture of  which  has  been  proposed  for  the  sake  of  their 
Thread :  Syrian  Swallow  Wort,  or  Virginian  Silk 
(Aschpias  Syriaca) — Common  Nettle  (  XJrtica  Dioi- 
ca) — Fullers"  Teasle  (Depsnnis  Fullnrum)  ;— Color- 
ing-Plants: Dyers'  Madder  (Riilna  Tiiiclori/viy-'Dy- 
ers'  Woad  (Isatis  Tii/ctorin)—'Dyers'  We].d(,Riseda 
Lutenla) — Bastard  SatiVon  (Cartliamus  Thictoriiisi); — 
The  Hop;  Tobacco;  Chiccoiy;  Carraway  (Carum 
Carui) ;  Common  Fennel  (Fa:inrAihivi  Vvlgare)  ; 
Anise  (Pi.nipindle  Anisum);  Culture  of  Fodder- 
Plants:  The  Potato — The  Field-Beet — The  Turaip 
(Brassica  i?a;)o)— Turnips  which  will  not  bear  Trans- 
planting—Turnips so  properly  called— Turnips  ad- 
mitting of  Transplantation— The  Turnip  Cabbasie — 
(Common  P^ed  and  White  Cabbage  (Brassica  Olira- 
cea  ;  var.  Capiinta) — Carrots? — The  Pai'snip — Maize, 
or  Indian  Corn  (Tea  Mais)  ,■— Herbage  Plants  :  Com- 
mon Purple  Clover  (Trrfnliiim  Pratnise,  var.  Sati- 
t'JHfl,)— White,  or  Dutch  Clover  {Trifulivm  Brpevs) — 
StrawbeTry  Trefoil  (Trifolium.  l''rng if trimi)— Lu- 
cerne (Medicagi)  Saliva) — Sainfoin  (Hidisarvvi  Oiio- 
hrychis)—Ye\\ovi  Sickle  Medick  (Mtdicago  Falcaia) 
—  Black  Medick  or  Nonsuch  {Medicago  Liipnliiia)— 
Corn  Spurry  (Spcrgnla  Arvoisis)— The  Tall-gi-ow- 
ing  Grasses — Ray  Grass  ( Solivm ^Pereniiit) — Common 
Oatlike  Grass   (AvKva  ElaiJor )—Tn\l  Fescue   Grass 


(Festuca  Elatior)  —  Coc]i's-fuot  Grass  (Dar/yUs  Glom- 
erata) — Dog-tail  Grass  (Cywosi/ruf  Cristatjis) — Com- 
tures  of  Ditterent  Kinds  of  Grain  ;  Culture  of  Hoed  '  mon  Cat's-tail  or  Timothy  Grass  (Phelevm  Frateiise) 
or  Weeded  Crops;  Vegetables  for  the  Mai'kct ;  Oil-  I — Woolly  Soft  Grass  (HoIcus  Sanatiis) — Meadow 
Plants;  Colza  and  Rape  (Autumnal  Varieties) —  •  Foxtail  Grass  (Alopecurus  Pratensis) — :Meadow 
Spring  Colza,  or  Spring  Rape — Mustard— Oily  Rad-     Grass  (P  ,a). 

ish  (Rapkanus  Chiiie.nsis  OU'ferus) — Cultivated  Gold  \  Sec.  VI.  The  Economy  of  Live  Stock  : — Horned 
of  Pleasure  (Myagrnm  Sativum) — Common  Poppy  ;  Cattle  ;  Breeding  Cattle — Feeding  of  Cattle  ;— The 
(Papaver  Sonniifervm)  ;■ — Thread  Plants:  Flax —  -Daily:  Cheese  Makine; — Fattening  of  Horned  Cat- 
Hemp  ('Ca?i?(ai?s  Sativa); — Other  Plants,  the  Cul-     tie;  Swine;  Sheep;  Horses. 

lE^  Tlie  subscription  price  to  the  Farmers'  Library  and  Monthly  Journal  of  Agricul- 
ture, containing  2  vols,  of  600  pages  each,  with  numerous  Engravings,  is  Five  Dollars  a  year. 
Where  five  persons  club  together  and  send  us  $20,  -we  send  five  copies.  Paj'm'^sit  is  invariably 
required  in  advance.  Money  may  be  remitted  through  the  Mail  at  our  risk.  The  Bank  notes  of 
any  State  of  specie  paying  Banks,  are  received  at  par.     Address 

GREELEY  &  McELRATH,  Publisheiis,  Tribune  Buildings,  New- York. 


LECTURES    TO   FARMERS 

ON 

AGRICULTUEAL  CHEMISTRY. 

By  ALEXANDER  PETZHOLDT. 

Thk.  taste  for  Scientific  Agriculture  in  the  United  States  has  created  a  demand  for  the  vei-y  in- 
foi-mation  wbi^h  these  Lectures  supply.  "The  motive,"  says  the  author,  "  which  has  ind'".>-e(l  me 
to  prepare  such  a  '"'ourse  of  Lectures,  i.«  the  complaint  I  have  heard  from  many  of  you,  tliat.  be- 
ing unacquainted  with  the  elements  of  Chemistrj',  you  have  found  it  difficult  to  understand  the 
questions  which  are  at  the  present  moment  so  warmly  discirssed,  respecting  the  theory  and  ]irac- 
tice  of  Agriculture."  This  work  being  less  scientific  and  technical  in  its  language  than  Liebis's 
work,  is  OTi  tliat  account  better  adapted  for  the  use  of  general  Farmers,  and  ought  to  be  first  read. 
Tlie  author  in  his  Preface  says  that  a  ■'  perusal  of  this  work  with  ordinary  attention  will  fumith 
the  necessary  amount  of  chemical  information  for  the  purposes  of  the  Farmer." 


In  reference  to  the  first  two  volumes  of  the  Farmers'  Library  and  Monthly  Jovrnnl  nf  Ai^'ricn!- 
ture,  now  bound  up  and  ready  for  sale,  the  Hon.  N.  S.  Benton,  Secretary  of  State  of  tlie  State  of 
New  York,  writes  to  the  publishers  as  follows: — 

Secretary's  Office,  Department  of  Common  Schools, 

Albany,  July  15,  lS4fi. 
I  have  examined,  with  as  much  care  and  attention  as  my  time  would  permit,  the  first  volumes  of  the 
JOURNAL  OF  AGRICULTURE  AND  THE  FARMERS'  LIBRARY,  published  by  Messrs  Greeley  & 
McElrath,  New  York,  and  do  not  perceive  any  objections  to  their  introduction  into  the  School  District 
Libraries  of  the  State  ;  and  1  can  have  no  doubt  this  work  would  prove  valuable  acquisitions  in  all,  but 
especially  to  those  where  the  subject  of  agriculture  excites  the  attention  of  the  inhabitants  of  the  district. 

N.  S.  BENTON,  Supt.  Com.  Schools. 

The  Deputy  Superintendent  of  the  Common  Schools  of  the  State  of  New  York,  writes  as  fol- 
lows : — 

Secretary's  Office,  Department  of  Common  Schools, 
Messrs.  Greeley  <fe  McElrath  : —  Albany,  July  9,  1846. 

Gentlemen  :  I  should  be  happy  to  see  this  work  (the  Farmers'  Library  and  Journal  of  Agricul- 
ture) in  every  School  Library  in  the  State  ;  and  I  hope  you  will  be  able  to  afford  it  at  a  price  which  will 
place  it  at  the  command  of  the  rural  districts  especially,  where  I  am  sure  it  can  not  fail  of  being  highly 
appreciated  and  extensively  read.  Works  of  l^iis  description  are,  in  my  judgment,  eminently  suitable  for 
our  District  Libraries  ;  and  I  know  of  none  more  useful  or  practical  than  the  present.  Its  execution  is 
exceedingly  creditable  to  tlie  publishers  ;  and  the  vast  amount  of  interesting  matter  comprised  in  its 
pages  can  not  fail  of  insuring  it  a  wide  circulation  among  the  agricultural  community — the  bulwark  of 
the  State.  Very  respectfullv, 

b.  S.  RANDALL,  Dep.  Supt.  Com.  Schools. 


~-) 


THE  FAliMERS'  LIBRARY 

AND 

MO]XTHLY  JOUENAL  OF  AGEICULTUIIE. 

The  first  year  of  this  great  Agi-icultural  Periodical  closes  with  the  June  number,  1846.  The 
pages  of  the  Library  portion  are  occupied  with  Petzholdt's  Agricultural  Chemistry  and  Von 
Thaor's  Principles  of  Agriculture.  The  pages  of  the  Monthly  T.  .urnal  portion  of  the  work  are  very 
diversified  in  their  subjects.     The  following  are  some  of  th'    eading  articles  : 


No.  I — (.Iuly). — Memoir  of  the  late  Stephen  Van 
Rensselaer  (with  a  tine  steel  portrait) ;  Deep  Tiow- 
in;: — An  Kxperiraent  illustratins  its  Effects :  Biitish 
.-Viri'ic-ultuiT.l  Dissertations ;  Prize  Essay  on  Fai'm 
Wanagenicnt,  (with  an  engraved  Plan  for  laying  out  a 
farm) ;  Fall  Plowing ;  On  the  Value  and  the  Progress 
of  Agriculiui-al  Science,  with  Extracts — from  J.  S. 
\V<ulsworth  ;  The  Poetry  of  Rural  Life  ;  Claims  of 
Agriculture  upon  the  Business  Community;  Gunno 
—  Recent  Experiments  in  Maryland  and  Virginia; 
.South-Down  Shf.'ep  (with  lithographic  portraits) ; 
Letter  fi'om  Hon  Andrew  Stevenson  of  Virginia; 
Soutliern  Apiculture — Remarks  of  the  Editor;  The 
Si'k  Plant  of  Tripoli  twiih  a  lithographic  illustration) 
— Letter  from  D.  S.  McCauley  to  Francis  Markoe ; 
Culture  of  Silk  in  Soulli  Carolina ;  A  New  Vegeta- 
ble 'Kohl  Rabi)  and  New  Grasses  (Tussac  Grass i — 
Recommended  to  be  imported ;  Agricultural  Ma- 
chines paientcds;  Effei  t.'^  of  Electricity  on  Vegetation; 
The  nisease  in  Potatoes — Various  Theories  ;  Notices 
of  titw  Books ;  Great  Sale  of  Cattle  at  Albany ; 
Items,  &.C. 

No.  IJ —  August). — Lady  Suffolk  (witli  aporti-ait); 
A  Dissertation  on  Horse-Breeding,  and  on  the  Trot- 
ting Horses  of  the  U.  S. ;  Obituary  Notice  of  Gen.  T. 
M.  Forman,  of  Sid. ;  Turnip  Culture  in  England  ; 
Under-Draining;  Irrigation  ;  Water-Mesdows;  Ento- 
molog}' ;  Canada  Thistle  (illustrated);  Comparative 
Value  of  Different  Kinds  of  Sheep  for  the  New-York 
Farmer ;  On  the  Preservation  of  Health  ;  The  Cause 
of  Education;  Agricultural  Associations  and  Science; 
Draining- Tile;  Lune  as  a  Fertilizer ;  XVIIlth  Annual 
Fair  of  the  American  Institute;  New-York  State  Ag- 
ricultural Society  Cattle  Show  at  Utica  ;  Good  Signs 
for  the  South,  &c.  &c. 

No.  Ill— (September^. — Brief  Sketch  of  the  (.-iuali- 
ties  of  the  Short-Horned  Bull  (with  a  portrait) — On 
the  Good  and  Bad  Points  of  Cattle  ;  St.  John's  Day 
Rye  and  Lucerne;  N.  Y.  State  Agricultural  Fair; 
Sugar — its  Culture  and  Manufacture  ;  Comparison  of 
Guano  with  otlier  Manures  ;  Mismanacement  of  Sta- 
ble-Dung Manure  ;  Entomology  ;  Cheshire  Cheese — 
A  Prize  Essay,  by  Henry  White  ;  Silk  Plant— Guano; 
Native  or  Wild  Maize ;  Thoughts  on  Trees  and 
Flowers ;  The  Clergy — their  power  to  improve  the 
Public  Taste  for  Agriculture  and  Horticulture— Let- 
ter from  Rev.  J.  O.  Choules  ;  The  Poeti-y  of  Rural 
Life  ;  Trials  of  Sulphuric  Acid  and  Bones  for  Tuniips; 
Use  of  Sulphuric  Acid  with  Bones  as  Compost;  Cot- 
tun  Plant  (illustrated-,  &c.  &c. 

No.  IV  — (OCTOBEKi. —  Memoir  of  Liebig  (with  a 
portraiti;  The  Sort  of  Information  wanted  at  the 
South  ;  To  Prevent  Smut  in  Wheat ;  Memoir  of  the 
Cotton  Plant,  by  W.  B.  Peabrook  ;  The  Central  or 
Red-Land  District  of  Virginia — Letter  from  Hon.  W. 
L.  Goirgin  ;  Various  Opinions  on  ^^oiIing  ;  Principles 
to  observe  in  the  erection  of  Farm  Houses;  Mannge- 
Bient  of  Farms — Mr  Hammond's  Fai-m ;  Atmosphere 
of  Stables;  Rellecnons  on  the  Progress  of  Agiicul- 
lural  Im;)rovement,  and  the  Political  and  Moral  In- 
fiucnce  of  Rural  Life — Letter  from  Gen.  Dearboni : 
ProiiTcss  of  Aixricultural  Improvement — Letter  from 
.Iiidge  Ropt ;  Improvement  in  the  mode  of  attaching 
L'orses  to  Wagons ;  Paring  and  Bunting  ;  The  Cen- 
ter of  Gravity  (illustrated) ;  A  Review  on  the  Past, 
Present,  -and  Fttture  State  of  the  Wool  Market ;  List 
of  Premiums  awarded  by  the  New- York  State  Agri- 
cultural Fair,  &c   &c. 

Ko.  V — (NovEMBEK).  Memoir  of  Hon.  Puchard 
I'pters  of  Pa.  (with  a  portrait) ;  Tunisian  Sheep  (with 


portraits)  ;  History  and  Uses  of  the  Cotton  Plant ; 
I.e'fe-  Tiom  Dr.  J.  Johnson  of  S.  C.  on  the  Silk  Plant ; 
1  noughts  on  Transplanting  Trees  :  Agriculiural  Ad- 
dress before  the  (iueens  Co.  Ag.  So.  by  J.  .-'.  f^kin- 
ner  ;  Guano  as  a  Manure  ;  Liebig's  Explanation  of 
the  Principles  and  use  of  Artificial  Manures  ;  Wine 
Making,  by  Rev.  S.  Weller,  with  Notes  by  S.  Clark  ; 
How  to  keep  Farm  Registers  ;  Entomology  ;  Man- 
agement of  Bees  ;  Sulphuric  Acid  and  Bones ;  The 
Fair  ot  the  American  Institute ;  t-hecp  and  Chest- 
nuts, &-C.  &c. 

No.  VI— (December).— Poultry  (with illustrations); 
Successful  Experiments  in  r^oiling;  Agricultural 
Products  of  the  United  .-tatesand  Great  Britain  ;  The 
Potato  Mun-ain  ;  Consumption  of  .'^usar  in  Europe 
and  North  America  ;  Wayes  and  Condition  of  Wo- 
men and  Children  emphiyed  in  the  ALTicuhnral  La- 
bor in  England;  History  and  u.->e  ot  the  Cotton 
Plant  (concluded  ■ ;  Wool-growing  at  the  .'^outh  ;  On 
Breeding  Horses;  Education  iti  Virdnia;  Potato 
f^tarch;  The  Inclined  Plane  (with  illushations);  Pea 
Culture  in  the  South  ;  societies  for  the  Promotion 
of  Agriculture,  Horticulture,  &c. ;  Agi-icullural  Pre- 
miums ;  yheep  Husbandry;  Peters's  Agricultural 
Account  Book  ;  Exposition  of  the  Condition  and  Re- 
sources of  Delaware,  &c. 

No.  VII— (January).- Farm  Buildings  (with  illus- 
trations) ;  Treatise  on  Milch  Cows,  whereby  the 
Quantity  and  (3,uality  of  Milk  which  any  Cow  will 
give  may  be  accurately  detennined  .  with  numerous 
illustrations)— by  U  Fr.  Guenon  :  Maryland  Farmers' 
Club  on  the  Right  Tack  ;  The  Mode  in  which  Lime 
Operates  on  t^oil ;  Poultry  and  Useful  Recipes ; 
Thoughts  on  the  Distribution  of  Labor  ;  Jerusalem 
Artichoke  ;  <  ellars  vs.  spring-Houses  for  Dairies  ; 
Flax  and  Hemp  Husbandry;  One-Horse  <  'arts  (with 
illustrations)  ;  The  Hydraulic  Ram  (with  illustra- 
tions) ;  Comparative  Views  of  the  Pi-ogress  of  Popu- 
lation in  different  Regions  of  the  United  Mates  ;  The 
Importance  of  Draining  Land,  &c. 

No.  VIII— (February).— Ti-eatise  on  Milch  Tows 
(with  illustrations) — continued  ;  The  Potato  Disease  ; 
Characteristics  of  different  Bi-eeds  of  Horses  -by  Hon! 
Zadock  Pratt ;  On  Fattening  f'attle  ;  The  Language 
of  Birds— Character  and  Habits  of  the  Whip-poor- 
will;  The  Importance  of  acquiring  a  Knowledge  of 
the  Natural  Science  :  "Lime  Enricheth  the  Father 
hiu  Impoverieheth  the  Son  ";  (  apital  needed  for  Aff- 
ricultural  Improvt'm.ent  ;  The  Use  of  .-alt  to  Man  and 
Animals;  On  the  Curina  of  Provisions  for  the  British 
Markets;  Sketch  of  Belgian  Husbandry  ,  The  Flower 
Garden,  &c.  cfcc. 

No.  IX— (March\— Smithsonian  Fund  ;  The  Pro- 
per Position  of  Country  Dvvclling-Houses  and  Barns; 
Raising  Potatoes  frr.m  -eed;  .-chcme  of  Reducing 
the  Quantity  of  Cotton  ;  .-ouihern  Hemp,  or  Kear- 
Grass  ;  Insects  Injurious  to  Vegetation  ;  Importing 
Societies;  Treaii.se  on  Milch"  Cows — continued; 
Quaker  or  Friends' Farming;  Flooding  Mea.lows  •' 
The  shepherd's  Dog,  ifcc.  &c.  ' 

No.  X  — (April \— Guano— its   Nature  and  Use 

by  Prof  Hardy  ;  Prospects  in  Virginia  for  New  Set 
tiers;  The  Bread-Fniif  Tree 'with  illustrations)  ;  Su- 
gar, and  its  Effects  on  Man  and  Animals  ;  The'  Sci- 
ence of  Botany  and  Horticulture;  Ammonia  and 
Water  in  Guano  ;  General  Treatment  of  Greenhouse 
Plants  ;  Eft'ects  of  Drouth  on  Indian  ( 'orn  ;  Philadel- 
phia Butter;  Treatise  on  Milch  cow.s — concluded- 
Labor  and  Machinery  ;  The  Diseases  of  the  Horse  ; 
Insects  most  Injurious  to  Vegetables  and  Animals,  &c. 


(  l^^  Each  year's  Numbers  contain  two  large  octavo  volumes  of  600  pages  each.  All  the  Num- 
J  bers  of  the  First  year  can  still  be  purcha.sed.  The  First  Number  of  the  Second  year  commences 
)    '.vith  July,  18-16.         GREELEY  &  McELRATH,  Publishers,  Tribune  Buildings.  New- York. 


D'ISRAELI'S 

CURIOSITIES   OF  LITERATURE.     • 

Every  body  knows  that  this  is  a  very  curious  book,  because  snch  is  its  general  reputation.  But  it  is 
act  known  to  every  one  why  it  is  so  curious,  because  comparatively  few  have  bad  an  opportunity  of  es- 
amining  it  for  themselves.  We  give  below  the  headings  of  the  different  matters  discussed  or  embraced 
in  this  interesting  volume,  so  that  authors,  literai-y  and  professional  gentlemen,  and  others,  may  judge  for 
themselves,  to  some  extent,  at  least,  whether  or  not  they  can  longer  conveniently  dispense  with  the  oppor- 
tunity of  personally  consulting  the  work.  Did  any  one  ever  see  such  a  medley  of  oddities,  or  such  a 
groupin"'  of  the  queer  things  growing  out  of  literary  productions  and  their  authors  as  are  contained  in 
what  follows  ? 


FIRST  SERIES. 

Libraries — The  Bibliomania — Literary  Journals — Re- 
covery of  Manuscripts — Sketches  of  Criticism — The  Per- 
secuted Learned— Poverty  of  the  Learned— Imprison- 
ment of  the  Learned — Amusements  of  the  Learned — Por- 
traits of  Authors — Destruction  of  Books — Some  Notices 
of  Lost  Works — Quodlibets,  or  Scholastic  Disquisitions — 
Fame^  Contemned— The  Six  Follies  of  Science— Imita- 
tors—Cicero's  Puns- Prefaces— The  Ancients  and  Mod- 
ems  Some  Ingenious  Thoughts — Early  Printing — Errata 

—Patrons— Poets,  Philosophers  and  Artists  made  by  Ac- 
cident— Inequalities  of  Genius — Conception  and  Expres- 
eion- Geosi'aphical   Diction— Legends— The   Port-Royal 

Society The   progress   of  Old  Age  in  New   Studies — 

Spanish  Poetry — St.  Evremond — RIen  of  Genius  deficient 
in  Conversation— Vida — The  Scuderies— De  La  Roche- 
foucaull — Prior's  Hans  Carvel— The  Student  in  the  Me- 
tropolis—The Talmud— Rabbinical  Stories— On  the  Cus- 
tom of  Saluting  after  Sneezing — Bonaventure  de  Periers 
—Grotius— Noblemen  turned  critics— Literary  Impos- 
tures  Cardinal  Richeheu — Aristotle  and  Plato — Abelard 

and  Eloisa — Physiognomy — Cbai-acters  described  by  Mu- 
sical Notes — Milton— Origin  of  Newspapers— Trials  and 
Proofs  of  Guilt  in  Superstitious  Ages— Inquisition— Singu- 
larities observed  by  various  Nations  in  their  Repasts — 
Monarchs — Titles  of  Illustrious,  Highness,  and  Excel- 
lence  Titles  of  Sovereigns — Royal  Divinities — Dethroned 

Monarchs— Feudal  Customs — Joan  of  Arc— Gaming— 
The  Arabic  Chronicle— Metempsychosis— Spanish  Eti- 
quette—The Goths  and  Huns— Of  Vicars  of  Bray— Doug- 
las—Critical  History  of  Poverty— Solomon  and  Sheba— 
Hell— The  Absent  Man — Wax-Work— Pasquin  and  Mar- 
forio — Female  Beauty  and  Ornaments — Modern  Platon- 

isjji Anecdotes  of  Fashion — A  Senate  of  Jesuits — The 

Lover's  Heart— The  History  of  Gloves— Relics  of  Saints 
—Perpetual  Lamps  of  the  Ancients— Natural  Produc- 
tions resembling  Artificial  Compositions— The  Poetical 
Garland   of  Julia — The   Violet -Tragic   Actors — Jocular 

Preachers Masterly   Imitators — Edward    the    Fourth— 

Elizabeth— The  Chinese  Language— Medical  Music— Mi- 
nute Writing — Numeral  Figures— English  Astrologers  — 
Alchj-my- Titles  of  Books— Literary  Follies— Literary 
Controversy— Literary  Blunders  — A  Literary  Wife- 
Dedications— Philosophical  Descriptive  Poems— Pam- 
phlets-Little Books— A  Catholic's  Refutation— The  Good 
Advice  of  an  old  Literary  Sinner— Mysteries,  Moralities, 
Farces  and.Sotties-Love  and  Folly,  an  Ancient  Morality 
— Reliffious  Nouvellettes— '  Critical  Sagacity,'  and  Happy 
Conjecture  ;  or,  Bentley's  Milton— A  Jansenist  Dictionary 
—Manuscripts  and  Books— The  Turkish  Spy— Spenser, 
Jonson,  and  Shakspeare— Ben  JoHson,  Feltham  and  Ran- 
dolph—Ariosto  and  Tasso— Venice— Bayle— Cervantes— 
Maaliabechi — Abridgers— Professors  of  Plagiarism  and 
Obscurity— Literary  Dutch— The  Productions  of  the  Mind 
not  seizable  by  Creditors— Critics — Anecdotes  of  Authors 
Censured Virginity— A  Glance  into  the  French  Acad- 
emy  Poetical  and  Grammatical  Deaths — Scarron— Peter 

Comeille— Poets— Romances — The  Astrea — Poets  Lau- 
reate—An^elo  Politian — Original  Letter  of  Queen  Eliza- 
beih— Anne  Bullen— James  I. — General  Monk  and  his 
Wife— Philip  and  Mary— Charles  the  First — Duke  of 
Buckingham— The  Death  of  Charles  IX.— Royal  Promo- 
tions—Nobility— Modes  of  Salutation,  and  Amicable  Cere- 
monies,  observed  in  various    Nations — Singularities   of 

■yVar Fire,  and  the   Origin  of  Fire-Works — The  Bible 

Prohibited  and    Improved— Origin  of  the  Materials  of 


Writing— Anecdotes  of  European  Manners — Tlie  Early 
Drama— The  Marriage  of  the  Arts — A  Contrivance  in 
Dramatic  Dialogue — The  Comedy  (jf  a  Madman— Soli- 
tude— Literaiy  Friendships — Anecdotes  of  Abstraction  of 
Mind — Richardson — Theological  Style— -Influence  of 
Names— The  Jews  of  York — The  Sovereignty  of  the  Seas 
— On  the  Custom  of  Kissing  Hands — Popes— Literary 
Composition — Poeiical  Imitations  and  Similarities — Ex- 
planation of  theFac-Simile — Literary  Fashions — The  Pan- 
tomimical  Characters — Extempore  Comedies — Massinger, 
Milton,  and  the  Italian  Theatre—  Songs  of  Trades,  or  Songs 
for  the  People— Introducers  of  Exotic  Flowers,  Fruits,  etc. 
— Usurers  of  the  Seventeenth  Century — Chidiock  'J'itch- 
bourae — Elizabeth  and  her  Parliament— Anecdotes  of 
Prince  Henry,  the  Son  of  James  I.,  when  a  Child — The 
Diaiy  of  a  Master  of  the  Ceremonies — Diaries :  Moral, 
Historical  and  Critical — Licensers  of  the  Press — Of  Ana- 
grams and  Echo  Verses — Oithogi-aphy  of  Proper  Names — 
Names  of  our  Streets — Secret  Histoiy  of  Edv/ard  Vere, 
Earl  of  Oxford — Ancient  Cookery  and  Cooks — Ancient 
and  Modern  Satunialia — Reliqute  Gethiniana^ — Robinson 
Crusoe — Catholic  and  Protestant  Dramas— The  History  of 
the  Theah-e  during  its  suppression — Drinking-Customs  in 
England  — Literary  Anecdotes — Condemned  Poets — Aca- 
jou and  Zirphile — Tom  O'Bedlams — Introduction  of  Tea, 
Coffee  and  Chocolate — Charles  the  First's  Love  of  the 
Fine  Arts — The  Secret  History  of  Charles  I.  and  his  Queen 
Henrietta — The  Minister ;  the  Cardinal  Duke  of  Richelieu 
— The  Minister ;  Duke  of  Buckingham,  Lord  Admiral. 
Lord  General,  (Sec.  &.c.  &c. — Felton,  the  Political  Assassin 
— Johnson's  Hints  for  the  Life  of  Pope. 

SECOND  SERIES. 

Modem  Literature — Characteristics  of  Bayle — Cicero 
viewed  as  a  Collector — History  of  Caraccas — English 
Academy  of  Literature — Quotation — Origin  of  Dante's 
Infei-no — Of  a  History  of  Events  which  have  not  hap- 
pened— Of  False  Political  Reports — Of  Supjiressors  and 
Dilapidators  of  Manuscripts — Parodies — Anecdotes  of  the 
Fairfax  Family — Medicine  and  Morals — Psalm  Sinsring — 
On  the  Ridiculous  Titles  assumed  by  the  Italian  Acade- 
mies— On  the  Hero  of  Hudibras ;  Butler  Vindicated — 
Shenstone's  School  Mistress— Ben  Jonson  on  Translation 
— The  Loves  of  the  Lady  Arabella — Domestic  History  of 
Sir  Edward  Coke— Of  Coke's  Stjie  and  his  Conduct — Se- 
cret History  of  Authors  who  have  ruined  their  Booksell- 
ers—Local  Descriptions — Masques — Of  Des  Maizeaux 
and  the  Secret  History  of  Anthony  Collins's  Manuscripts 
— History  of  New  Words — Political  Nick-Names — Domes- 
tic Life  of  a  Poet — Shenstone  Vindicated— Secret  History 
of  the  Building  of  Blenheim— Secret  History  cf  Sir  Wal- 
ter Raleigh — Literary  Unions-  Secret  Histtrj  of  Ra- 
leigh's Histoiy  of  the  World  and  Vasari's  Lives— Of  a  Bi- 
ography Painted — Cause  and  Pretext — Political  Forgeries 
and  Fictions — Expression  of  Sujtpressed  Opinion— Auto- 
graphs— History  of  Writing-Masters — Italian  Historians — 
Palaces  built  by  Ministers — "  Taxation  no  Tyranny  " — 
Book  of  Death— History  of  the  Skeleton  of  Death— Rival 
Biographers  of  Heylin — Of  Lenglet  du  Fresnoy — Diction- 
ary of  Ti'evoux — Quadrio's  Account  of  English  Poetry — 
Political  Religionism— Toleration— Apology  for  the  Parisian 
Massacre — Prediction — Dreams  at  the  Dawn  of  Philoso- 
phy—Puck the  Commentator— Literai-y  Forgeries — Lit- 
erary Filchers — Lord  Bacon  at  Home— Secret  History  o( 
the  Death  of  Queen  Elizabeth— James  the  First  as  a  Fa 
ther  and  Husband — The  Man  of  One  Book— A  Bibliog- 


noste — Secret  Historj' of  an  Elective  Monarchy  ;  n  Polit- 
icul  Sketch — Buildinas  in  the  Sleiropolis  and  Residence 
in  the  Country— Royal  Proclamalions— Time  Sotirces  of 
Secret  History — Literary  Residences — AVhcther  Allowa- 
ble to  Kuin  One's  Self— Discorerics  of  Secluded  Men — 
Senlimental  Biography— Literary  Parallels — 'i'he  Pearl 
Bible  and  Six  Thousand  Errata — View  of  a  Particular  Pe- 
riod of  the  State  of  Kclision  in  our  Civil  Wars — Bucking- 
ham's Political  Coquetry  with  the  Puritans— Sir  Edward 
Coke's  exceptions  against  the  High  Sherift''s  Oath — Se- 
cret History  of  Charles  the  First  and  his  Parliaments — 
The  Rump — Life  and  Habits  of  a  Literary  Antiquary — 
Oldys  and  his  MSS. 


THE  LITERARY  CHARACTER,  &c.  WITH  THE  HIS- 
TORY OF  MEN  OF  GENIUS. 

On  Literary  Characters— Youth  of  Genius — The  First 
Studies — The  In-itabilil)' of  Genius — The  Spirit  of  Litera- 
ture, and  the  Spirit  of  Society  -  Literary  Solitude — Med- 
itations of  Genius— Tlie  Enthusiasm  of  Genius— Literary 
Jealousy — Want  of  Mutual  Esteem — Self  Praise — The  Dq. 
mestic  Life  of  Genius— The  Matrimonial  State — Lileiaiy 
p'riendships — The  Literary  and  Personal  Character-^ Tlie 
Man  of  Letters — Literary  Old  Age — Literary  Honors — The 
Influence  of  Authors. 


The  new  American  Edition  of  this  ^vovk  contains,  also, 


CURIOSITIES  OF  AMERICAN  LITERATURE. 

BY  RUFUS  W.  GRISWOLD. 


'•American  Taxation  " — Authorship  of  the  Declaration 
of  Independence — American  Cadmus — Anagrams — Ethan 
Alleti — Bay  Psalm  Book — Anne  Bradstreet;  her  Poems — 
"  Baltie  of  Bunker  Hill " — Ballad  on  the  Burning  of 
Charlestown-" Ballad  of  the  Tea  Party" — '-Battle  of 
Trenton  " — "  Brave  Pawling  and  the  Spy  " — "  Bokl  Haw- 
thorne " — "  Battle  of  the  Kegs  " — Bj-les,  Mather,  and  Jo- 
seph Green — Alexander  H.  Bogart — Life,  Writings  and 
Opinions  of  Barlow — Beveridge  :  his  Latin  Poems — Cu- 
rious Account  of  the  Battle  of  Saratoga — The  Cow  Chase, 
written  by  Andre — Cherokee  Alphabet ;  invention  of— 
Correspondence  of  Dr.  Mayhew  -  Controversial  Menda- 
,-ity — Literai-y  Confederacies — Dunton's  "  Life  and  Er- 
rors " — Dedication  of  the  Indian  Bible — "  Discourse  con- 
?erning  the  Currencies  in  the  British  Plantations  in 
America — Lord  Timothy  Dexter;  his  "  Pickle  for  the 
Knowing  Ones" — Dedications  and  Introductory  Poems — 
Dr.  Dwight  and  Mr.  Dennie — Thomas  Dudley  ;  Epitaph 
on — Eliot  and  his  Indian  Translations— Epitaphs,  Ana- 
jrams  and  Elegies  of  the  Puritans — Elegy  on  Thomas 
?hepard.by  Urian  Oakes — Editorial  Recantations — Framp- 
ItKi's  "New  Found  World" — ^"Free  America,"  by  General 
vVaiTen — "Fate  of  John  Burgoyne,"  a  Ballad — Peter 
Foulcer :  his  "  Looking-Glass  for  the  Times  " — Fabrica- 
tion of  Authorities — Joseph  Green  and  Byles — History  of 
f.'onnecticut,  by  Dr.  Peters — Francis  Hopkinson— Rev. 
Thomas  Hooker,  Elegy  on,  by  Cotton  ;    Lament  for  his 


Death— Josselyn's  two  Voyages  to  America— Keith"? 
"  Travels  from  New-Hampshire  tf)  Caratuck," — Love 
well's  Fight,  Ballad  on — Cotton  Mather :  his  Life  and 
Character  ;  his  connection  with  the  Witchcraft  Delusion; 
Grahame's  Opinion  of  his  "Magnalia"- — Minstrelsy  of  the 
Indian  Wars  and  the  Revolution — Dr.  Jonathan  Mayhew 
— "  New-England's  Prospect," — "  The  North  Campaign,"  a 
Ballad — William  Penn  and  John  Locke — Poetry  of' Gov- 
ernor Wolcott — Poem  by  Allen  on  the  Boston  Massacre — 
The  Pah-iot's  Appeal — "  Progress  of  ."^ir  Jack  Brag  " — 
Robert  Treat  Paine ;  High  Prices  paid  for  his  Poems  ; 
Rapidity  with  which  he  wrote— Rare  and  Curious  Books 
by  the  Early  Travelers  in  America — Rogers's  "  Concise 
Account  of  North  America  " — Rivington  and  Freneau — ■ 
Rivington's  Confessions;  Last  Will  and  Testament;  Epi- 
grams on — Edward  Randolph — "Randolph's  Welcome" 
— James  Ralph — Rapid  Composition — "  Simple  Cobler  of 
Aggawam  '' — Satirical,  Dramatic,  and  other  Poems,  wTJt- 
ten  during  the  Revolution — "  Song  for  the  Sons  of  Lib- 
erty " — Dr.  J.  M.  Sewall ;  his  Writings — Sands  ;  Fabrica- 
tion   of   Authorities — Benjamin    Towne '•  Virgo    Tri- 

umphans  " — "  Virginia  Richly  Valued,"  &c.- — Verses  ou 
the  Massacre  of  Wyoming — Pioger  Williams  and  his  Con- 
troversies— Nathaniel  Ward  ;  his  "  Simple  Cobler  of  Ag- 
gawam " — War  Song,  written  in  1776 — Michael  Wiggles- 
worth  ;  Extracts  from  his  "  Day  of  Doom  ;"  Epitaphon — 
Dr.  Witherspoon  and  Benjamin  Towne. 


|^°  Tlie  above  work  is  published  complete  in  one  very  large  royal  octavo  volume,  handsomely  bound 
in  fall  cloth,  lettered  and  gilt  backs,  with  a  Portrait  of  the  author,  and  is  sold  at  the  low  price  of  $2  50.  It 
is  publislied  in  New-York  by  Greeley  &  McElrath,  but  can  be  purchased  through  any  Bookseller. 

GERMAN  LANGUAGE. 

A  PHRASE  BOOK  IN  ENGLISH  AND  GERMAN,  with  a  Literal  Translation  of  the  German  into 
English,  together  with  a  Complete  Explanation  of  the  Sounds  and  the  Accentuation  of  the  German : 
By  MoRiTZ  Ertheiler.     (For  Schools  a-nd  Private  Learners.)     Price  25  cents,  or,  bound,  37^  cents. 


A    TREATISE    OxN    MILCH    COWS, 

Whereby  the  Quality  and  Quantity  of  Milk  which  any  Cow  will  give  may  be  accurately  determined,  by  observ- 
ing Natural  Marks  or  External  Indications  alone  ;  the  length  of  time  she  will  continue  to  give  Milk,  &c.,  &c. 
By  M.  FRANCIS  GUENcJN,  of  Liborne,  France.  Translated  for  *;he  Farmers'  Library,  from  the  French,  by 
N.  P.  Trist,  Esq.,  late  United  States  Consul  at  Havana.     With  Introductory  Remarks  and  Observations  on 

Tile   Coiv  and   tlie   Dairy, 

BY    JOHN    S.    SKINNER,    EDITOR    OF    THE    FARMERS'    LIBRARY. 

ILLUSTRATED   WITH   NUMEROUS   ENGRAVINGS, 

IJO'  Price  for  single  copies,  neatly  done  up  in  paper  covers,  37^  cents.  Library  edition,  full  bound  in  cloth 
and  lettered,  62  1-2  cents.     The  usual  discount  to  Booksellers,  Agents,  Country  Merchants,  and  Pedlars. 

This  extraordinary  Book  has  excited  the  attention  of  the  ablest  Agriculturists  of  the  country.  Five  Thou- 
sand copies  were  sold  in  the  first  four  weeks  of  its  publication  in  New  York.  The  Publishers  have  received 
tmmerous  testimonials  as  to  the  usefulness  and  accuracy  of  Guenon's  Theory,  while  others,  from  partial  ex- 
periments, have  doubted  its  accuracy.  The  practical  remarks,  and  the  useful  information  contained  in  the 
first  part  of  the  Book  is  worth  more  to  any  Farmer  than  the  whole  cost. 

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Now  puhlishijig  in  Monthhj  Parts,  i?i  the  FARMERS'  LIBRARY, 
Price  50  cents  each,  or  ^5  per  annum, 

THE  BOOK  OF  THE  FAM: 

BEING  A  SYSTEMATIC  WORK  ON 

PRACTICAL  AGRICULTURE, 

ON  AN  ENTIRELY  NEW  AND  ORIGINAL  PLAN. 

BY  HENRY  STEPHENS, 

Editor  of  "  The  Quarterly  Journal  of  Agriculture,"  and  "  Prize  Essays  and  Transactions  of  the  Highland 
and  Agricultural  Society  of  Scotland." 

ILLUSTRATED  WITH  PORTRAITS  OF  ANIMALS, 

PAINTED  FROM  THE  LIFE— BEAUTIFULLY  ENGRAVED  ;   AND  NUMEROUS  WOODCUTS  AND 

PLATES  OF  AGRICULTURAL  IMPLEMENTS, 

So  particularized  as  to  enable  Country  Mechanics  to  construct  them  from  the  descriptions. 

Of  the  style,  costliness,  and  value  of  this  celebrated  work,  some  idea  may  be 
formed,  when  we  state  that,  in  the  first  place,  it  contains  more  than  1400  pages, 
with  upward  of  Six  Hundred  Engravings!  and,  further,  that  in  England  it  re- 
quired more  than  two  years  to  publish  it,  and  cost  there  $24.  This  neat  work  is 
now  publishing  in  the  Farmers'  Library.  No  farmer  who  thirsts  for  knowledge 
himself,  or  who  aspires  to  have  his  son  rise  to  the  true  "  post  of  honor,"  the  digni- 
fied station  of  an  intellectual  and  accomplished  agriculturist,  can  justifiably  deny 
himself  such  a  work  as  is  found  in  the  Farmers'  Library  and  Monthly  Jour- 
nal OF  A.GRICULTURE. 

Among  the  Six  Hundred  Engravings  which  will  be  published  in  this  BOOK  OF 
THE  FARM,  we  have  only  room  to  mention  the  following : — 

Views  of  Farmsteads,  or  Farm  Buildings ;  Fine  Specimens  of  Cattle,  Horses, 
Oxen,  Swine,  Cows,  Sheep,  fee. ;  Thrashing-Machines  ;  Sowing-Machines  ;  Grub- 
bers ;  The  Farm-House  ;  Servant's  Houses  ;  Fences;  Thorn  Hedges;  Field  Gates  ; 
Stone  Dykes  ;  Embankments  ;  Draining — an  Open  Drain  in  Grass :  Covered  do. ; 
Planks  and  Wedges  to  prevent  Sides  of  Drains  falling  in,  &:c.,  &c.,  &c.  AGRI- 
CULTURAL IMPLEMENTS  of  all  kinds;  Various  Kinds  of  Plows:  Sections 
and  Parts  of  do. ;  Shovels  ;  Scoops  ;  Spades  ;  Plumb-Level ;  Swing-Trees  for  two 
Horses,  for  three  Horses,  for  four  Horses ;  Harrows  ;  Horse-Hoes  ;  Rollers  ;  Straw- 
Racks;  Water-Troughs;  Straw-Cutters;  Shepherd's  Crook;  Snowl'iw;  Dung- 
Hawk;  Scythe  and  Bend  Sned;  Bull's  Ring ;  Bullock  Holder;  Rakes  Form  of 
Haystacks;  Corn-Bruisers;  Riddles;  Rope-Spinners;  Ladders;  Bean-,  41 ;  In- 
strument for  Topping  Turneps  ;  Turnep-Trough  for  Feeding  Sheep  ;  Movah  Shed 
for  Sheep  ;  Oil-Cake  Breaker  ;  Wheelbarrow  ;  Turnep  Slicer  for  Sheep  ;  Fi^^  "ing 
for  relieving  Cattle  of  Choking;  the  Milking-Pail ;  Curd-Cutter;  Cheese-Vat; 
Churns;  Cheese-Press;  &c.,  &c.  Horse-Cart;  Liquid-Manure  Cart ;  Single-horse 
Tilt-Cart,  &c.,  &c.,  &c.  Various  Operations  connected  with  the  Culture  of  Grain 
&c.,  &c.,  &c.  Also,  Plans  for  Irrigation;  Insects  aff'ecting  Live  Stock  and  Crops; 
Harness,  Bridle-Bit,  Collars,  &c.,  &c.,  &c.,  &c. 

[C7=  This  great  Work  is  now  publishing  in  the  FARMERS'  LIBRARY  AND 
MONTHLY  JOURNAL  OF  AGRICULTURE,  the  subscription  price  of  which 
is  $5  per  annum.  Every  farmer,  and  every  gentleman  who  owns  land  or  culti- 
vates a  garden,  is  earnestly  requested  to  examine  this  Work. 

GREELEY  &  McELRATH,  Publishers. 

New  York,  July  1,  1846. 


STEPHENS'S  BOOK  OF  THE  PARI. 

600   ENGBAVIKGS! 

The  following  general  titles  of  chapters  and  parts  of  the  Work  of  Mr.  Stephens,  will 
give  only  a  very  imperfect  notion  of  the  variety  and  extent  of  the  entire  contents. 


CONTENTS  OF  STEPHENS'S 


The  difficulties  which  the  young  farmer  lias 
to  encounter  at  the  outset  of  learning  prac- 
tical Husbandry. 

The  means  of  overcoming  those  difficuUies. 

The  liind  of  information  to  be  found  in  ex- 
istent works  on  Agriculture. 

The  construction  of  "The  Book  of  the 
Farm." 

The  existing  methods  of  learning  practical 
Husbandry. 

The  establishment  of  scientific  institutions 
of  practical  Agriculture. 

The  evils  attendant  on  landowners  neglect- 
ing to  learu  practical  Agi'iculture. 

Experimental  farms  as  places  for  instruc- 
tion isi  farming. 

A  few  words  to  young  farmers  who  intend 
emigrating  as  agricultural  settlers  to  the 
colonies. 

The  kind  of  education  best  suited  to  young 
farmers. 

The  different  kinds  of  farming. 

Choosing  the  kind  of  farming. 

Selecting  a  tutor  farmer  for  teachiugfarming. 

The  pupilage. 

Dealing  with  the  details  of  farming. 

WINTER. 

The  steading  or  farmstead. 

The  farm-house. 

The  persons  who  labor  the  farm. 

The  weather  in  winter. 

Climate. 

Observing  and  recording  facts. 

Soils  and  subsoils. 

Enclosures  and  shelter. 

Tlie  planting  of  thorn  hedges. 

The  plow. 

The  various  modes  of  plowing  ridges. 

Draining. 

Yoking  and  harnessing  the  plow,  and  of 
swing-trees. 

Plowing  stubble  and  lea-ground. 

Trench  and  subsoil  plowing,  and  moor-land 
pan. 

Drawing  and  stowing  turnips,  mangel-wur- 
zel, cabbage,  carrots,  and  parsnips. 

The  feeding  of  sheep  on  turnip-s  '^  winter. 

Driving  and  slaughtering  sheep. 

Rearing  and  feeding  cattle  on  turnips  in 
winter. 

Driving  and  .slaughtering  cattle. 

The  treatment  of  farm-horses  in  winter. 

Fattening,  driving,  and  slaughtering  swine. 

The  treatment  of  fowls  in  winter. 

Threshing  and  winnowing  grain,  and  of  the 
the  threshing-machine. 

The  wages  of  farm-servants. 

Com  markets. 

The  farm-smith,  joiner,  and  saddler. 

The  forming  of  dung-hills,  and  of  liquid  ma- 
nure tanks. 

Winter  irrigation. 

SPRING. 

Cows  calving,  and  of  calves. 

The  advantages  of  having  field-work  in  a 

forward  state. 
Cross-plowing,  drilling,  and  ribbing  land. 


BOOK  OF  THE  FARM. 

49.  Sowing  spring  wheat  and  grass  seeds. 

50.  Sowing  beans,  peas,  tares,  lucerne,  sainfoin, 

flax,  and  hemp. 

51.  Switching,  pruning,  and  water-tabling  thorn- 

hedges. 

52.  Hiring  farm-servants. 

53.  Sowing  oat-seed. 

54.  The  lambing  of  ewes. 

55.  The  training  and  working  the  shepherd's 

dog. 

56.  Sovv'ing  barley-seed. 

56.  Turning  dunghills  and  composts. 

57.  Planting  potatoes. 

58.  Breaking  in  young  draught-horses. 

59.  Sows  farrowing  or  littering. 

60.  The  hatching  of  fowls. 

SUMMER. 

61.  The  sowing  of  turnips,  mangel-wurzel,  rape, 

caiTots,  and  parsnips. 

62.  Repairing  the  fences  of  grass-fields,  and  the 

proper  construction  of  field-gates. 

63.  The  weaning  of  calves,  bulls,  and  the  graz- 

ing of  cattle  till  winter. 

64.  Mares  foaling,  stallions,  and  horses  at  grass. 

65.  Sheep-washing,  sheep-shearing,  and  wean- 

ing of  lambs. 

66.  Rolling  the  fleece,  and  the  qualities  of  wool. 

67.  The  making  of  butter  and  cheese. 

68.  Weeding  corn,  green  crops,  pastures,  and 

of  casualties  to  plants. 

69.  Hay-making. 

69.  Summer-fallowing,  and  liming  the  soil. 

70.  Building  stone-dykes. 

71.  Embankments  against  rivulets. 

72.  Forming  water-meadows 

73.  Breaking-in  young  saddle-horses. 

AUTUMN. 

74.  Pulling  flax  and  hemp,  and  of  the  hop. 

75.  Reaping  rj-e,  wheat,  barley,  oats,  beans,  and 

peas. 

76.  Carrying  in   and  stacking  wheat,   barley, 

oats,  beans,  and  peas. 

77.  Drafting  ewes  and  gimmers,  tupping  ewes, 

and  bathing  and  smearing  sheep. 

78.  Lifting  and  pitting  potatoes. 

79.  Sowing  annual  wheat,  and  the  twjstruction 

and  principles  of  agricultural  wheel-car- 
riages. 

80.  Eggs. 


81.  Rotation  of  crops. 

82.  Fertilizing  the  .soil  by  means  of  manures. 

83.  The  points  posses.sed  by  the  domesticated 

animals  most  desirable  for  the  farmer  to 
cultivate. 

84.  Making  experiments  on  farms. 

85.  Destroying  and  scaring  vermin  on  farms. 
8.5.  Looking  at  a  farm,  its  rent — its  lease — its 

stocking — the  capital  required  for  it. 

86.  Improving  waste  land. 

87.  Farm  book-keeping. 

88.  The  conveniences  of  the  cottages  of  farm- 

servants. 

89.  The  care  to  be  bestowed  on  the  preserva- 

tion of  implements. 
Index. 


Extracts  from  the  Critical  Notices  published  in  England  during  the  publication 
of  the  loork  in  London. 

From  the  London  Times. 
"  The  first  part  or  number  of  this  work  has  just  been  published  by  Messrs.  Blackwood.  It  is  written  by 
Mr.  Henry  Stephens,  a  gentleman  already  known  to  the  pulilic  in  his  editorial  character  in  the  Quarterly 
Journal  of  Agriculture.  The  great  merit  of  the  work,  as  far  as  it  has  yet  gone,  is  the  intelligible  manner  in 
which  it  is  written,  and  the  strong  good  sen.se  with  which  it  is  distinguished.  The  proposed  arrangement. 
Bet  forth  in  the  i)lan  of  the  work,  is  clear  and  satisfactory;  and  the  whole  number  is  va.!uable  as  being  the 
result  of  practical  experience  and  competent  theoretic  knowledge.  It  is  a  book  which  will  be  received  with 
gratitude  by  those  who  are  really  anxious  to  profit  by  instruction,  and  whose  anxiety  for  iinproveinent  is 
not  impeded  by  prejudice."  ..."  The  plan  of  the  work,  it  may  again  be  observed,  is  very  good— the 
reasoning  is  logical — the  assertions  are  the  results  of  accurate  exaininaiion  and  repeated  expencnce.  In 
addition  to  the  information  conveyed  in  the  letter-press,  the  book  is  ornamented  by  accurate  and  handsome 
plates  of  agricultural  animals,  implements  of  farming,  plans  of  farming,  &c.  &c." 

From  the  Newcastle  Courant. 
"  Mr.  Stephen.s's  work  is  divided  into  three  portions.  In  the  first,  the  pupil  is  shown  the  difficulties  he  has 
to  encounter  in  acquiring  a  competent  knowledge  of  farming  as  a  profession,  and  the  most  easy  and  eft'ect- 
ual  methods  of  overcoming  these,  'i'he  second  poition  details  the  various  kinds  of  fanning  practiced  in  the 
country,  and  points  out  that  which  the  Author  reckons  the  best  for  adoption  under  given  circumstances. — 
The  third  and  concluding  portion  accompanies  the  young  farmer  into  the  world,  where  it  acquaints  him 
how  to  look  about  for  a  proper  farm  for  himself" 

From  Felix  Farley's  Bristol  Journal. 
•^When  we  say  that  the  Author  is  Mr.  Henry  f^tephons,  we  are  safe  in  expressing  our  conviction  that  the 
results  of  his  penetration,  judgment,  and  experience,  so  placed  before  the  public,  will  confer  an  advantage 
on  the  agiieultural  interest  of  no  common  order.    We  therefore  predict  a  large  measure  of  success  to  the 
intended  work." 

jfram  The  Argus. 
"We  regard  It  as  a  national  work ;  and,  from  the  masterly  manner  In -which  Mr.  Stephens  handles  nis 
subjects,  we  feel  assured  it  must  become  a  standard  one.     His  thorough  practical  knowledge,  backed  by  his 
scientific  acquirements,  makes  the  Author's  fitness  for  the'  task  conspicuous ;  and  the  unpresuming  manner 
in  which  his  talent  is  displayed  enhances  its  value  s.ill  more  in  our  eyes." 
From  the  Midland  Comities  Herald. 
"The  entirely  practical  nature  of  this  work,  and  tlw  evident  care  with  which  it  is  produced,  will,  we 
think,  render  it  one  of  the  most  useful  public-itions  for  tlie  farmer  which  has  yet  appeared-" 

From  The  Times. 
"  The  great  merit  of  the  work,  as  far  as  it  has  yet  gone,  is  the  intelligible  manner  in  which  it  is  written, 
and  the  strong  good  sense  with  which  it  is  distinguished.     It  is  a  book  which  will  be  received  with  grati- 
tude by  those  who  are  really  anxious  to  profit  by  instruction,  and  whose  anxiety  for  improvement  is  not 
impeded  by  prejudice. 

From  the  Birmingham  Advertiser. 
"  The  farmers  of  England  would  do  well  to  possess  themselves  of  this  work,  for  the  variety  of  useful  in- 
formation, and  the  many  practical  suggestions  it  contains." 

From  The  Britannia. 
"  The  two  parts  now  before  us  are  models  of  clear,  sensible  composition,  and  fonn  such  an  introduction 
to  the  practice  of  farming  as  has  never  been  published  before.  The  author  brings  to  his  task  a  large  store 
of  knowledge,  sound  sense  and  a  lucid  style."  "We  are  quite  sure  that  never  was  any  work  more  called 
for,  by  the  intelligence  of  the  age  than  this  '  Book  of  the  Farm,'  and  believe  that  it  could  not  have  been 
entrusted  to  more  competent  hands,  or  produced  in  better  style.  We  strongly  recommend  it  to  all  classes 
of  agriculturists  as  a  publication  of  decided  utility,  and  likely  to  be  most  serviceable  to  tlrem  in  the  suc- 
cessful prosecution  of  tneir  labors." 

From  the  Sporting  Review. 
"  The  work  before  us  is  one  of  the  most  practical  results  of  so  patriotic  a  spirit.     It  is  a  most  wel- 
come addition  to  our  rural  literature.     As  it  proceeds,  we  hope  to  transfer  some  of  its  good  things  to 
our  pages. 

From  the  New  Farmers'  Journal. 
"On  all  these  important  points,  no  one  is  better  qualified  to  .fill  the  office  of  a  mentor  than  Mr.  Stephens, 
of  which  the  well-an-anged  plan,  and  judicious  execution,  of  the  book  before  us,  aft'ord  u-refragable 
testimony." 

The  Concluding  Paragraph. 

Mr.  Stephens,  the  Author  of  tbe  above  named  work,  was  engaged  for  several  years  in  writmg 
it.  Its  publication  was  commenced  in  London  in  January,  1842,  and  concluded  in  August,  1844. 
The  Author  closes  the  work  in  the  following  words: 

"  I  have  now  brought  to  a  termination  the  task  I  had  imposed  upon  myself  in  writing  this  work. 
If  you  will  but  follow  the  prescriptions  I  have  given  in  it,  for  conducting  the  larger  operations  of 
the  field,  and  for  treating  the  various  animals  of  the  fann  ;  and — not  to  mention  the  proper  plow- 
ing and  manuring  of  the  soil — as  the  practice  of  every  farmer  demonstrates  the  necessity  of  afford- 
ing due  attention  to  those  most  import-ant  because  fundamental  operations,  if  j-ou  finish  off  your 
fields  in  a  manner  indicating  care  and  neatness — plowing  round  their  margins,  and  turning  over 
the  corners ;  if  you  keep  your  fences  clean  and  in  a  state  of  repair — your  fields  free  of  weeds ;  if 
you  give  your  slock  abundance  of  fresh  food  at  regular  intervals  in  winter,  and  supply  them  with 
plenty  of  clean  water  on  fresh  pastures  in  summer ;  if  you  have  the  farm  roads  always  in  a  ser- 
viceable state,  and  everything  about  the  steading  neat  and  orderly  ;  if  you  exhibit  skill  and  taste 
in  all  these  matters,  and  put  what  is  called  a  fine  skin  on  your  farm,  you  will  not  fail  to  earn  for 
yourself  the  appellation  of  a  good  and  exemplary  farmer:  and  when  you  have  everything  about 
you  '  thus  well  disposed,'  you  will  find,  with  Hesiod  of  old,  that  profitably,  as  well  as  creditably, 
for  you  '  shall  glide  away  thy  rustic  year.'  '" 


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